Table of contents

We study the statistical properties of magnification perturbations by substructures in strong lensed systems using linear perturbation theory and an analytical substructure model including tidal truncation and a continuous substructure mass spectrum. We demonstrate that magnification perturbations are dominated by perturbers found within roughly a tidal radius of an image and that sizable magnification perturbations may arise from small, coherent contributions from several substructures within the lens halo. The rms fluctuation of the magnification perturbation is ~10%-20%, and both the average and rms perturbations are sensitive to the mass spectrum and density profile of the perturbers. Interestingly, we find that relative to a smooth model of the same mass, the average magnification in clumpy models is lower (higher) than that in smooth models for positive-parity (negative-parity) images. This is opposite from what is observed if one assumes that the image magnification predicted by the best-fit smooth model of a lens is a good proxy for what the observed magnification would have been if substructures were absent. While it is possible for this discrepancy to be resolved via nonlinear perturbers, we argue that a more likely explanation is that the assumption that the best-fit lens model is a good proxy for the magnification in the absence of substructure is not correct. We conclude that a better theoretical understanding of the predicted statistical properties of magnification perturbations by CDM substructure is needed in order to affirm that CDM substructures have been unambiguously detected.

There is reason to suspect that about half of the baryons in the universe are in pressure-supported plasma in the halos of normal galaxies, drawn in by gravity along with about half of the dark matter. We present a model for this substantial baryonic component, the galactic coronae, that fits the available observational constraints. This phenomenological approach requires departures from state-of-the-art numerical models of galaxy formation, but the adjustments are not so large as to seem unreasonable. In particular, massive coronae would have to be hotter than the kinetic temperature of the halo dark matter so as to produce acceptable central electron densities. This higher temperature might result from the difference of the fluid dynamics of the baryons and the collisionless dynamics of the dark matter during the assembly of the protogalaxy, in an analogy to what seems to happen in cluster formation. The cooling time of a massive corona would be longer than the gravitational collapse time but, in the inner parts, shorter than the Hubble time, making the corona thermally unstable to the formation of a cloudy structure that is settling and adding to the mass in interstellar matter and stars. Since in this picture the mass in the corona of a spiral galaxy is much larger than the mass in condensed baryons, the corona would be a substantial reservoir that could supply matter for star formation in isolated spirals continuing well past the present epoch.

We have developed a new halo-finding method called the physically self-bound (PSB) group-finding algorithm, which is designed to identify halos located in crowded regions. To demarcate subhalo regions, we introduce the tidal constraint combined with the self-boundedness check. In order to reduce overload and parallel finding, we first divide whole simulation particles into many local particle groups using two distinct methods. One adopts a density mesh used to group particles in connected local overdense cells, and the other applies the friends-of-friends (FoF) algorithm with a linking length (l_loc) 0.3 times the mean particle separation. Then we divide each local particle group into several subgroups enclosed by numerous density levels and identify tidally stable and self-bound subhalos around density peaks. Subhalo finding is done on a fine mesh with cell size equal to twice the force resolution. Particles in a density shell that surrounds only one density peak form a subhalo candidate. Particles located in the outer remaining density shells that surround more than one peak become member candidates of one of the subhalos. We determine the membership using the tidal boundary constraint. We have found that the mass function is very insensitive to l_loc when l_loc ≥ 0.3. At least 40% of subhalos do not seem to collapse to the most massive subhalo in a FoF group when the Press & Schechter collapse model is applied to measure the size of a collapsed structure in the Lagrangian space. We have applied our halo-finding method to a 1024³ particle simulation in a ΛCDM model and compare the halo mass functions with those previously found in the literature.

We propose to determine the optimal softening length in N-body halo simulations by minimizing the ensemble-average acceleration error at a small radius r₀. This strategy ensures that the error never exceeds the optimal value beyond r₀. Furthermore, we derive semianalytic formulae for calculating the acceleration error due to the discreteness of particles and softened gravity, which are validated by direct N-body force calculations. We estimate that current state-of-the-art halo simulations suffer ≳6% acceleration error at 1% of the halo virial radius. The error grows rapidly toward the center and could contribute significantly to the uncertainties of inner halo properties.

Simulations predict that the first stars in a ΛCDM universe formed at redshifts z > 20 in minihalos with masses of about 10⁶M_☉. We have studied their radiative feedback by simulating the propagation of ionization fronts (I-fronts) created by these first Population III stars (M_* = 15-500 M_☉) at z = 20, within the density field of a cosmological simulation of primordial star formation, outward through the host minihalo and into the surrounding gas. A three-dimensional ray-tracing calculation tracks the I-front once the H II region evolves a "champagne flow" inside the minihalo, after the early D-type I-front detaches from the shock and runs ahead, becoming R type. We take account of the hydrodynamic back-reaction by an approximate model of the central wind. We find that the escape fraction of ionizing radiation from the host halo increases with stellar mass, with 0.7 ≲ f_esc ≲ 0.9 for 80 ≲ M_*/M_☉ ≲ 500. To quantify the ionizing efficiency of these stars as they begin cosmic reionization, we find that for M_* ≳ 80 M_☉, the ratio of gas mass ionized to stellar mass is ~60,000, roughly half the number of ionizing photons released per stellar baryon. Nearby minihalos are shown to trap the I-front, so their centers remain neutral. This is contrary to the recent suggestion that these stars would trigger formation of a second generation by fully ionizing neighboring minihalos, stimulating H₂ formation in their cores. Finally, we discuss how the evacuation of gas from the host halo reduces the growth and luminosity of "miniquasars" that may form from black hole remnants of the first stars.

Using a large set of high-resolution numerical simulations incorporating nonequilibrium molecular hydrogen chemistry and a constant source of external radiation, we study gas collapse in previously photoionized minigalaxies with virial temperatures less than 10⁴ K in the early universe (redshifts z = 10-20). We confirm that the mechanism of positive feedback of ionizing radiation on star formation in minigalaxies proposed by Ricotti and coworkers can be efficient despite a significant flux of metagalactic photodissociating radiation. We derive critical fluxes for the Lyman-Werner background radiation sufficient to prevent the collapse of gas in minigalaxies as a function of the virial mass of the halo and redshift. In our model, the formation of minigalaxies in defunct H II regions is most efficient at large redshifts (z ≳ 15) and/or for large local gas overdensity δ ≳ 10. We show that nonequilibrium chemistry plays an important dynamical role not only during the initial evolutionary phase, leading to the gas becoming gravitationally unstable inside the minihalo, but also at the advanced stages of the core collapse, resulting in efficient gas accretion in the core region. We speculate on a possible connection between our objects and metal-poor globular clusters and dwarf spheroidal galaxies.

We present U and B galaxy differential number counts from a field of ~900 arcmin², based on GOYA Survey imaging of the HST Groth-Westphal strip. Source detection efficiency corrections as a function of the object size have been applied. A variation of the half-exposure image method has been devised to identify and remove spurious detections. Achieved 50% detection efficiencies are 24.8 mag in U and 25.5 mag in B in the Vega system. Number count slopes are d log N/dm = 0.50 ± 0.02 for B = 21.0-24.5 and d log N/dm = 0.48 ± 0.03 for U = 21.0-24.0. Simple number count models are presented that simultaneously reproduce the counts over 15 mag in U and B and over 10 mag in K_s, using a Λ-dominated cosmology and SDSS local luminosity functions. Only by setting a recent z_f ~ 1.5 formation redshift for early-type, red galaxies do the models reproduce the change of slope observed at K_s = 17.5 in NIR counts. A moderate optical depth (τ_B = 0.6) for all galaxy types ensures that the recent formation for elliptical galaxies does not leave a signature in the U or B number counts, which are featureless at intermediate magnitudes. No ad hoc disappearing populations are needed to explain the counts if number evolution is introduced using an observationally based z evolution of the merger fraction.

We study the physical and photometric properties of galaxies at z = 4 in cosmological hydrodynamic simulations of a ΛCDM universe. We focus on galaxies satisfying the ``B dropout" criteria of the Great Observatories Origins DEEP Survey (GOODS). Our simulations predict that high-redshift galaxies show strong correlations in star formation rate (SFR) versus stellar mass, and weaker correlations versus environment and age, such that B dropouts are predicted to be the most massive, most rapidly star-forming galaxies at z = 4, living preferentially in dense regions. The simulated rest-frame UV luminosity function (LF) and integrated luminosity density are in broad agreement with observations at z ~ 4. The predicted faint-end slope is intrinsically steep but becomes shallower and is in reasonable agreement with data once GOODS selection criteria are imposed. At the bright end, there may be a modest excess of bright, rapidly star-forming galaxies. The predicted rest-frame optical LF is approximately 1 mag brighter than the rest-frame UV LF. We predict that GOODS B dropouts represent less than 50% of the total stellar mass density formed in galaxies more massive than log(M_*/M_☉) > 8.7 by z = 4, mainly because of brightness limits in the HST ACS bands. Most of these results are somewhat sensitive to the effects of dust extinction. On average, simulated B dropouts are less metal enriched than observed low-redshift galaxies of similar stellar mass by ≈0.6 dex. Two simulated B dropouts exhibit SFRs exceeding 1000 M_☉ yr^-1, similar to observed submillimeter galaxies. These galaxies are quite massive but are not undergoing starbursts; their SFRs only mildly exceed (by ~2-3 times) the SFRs that would be expected for their stellar mass. Finally, the overall distribution of dust reddening and mean stellar age may be constrained from color-color plots although the specific value for each galaxy cannot.

The observed far greater number of giant arcs than that predicted by the ΛCDM cosmology has been a long-standing puzzle. We adopt high-resolution cosmological simulations to assess this problem and quantitatively show that this issue can be resolved by inclusion of high-redshift source galaxies. Our ΛCDM results reveal a much higher giant-arc probability (>10^-5) for source galaxies of z > 2 than that for z < 2. Together with the source redshift distribution derived from the NTT catalogs, we obtain the predicted giant-arc number counts, which turn out to be consistent with most observations. The fact that the high-redshift (z ≥ 1.5) galaxies are preferentially magnified by the foreground lenses with a surprisingly high efficiency explains why previous theoretical work substantially underestimated the giant-arc number density. In previous work, the source galaxies were assumed to be mostly around z = 1, an assumption commonly adopted in weak-lensing studies. In addition, although the lens mergers have been found to yield enhanced lensing cross sections, we find that merely 5% of giant arcs are associated with merging lenses. Our results support not only the ΛCDM cosmology but also the long-standing anticipation that galaxy clusters, mostly formed below z = 1, are powerful gravitational telescopes for probing the high-redshift protogalaxies. We illustrate how the patchy dust regions in interacting galaxies at z = 4 may appear as filamentary structures within a giant arc when observed at the submillimeter wave band.

Using a new model for quasar lifetimes and light curves derived from numerical simulations of galaxy mergers that incorporate black hole growth, we study the faint-end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our model motivates a new interpretation of the QLF in which the bright end consists of quasars radiating near their peak luminosities but the faint end is mostly made up of brighter peak luminosity quasars seen in less luminous phases of evolution. The faint-end slope of the QLF is then set by the behavior of the lifetime (light curve) of quasars with peak luminosities near the observed break when they are in less luminous stages of evolution. We determine the faint-end slope of the QLF from the quasar lifetime, based on a set of simulations that encompass a wide range of host galaxy, merger, black hole, and interstellar gas properties. Brighter peak luminosity (higher black hole mass) systems undergo more violent evolution, and gas is expelled and heated more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From our model, we predict the evolution of the faint-end slope of the QLF and find good agreement with observations. Although black holes grow in an antihierarchical manner (with lower mass black holes formed primarily at lower redshifts), in our picture the observed change in slope and differential or "luminosity-dependent density evolution" in the QLF is determined by the nontrivial, luminosity-dependent quasar lifetime and physics of quasar feedback, and not by changes in the shape of the underlying peak luminosity or active black hole mass distributions.

We report identification of the radio-loud narrow-line quasar SDSS J172206.03+565451.6, which we found in the course of a search for radio-loud narrow-line active galactic nuclei (AGNs). SDSS J172206.03+565451.6 is only about the fourth securely identified radio-loud narrow-line quasar and the second-most radio loud, with a radio index R_1.4 ≈ 100-700. Its black hole mass, M_BH ≃ (2-3) × 10⁷M_☉ estimated from Hβ line width and 5100 Å luminosity, is unusually small given its radio loudness, and the combination of mass and radio index puts SDSS J172206.03+565451.6 in a scarcely populated region of M_BH-R diagrams. SDSS J172206.03+565451.6 is a classical narrow-line Seyfert 1-type object with FWHM_Hβ ≃ 1490 km s^-1, an intensity ratio of [O ]/Hβ ≃ 0.7, and Fe II emission complexes with Fe λ4570/Hβ ≃ 0.7. The ionization parameter of its narrow-line region, estimated from the line ratio [O ]/[O ], is similar to Seyferts, and its high ratio of [Ne ]/[Ne ] indicates a strong EUV-to-soft X-ray excess. We advertise the combined usage of [O ]/[O ] and [Ne ]/[Ne ] diagrams as a useful diagnostic tool to estimate ionization parameters and to constrain the EUV-soft X-ray continuum shape relatively independently from other parameters.

Using radio observations by FIRST and NVSS, we build a sample of 151 radio-variable quasars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Six (and probably another two) of these objects are classified as broad absorption line (BAL) quasars and show radio flux variations of a few tens of percent within 1.5-5 yr. Such large amplitudes of variations imply brightness temperatures much higher than the inverse Compton limits (10¹² K) in all these BAL quasars, suggesting the presence of relativistic jets beaming toward the observer. The angles between the outflow and the jet are constrained to be less than ~20°. Such BAL quasars with polar outflows are beyond the simple unification models of BAL and non-BAL quasars, which hypothesize BAL quasars as otherwise normal quasars seen nearly edge-on.

In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a knot at one end plus an extended tail, resembling a tadpole. These "tadpole galaxies" appear dynamically unrelaxed—presumably in an early merging stage—where tidal interactions likely created the distorted knot-plus-tail morphology. Here we systematically select tadpole galaxies from the HUDF and study their properties as a function of their photometric redshifts. In a companion HUDF variability study presented in this issue, Cohen et al. revealed a total of 45 variable objects believed to be active galactic nuclei (AGNs). Here we show that this faint AGN sample has no overlap with the tadpole galaxy sample, as predicted by recent theoretical work. The tadpole morphology—combined with the lack of overlap with the variable objects—supports the idea that these galaxies are in the process of an early-stage merger event, i.e., at a stage that likely precedes the "turn-on" of any AGN component and the onset of any point-source variability. We show that the redshift distribution of tadpole galaxies follows that of the general field galaxy population, indicating that—if most of the tadpole galaxies are indeed dynamically young—the process of galaxy assembly generally kept up with the reservoir of available field galaxies as a function of cosmic epoch. These new observational results highlight the importance of merger-driven processes throughout cosmic history and are consistent with a variety of theoretical and numerical predictions.

We present a photometric search for objects with point-source components that are optically variable on timescales of weeks to months in the Hubble Ultra Deep Field (HUDF) to i = 28.0 mag. The data are split into four substacks of approximately equal exposure times. Objects exhibiting the signature of optical variability are selected by studying the photometric error distribution between the four different epochs and selecting 622 candidates as 3.0 σ outliers from the original catalog of 4644 objects. Of these, 45 are visually confirmed as free of contamination from close neighbors or various types of image defects. Four lie within the positional error boxes of Chandra X-ray sources, and two of these are spectroscopically confirmed AGNs. The photometric redshift distribution of the selected variable sample is compared to that of field galaxies, and we find that a constant fraction of ~1% of all field objects show variability over the range of 0.1 ≲ z ≲ 4.5. Combined with other recent HUDF results, as well as those of recent state-of-the-art numerical simulations, we discuss a potential link between the hierarchical merging of galaxies and the growth of AGNs.

As a result of deep hard X-ray observations by Chandra and XMM-Newton, a significant fraction of the CXRB has been resolved into individual sources. These objects are almost all AGNs, and optical follow-up observations find that they are mostly obscured type 2 AGNs, have Seyfert-like X-ray luminosities, and peak in redshift at z ~ 0.7. Since this redshift is similar to the peak in the cosmic star formation rate, this paper proposes that the obscuring material required for AGN unification is regulated by star formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of type 2 to type 1 AGNs that is a function of both z and 2-10 keV X-ray luminosity, L_X. The evolutionary models are constrained by parameterizing the observed type 1 AGN fractions from the recent work by Barger et al. The parameterization that simultaneously best accounts for Barger's data, the CXRB spectrum, and the X-ray number counts has a local, low-L_X type 2/type 1 ratio of 4 and predicts a type 2 AGN fraction that evolves as (1 + z)^0.3. This particular evolution predicts a type 2/type 1 ratio of 1-2 for log L_X > 44, and thus the deep X-ray surveys are missing about half the obscured AGNs with these luminosities. These objects are likely to be Compton thick. Overall, these calculations show that the current data strongly support a change to the AGN unification scenario in which the obscuration is connected with star formation in the host galaxy rather than a molecular torus alone. The evolution of the obscuration implies a close relationship between star formation and AGN fueling.

We present results from a 35 ks Chandra ACIS-I observation of the hot intracluster medium (ICM) around the FR II radio galaxy 3C 388. 3C 388 resides in a cluster environment with an ICM temperature of ~3.5 keV. We detect cavities in the ICM coincident with the radio lobes. The enthalpy of these cavities is ~1.2 × 10⁶⁰ ergs. The work done on the gas by the inflation of the lobes is ~3 × 10⁵⁹ ergs, or ~0.87 keV per particle out to the radius of the lobes. The radiative timescale for gas at the center of the cluster at the current temperature is a few Gyr. The gas in the core was probably cooler and denser before the outburst, so the cooling time was considerably shorter. We are therefore likely to be witnessing the quenching of a cluster cooling flow by a radio galaxy outburst. The mechanical power of the lobes is at least 20 times larger than the radiative losses out to the cooling radius. Outbursts of similar power with a ~5% duty cycle would be more than sufficient to continually reheat the cluster core over the Hubble time and prevent the cooling of any significant amount of gas. The mechanical power of the outburst is also roughly 2 orders of magnitude larger than either the X-ray luminosity of the active nucleus or the radio luminosity of the lobes. The equipartition pressure of the radio lobes is more than an order of magnitude lower than that of the ambient medium, indicating that the pressure of the lobe is dominated by something other than the relativistic electrons radiating at GHz frequencies.

The MAGIC Cerenkov telescope has observed very high energy (VHE) γ-ray emission from the active galactic nucleus 1ES 1959+650 during 6 hr in 2004 September and October. The observations were carried out alternating with observations of the Crab Nebula, whose data were used as a reference source for optimizing γ -ray/hadron separation and for flux comparison. The data analysis shows VHE γ-ray emission of 1ES 1959+650 with ~8 σ significance, at a time of low activity in both optical and X-ray wavelengths. An integral flux above ~180 GeV of about 20% that of the Crab Nebula was obtained. The light curve, sampled over 7 days, shows no significant variations. The differential energy spectrum between 180 GeV and 2 TeV can be fitted with a power-law of index -2.72 ± 0.14. The spectrum is consistent with the slightly steeper spectrum seen by HEGRA at higher energies, also during periods of low X-ray activity.

We present the results of two complementary investigations into the nature of strong (rest equivalent width, W_r > 1.0 Å) Mg II absorption systems at high redshift. The first line of questioning examines the complete Sloan Digital Sky Survey Data Release 3 set of quasar spectra to determine the evolution of the incidence of strong Mg II absorption. This search resulted in 7421 confirmed Mg II systems of W_r > 1.0 Å, yielding a >95% complete statistical sample of 4835 absorbers (systems detected in S/N > 7 spectral regions) spanning a redshift range 0.35 < z < 2.3. The redshift evolution of the comoving line-of-sight number density, ℓ_Mg(X), is characterized by a roughly constant value at z > 0.8, indicating that the product of the number density and gas cross section of halos hosting strong Mg II is unevolving at these redshifts. In contrast, one observes a decline in ℓ_Mg(X) at z < 0.8, which we interpret as a decrease in the gas cross section to strong Mg II absorption and therefore a decline in the physical processes relevant to strong Mg II absorption. Perhaps uncoincidentally, this evolution roughly tracks the global evolution of the star formation rate density. Dividing the systems in W_r subsamples, the ℓ_Mg(X) curves show similar shape with lower normalization at higher W_r values and a more pronounced decrease in ℓ_Mg(X) at z < 0.8 for larger W_r systems. We also present the results of a search for strong Mg II absorption in a set of 91 high-resolution quasar spectra collected on the ESI and HIRES spectrographs. These data allow us to investigate the kinematics of such systems at 0.8 < z < 2.7. In this search a total of 22 systems of W_r > 1.0 Å were discovered. These systems are characterized by the presence of numerous components spread over an average velocity width of Δv ≈ 200 km s^-1. Also, absorption due to more highly ionized species (e.g., Al III, C IV, Si IV) tends to display kinematic profiles similar to the corresponding Mg II and Fe II absorption. We consider all of these results in light of two competing theories previously introduced to explain strong Mg II absorption: post-starburst, supernova-driven galactic winds and accreting gas in the halos of massive galaxies. The latter model is especially disfavored by the absence of evolution in ℓ_Mg(X) at z > 1. We argue that the strong Mg II phenomenon primarily arises from feedback processes in relatively low mass galactic halos related to star formation.

We present the analysis of the velocity structure of the intracluster gas near the core of Abell 3526 obtained with two off-center Chandra observations, specifically designed to eliminate errors due to spatial variations of the instrumental gain. We detected a significant velocity gradient along the northeast-southwest direction, roughly perpendicular to the direction of the incoming subgroup Cen 45, in agreement with previous ASCA SIS measurements. The presence of gas bulk velocities is observed both with and without the inclusion of the Fe K line complex in the spectral fittings. The configuration and magnitude of the velocity gradient is consistent with near transonic circulatory motion, either bulk or eddylike. The velocity difference obtained using the best calibrated central regions of ACIS-S3 is found to be (2.4 ± 1.0) × 10³ km s^-1 for rectangular regions 24 × 3' roughly diametrically opposed around the cluster's core. There are also indications of a high-velocity zone toward the southern region with similar magnitudes. The detection of velocity gradients is significant at >99.4% confidence, and simulations show that intrachip gain fluctuations >1800 km s^-1 are required to explain the velocity gradient by chance. The measurements suggest that >1% of the total merger energy can still be bulk kinetic 0.4 Gyr after the merging event. This is the first direct confirmation of velocity gradients in the intracluster gas with independent instruments and indicates that strong departure from hydrostatic equilibrium is possible even for cool clusters that do not show obvious signs of merging.

This paper presents both stellar mass and H II region diagnostics based on dusty, radiation-pressure-dominated photoionization models for compact and ultracompact H II regions, and compares these with observational constraints. These models successfully reproduce the observed relationship between the density and the thickness of the ionized layer. The absorption of ionizing photons in the dusty ionized plasma makes denser ionized regions thinner than simple photoionization models would predict, improving the fit with the observations. The models provide a good fit to observed diagnostic plots involving ratios of infrared emission lines, all accessible with the IRS instrument of the Spitzer Space Telescope. These give the effective temperature to an accuracy of about 2500 K and the mass of the ionizing star to a precision of about ±30%. The S IV/S III ratio is sensitive to foreground extinction as well as to stellar effective temperature or mass. From this ratio, we determine that the mean extinction to observed compact H II regions is typically A_V ~ 30 mag. The electron temperature depends on the chemical abundances, the pressure, and the effective temperature of the exciting star. We use these models to rederive the slope of the Galactic abundance gradient, with the result that d log(O/H)/dR_G = 0.06 ± 0.01 dex kpc^-1, bringing the Galactic abundance gradient derived from compact H II regions into closer agreement with those based on other techniques. The shape of the far-IR SED of compact H II regions can be used to constrain the mean pressure or density in the H II region. The Spitzer MIPS instrument should be very helpful in this regard.

We present numerical simulations of the propagation of ultra-high-energy cosmic rays (UHECRs) above 10¹⁹ eV in a structured extragalactic magnetic field (EGMF) and simulate their arrival distributions at the Earth. We use the IRAS PSCz catalog in order to construct a model of the EGMF and source models of UHECRs, both of which reproduce the local structures observed around the Milky Way. We also consider modifications of UHECR arrival directions by the Galactic magnetic field. We follow an inverse process of their propagation from the Earth and record the trajectories. This enables us to calculate only trajectories of UHECRs arriving at the Earth, which saves CPU time. From these trajectories and our source models, we construct arrival distributions of UHECRs and calculate their harmonic amplitudes and two-point correlation functions. We estimate the number density of sources that best reproduces the Akeno Ground Air Shower Array (AGASA) observation. As a result, we find that the most appropriate number density of the sources is ~5 × 10^-6 Mpc^-3. This constrains the source candidates of UHECRs. We also demonstrate sky maps of their arrival distribution with the event number expected by future experiments and examine how the EGMF affects their arrival distribution. A main result is the diffusion of clustering events, which are obtained from calculations in the absence of the EGMF. This tendency allows us to reproduce the observed two-point correlation function better.

Using the Florida Multi-object Imaging Near-IR Grism Observational Spectrometer (FLAMINGOS), we have conducted the FLAMINGOS Extragalactic Survey (FLAMEX), a deep imaging survey covering 7.1 deg² within the 18.6 deg² NOAO Deep Wide-Field Survey (NDWFS) regions. FLAMEX is the first deep, wide-area, near-infrared survey to image in both the J and K_s filters, and is larger than any previous NIR survey of comparable depth. The intent of FLAMEX is to facilitate the study of galaxy and galaxy cluster evolution at 1 < z < 2 by providing rest-frame optical photometry for the massive galaxy population at this epoch. This effort is designed to yield a public data set that complements and augments the suite of existing surveys in the NDWFS fields. We present an overview of FLAMEX and initial results based on ~150,000 K_s-selected sources in the Boötes field. We describe the observations and reductions, quantify the data quality, and verify that the number counts are consistent with results from previous surveys. Finally, we comment on the utility of this sample for detailed study of the ERO population, and present one of the first spectroscopically confirmed z > 1 galaxy clusters detected using the joint FLAMEX, NDWFS, and Spitzer IRAC Shallow Survey data sets.

Using mid-IR and optical data, we deduce the total infrared (IR) luminosities of galaxies in the Coma Cluster and present their IR luminosity function (LF). The shape of the overall Coma IR LF does not show significant differences from the IR LFs of the general field, which indicates the general independence of global galaxy star formation from environment up to densities ~40 times greater than in the field (we cannot test such independence above L_IR ≈ 10⁴⁴ ergs s^-1). However, a shallower faint-end slope and a smaller L are found in the core region (where the densities are still higher) compared to the outskirt region of the cluster, and most of the brightest IR galaxies are found outside the core region. The IR LF in the NGC 4839 group region does not show any unique characteristics. By integrating the IR LF, we find a total star formation rate in the cluster of about 97.0 M_☉ yr^-1. We also studied the contributions of early- and late-type galaxies to the IR LF. The late-type galaxies dominate the bright end of the LF, and the early-type galaxies, although only making up a small portion (≈15%) of the total IR emission of the cluster, contribute greatly to the number counts of the LF at L_IR < 10⁴³ ergs s^-1.

We use HST ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster Survey to investigate the nature of diffuse star clusters (DSCs). Compared to globular clusters (GCs), these star clusters have low luminosities (M_V > -8) and a broad distribution of sizes (3 < r_h < 30 pc), but they are principally characterized by their low mean surface brightnesses, which can be more than 3 mag fainter than a typical GC (μ_g > 20 mag arcsec^-2). The median colors of diffuse star cluster systems (1.1 < g - z < 1.6) are redder than metal-rich GCs and often as red as the galaxy itself. Most DSC systems thus have mean ages older than 5 Gyr or else have supersolar metallicities, implying that diffuse star clusters are likely to be long-lived. Twelve galaxies in our sample contain a significant excess of diffuse star cluster candidates; nine are morphologically classified as lenticulars (S0s), and five visibly contain dust. We also find DSCs in the halo of the giant elliptical M49, near the companion galaxy VCC 1199. Most DSC systems appear spatially associated with galactic disks, but substantial DSC populations are not present in all lenticular galaxies, and environment is not a good predictor of their existence. Diffuse star clusters are similar to and include the locus of "faint fuzzies" identified in other nearby galaxies. Unlike luminous GCs, whose sizes are constant with luminosity, DSCs are bounded at the bright end by an envelope of nearly constant surface brightness. We suggest that populations of diffuse star clusters preferentially form, survive, and coevolve with galactic disks. Their properties are broadly consistent with those of merged star cluster complexes, and we note that despite being 3-5 mag brighter than DSCs, ultracompact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters, and if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find.

The 1 < z < 2 redshift window hosts the peak of the star formation and metal production rates. Studies of the metal content of the star-forming galaxies at these epochs are, however, sparse. We report VLT ISAAC near-infrared spectroscopy for a sample of five [O II]-selected, M_B,AB ≲ -21.5, z ~ 1.4 galaxies, by which we measured Hβ and [O III] λ5007 emission-line fluxes from J-band spectra, and Hα line fluxes plus upper limits for [N II] λ6584 fluxes from H-band spectra. The z ~ 1.4 galaxies are characterized by the high [O III]/[O II] line ratios, low extinctions, and low metallicities that are typical of lower luminosity CADIS galaxies at 0.4 ≲ z ≲ 0.7 and of more luminous Lyman break galaxies at z ~ 3 but not seen in CFRS galaxies at 0.4 ≲ z ≲ 0.9. This type of spectrum (e.g., high [O III] λ5007/[O II] λ3727) is seen in progressively more luminous galaxies as the redshift increases. These spectra are caused by a combination of high-ionization parameter q and lower [O/H]. PÉGASE2 chemical evolution models are used to relate the observed metallicities and luminosities of z ~ 1.4 galaxies to galaxy samples at lower and higher redshifts. Not surprisingly, we see a relationship between redshift and inferred chemical age. We suppose that the metal-enriched reservoirs of star-forming gas that we are probing at intermediate redshifts are being mostly consumed to build up both the disk and the bulge components of spiral galaxies. Finally, our analysis of the metallicity-luminosity relation at 0 ≲ z ≲ 1.5 suggests that the period of rapid chemical evolution may take place in progressively lower mass systems as the universe ages. These results are consistent with a "downsizing"-type picture, in the sense that particular signatures (e.g., high [O III]/[O II] or low [O/H]) are seen in progressively more luminous (massive) systems at higher redshifts.

It is well established that strong bars rotating in dense halos generally slow down as they lose angular momentum to the halo through dynamical friction. Angular momentum exchanges between the bar and halo particles take place at resonances. While some particles gain and others lose, friction arises when there is an excess of gainers over losers. This imbalance results from the generally decreasing numbers of particles with increasing angular momentum, and friction can therefore be avoided if there is no gradient in the density of particles across the major resonances. Here we show that anomalously weak friction can occur for this reason if the pattern speed of the bar fluctuates upward. After such an event, the density of resonant halo particles has a local inflexion created by the earlier exchanges, and bar slowdown can be delayed for a long period; we describe this as a metastable state. We show that this behavior in purely collisionless N-body simulations is far more likely to occur in methods with adaptive resolution. We also show that the phenomenon could arise in nature, since bar-driven gas inflow could easily raise the bar pattern speed enough to reach the metastable state. Finally, we demonstrate that mild external or internal perturbations quickly restore the usual frictional drag, and it is unlikely therefore that a strong bar in a galaxy having a dense halo could rotate for a long period without friction.

We model gravitational instability in a wide range of isolated disk galaxies, using GADGET, a three-dimensional, smoothed particle hydrodynamics code. The model galaxies include a dark matter halo and a disk of stars and isothermal gas. Absorbing sink particles are used to directly measure the mass of gravitationally collapsing gas. Below the density at which they are inserted, the collapsing gas is fully resolved. We make the assumption that stars and molecular gas form within the sink particle once it is created and that the star formation rate is the gravitational collapse rate times a constant efficiency factor. In our models, the derived star formation rate declines exponentially with time, and radial profiles of atomic and molecular gas and star formation rate reproduce observed behavior. We derive from our models and discuss both the global and local Schmidt laws for star formation: power-law relations between surface densities of gas and star formation rate. The global Schmidt law observed in disk galaxies is quantitatively reproduced by our models. We find that the surface density of star formation rate directly correlates with the strength of local gravitational instability. The local Schmidt laws of individual galaxies in our models show clear evidence of star formation thresholds. The variations in both the slope and the normalization of the local Schmidt laws cover the observed range. The averaged values agree well with the observed average and with the global law. Our results suggest that the nonlinear development of gravitational instability determines the local and global Schmidt laws and the star formation thresholds. We derive from our models the quantitative dependence of the global star formation efficiency on the initial gravitational instability of galaxies. The more unstable a galaxy is, the quicker and more efficiently its gas collapses gravitationally and forms stars.

We present the elemental abundances of HE 1327-2326, the most iron-deficient star known, determined from a comprehensive analysis of spectra obtained with the Subaru Telescope High Dispersion Spectrograph. HE 1327-2326 is either in its main-sequence or subgiant phase of evolution. Its non-LTE-corrected iron abundance is [Fe/H] = -5.45, 0.2 dex lower than that of HE 0107-5240, the previously most iron-poor object known, and more than 1 dex lower than those of all other metal-poor stars. Both HE 1327-2326 and HE 0107-5240 exhibit extremely large overabundances of carbon ([C/Fe] ~ +4). The combination of extremely high carbon abundance with outstandingly low iron abundance in these objects clearly distinguishes them from other metal-poor stars. The large carbon excesses in these two stars are not the result of a selection effect. There also exist important differences between HE 1327-2326 and HE 0107-5240. While the former shows remarkable overabundances of the light elements (N, Na, Mg, and Al), the latter shows only relatively small excesses of N and Na. The neutron-capture element Sr is detected in HE 1327-2326, but not in HE 0107-5240; its Sr abundance is significantly higher than the upper limit for HE 0107-5240. The Li I λ6707 line, which is detected in most metal-poor dwarfs and warm subgiants having the same temperature as HE 1327-2326, is not found in this object. The upper limit of its Li abundance [log < 1.5] is clearly lower than the Spite plateau value. These data provide new constraints on models of nucleosynthesis processes in the first-generation objects that were responsible for metal enrichment at the earliest times. We discuss possible scenarios to explain the observed abundance patterns.

We present measurements of the neutron-capture elements Rb and Pb in five giant stars of the globular cluster NGC 6752 and Pb measurements in four giants of the globular cluster M13. The abundances were derived by comparing synthetic spectra with high-resolution, high signal-to-noise ratio spectra obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope. The program stars span the range of the O-Al abundance variation. In NGC 6752, the mean abundances are [Rb/Fe] = -0.17 ± 0.06 (σ = 0.14), [Rb/Zr] = -0.12 ± 0.06 (σ = 0.13), and [Pb/Fe] = -0.17 ± 0.04 (σ = 0.08). In M13 the mean abundance is [Pb/Fe] = -0.28 ± 0.03 (σ = 0.06). Within the measurement uncertainties, we find no evidence for star-to-star variation for either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe], [Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample of field stars at the same metallicity. For M13 and NGC 6752 the Pb abundances are in accord with predictions from a Galactic chemical evolution model. If metal-poor intermediate-mass asymptotic giant branch stars did produce the globular cluster abundance anomalies, then such stars do not synthesize significant quantities of Rb or Pb. Alternatively, if such stars do synthesize large amounts of Rb or Pb, then they are not responsible for the abundance anomalies seen in globular clusters.

We present measurements of the oxygen abundance of the Milky Way's ISM by observing the K-shell X-ray photoionization edge toward galaxy clusters. This effect is most easily observed toward objects with Galactic columns (n_H) of a few times 10²¹ cm^-2. We measure X-ray column densities toward 11 clusters and find that at high Galactic columns above approximately 10²¹ cm^-2 the X-ray columns are generally 1.5-3.0 times greater than the 21 cm H I columns, indicating that molecular clouds become an important contributor to n_H at higher columns. We find the average ISM oxygen abundance to be (O/H) = (4.85 ± 0.06) × 10^-4, or 0.99 solar when using the most recent solar photospheric values. Since X-ray observations are sensitive to the total amount of oxygen present (gas+dust), these results indicate a high gas to dust ratio. Also, the oxygen abundances along lines of sight through high Galactic columns (n_H) are the same as abundances through low columns, suggesting that the composition of denser clouds is similar to that of the more diffuse ISM.

We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H₂ absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H₂ is intricate, and it is important to validate these numerical simulations in better understood environments. This paper studies a well-defined line of sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM toward HD 185418, a line of sight with many observables. We deduce density, temperature, local radiation field, cosmic-ray ionization rate, and chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density and similar abundances, and we require a cosmic-ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.

The spectral energy distribution of the dark cloud LDN 1622, as measured by Finkbeiner using WMAP data, drops above 30 GHz and is suggestive of a Boltzmann cutoff in grain rotation frequencies, characteristic of spinning dust emission. LDN 1622 is conspicuous in the 31 GHz image we obtained with the Cosmic Background Imager, which is the first centimeter-wave resolved image of a dark cloud. The 31 GHz emission follows the emission traced by the four IRAS bands. The normalized cross-correlation of the 31 GHz image with the IRAS images is higher by 6.6 σ for the 12 and 25 μm bands than for the 60 and 100 μm bands: C₁₂₊₂₅ = 0.76 ± 0.02, and C₆₀₊₁₀₀ = 0.64 ± 0.01. The mid-IR-centimeter-wave correlation in LDN 1622 is evidence for very small grain (VSG) or continuum emission at 26-36 GHz from a hot molecular phase. In dark clouds and their photon-dominated regions (PDRs), the 12 and 25 μm emission is attributed to stochastic heating of the VSGs. The mid-IR and centimeter-wave dust emissions arise in a limb-brightened shell coincident with the PDR of LDN 1622, where the incident UV radiation from the Ori OB 1b association heats and charges the grains, as is required for spinning dust.

We present polarization maps of G30.79 FIR 10 (in W43) from thermal dust emission at 1.3 mm and from CO J = 2 → 1 line emission. The observations were obtained using the Berkeley-Illinois-Maryland Association array in the period 2002-2004. The G30.79 FIR 10 region shows an ordered polarization pattern in dust emission, which suggests an hourglass shape for the magnetic field. Only marginal detections for line polarization were made from this region. Application of the Chandrasekhar-Fermi method yielded B_pos ≈ 1.7 mG and a statistically corrected mass to magnetic flux ratio λ_C ≈ 0.9, or essentially critical.

We present long-slit H- and K-band spectroscopy of the low-mass outflow source SVS 13, obtained with the adaptive-optics-assisted imager-spectrometer NACO on the VLT. With a spatial resolution of <025 and a pixel scale of 0027, we precisely establish the relative offsets of H₂, [Fe II], CO, H I, and Na I components from the source continuum. The H₂ and [Fe II] peaks are clearly associated with the jet, while the CO, H I, and Na I peaks are spatially unresolved and coincident with the source, as is expected for emission associated with accretion processes. The H₂ profile along the slit is resolved into multiple components, which increase in size, although they decrease in intensity, with distance from the source. This trend might be consistent with thermal expansion of packets of gas ejected during periods of increased accretion activity. Indeed, for the brightest component nearest the source, proper-motion measurements indicate a tangential velocity of 0028 yr^-1. It therefore seems unlikely that this emission peak is associated with a stationary zone of warm gas at the base of the jet. However, the same cannot be said for the [Fe II] peak, for which we see no evidence for motion downwind, even though radial velocity measurements indicate that the emission is associated with higher jet velocities. We postulate that the [Fe II] could be associated with a collimation shock at the base of the jet.

We have observed a high-mass protobinary system in the hot core W3(H₂O) with the BIMA array. Our continuum maps at wavelengths of 1.4 and 2.8 mm both achieve subarcsecond angular resolutions and show a double-peaked morphology. The angular separation of the two sources is 119, corresponding to 2.43 × 10³ AU at the source distance of 2.04 kpc. The flux densities of the two sources at 1.4 and 2.8 mm have a spectral index of 3, translating to an opacity law of κ_ν ∝ ν. The small spectral indices suggest that grain growth has begun in the hot core. We have also observed five K components of the methyl cyanide (CH₃CN) J = 12 → 11 transitions. A radial velocity difference of 2.81 ± 0.10 km s^-1 is found toward the two continuum peaks. Interpreting these two sources as binary components in orbit about each other, we find a minimum mass of 22 M_☉ for the system. Radiative transfer models are constructed to explain both the continuum and methyl cyanide line observations of each source. Power-law distributions of both density and temperature are derived. Density distributions close to the free-fall value, r^-1.5, are found for both components, suggesting continuing accretion. The derived luminosities suggest that the two sources have equivalent zero-age main-sequence (ZAMS) spectral type B0.5-B0. The nebular masses derived from the continuum observations are about 5 M_☉ for source A and 4 M_☉ for source C. A velocity gradient previously detected may be explained by unresolved binary rotation with a small velocity difference.

Gravitational lensing of high-redshift supernovae is potentially an important source of uncertainty when cosmological parameters are being derived from the measured brightness of Type Ia supernovae, especially in deep surveys with scarce statistics. Photometric and spectroscopic measurements of foreground galaxies along the lines of sight of 33 supernovae discovered with the Hubble Space Telescope, both core-collapse and Type Ia, are used to model the magnification probability distributions of the sources. Modelling galaxy halos with SIS or NFW profiles and using M/L scaling laws provided by the Faber-Jackson and Tully-Fisher relations, we find clear evidence for supernovae with lensing (de)magnification. However, the magnification distribution of the Type Ia supernovae used to determine cosmological distances matches very well the expectations for an unbiased sample; i.e., their mean magnification factor is consistent with unity. Our results show that the lensing distortions of the supernova brightness can be well understood for the GOODS sample and that correcting for this effect has a negligible impact on the derived cosmological parameters.

The capture and inspiral of compact stellar objects into massive black holes is an important source of low-frequency gravitational waves (with frequencies ~1-100 mHz), such as those that might be detected by the planned Laser Interferometer Space Antenna (LISA). Simulations of stellar clusters designed to study this problem typically rely on simple treatments of the black hole encounter that neglect some important features of orbits around black holes, such as the minimum radii of stable, nonplunging orbits. Incorporating an accurate representation of the orbital dynamics near a black hole has been avoided due to the large computational overhead. This paper provides new, more accurate expressions for the energy and angular momentum lost by a compact object during a parabolic encounter with a nonspinning black hole, and the subsequent inspiral lifetime. These results improve on the Keplerian expressions that are now commonly used and will allow efficient computational simulations to be performed that account for the relativistic nature of the spacetime around the central black hole in the system.

We study observational constraints on neutron star (NS) kicks for isolated pulsars and for NSs in binary systems. We are particularly interested in the evidence of kick-spin alignment/misalignment and its dependence on the NS initial spin period. For several young pulsars, X-ray observations of compact nebulae showed that pulsar proper motion is aligned with the spin direction as defined by the symmetry axis of the nebula. We also critically examine the measurements of the proper motion and the projected spin axis from a large sample of pulsars with well-calibrated polarization data. We find that among the two dozen pulsars for which reliable measurements are available, there is a significant correlation between the spin axis and the proper motion. For various NS binaries, including double NS systems, binaries with massive main-sequence star companions, and binaries with massive white-dwarf companions, we obtain constraints on the kick magnitudes and directions from the observed orbital characteristics of the system. We find that the kick velocity is misaligned with the NS spin axis in a number of systems, and the NS spin period (when available) in these systems is generally longer than several hundred milliseconds. These constraints, together with the spin-kick alignment observed in many isolated pulsars, suggest that the kick timescale is hundreds of milliseconds to 1 s, so that spin-kick alignment or misalignment can be obtained depending on the initial spin period of the NS. We discuss the implication of our result for various NS kick mechanisms.

We solve for the evolution of the vertical extent of the convective region of a neutron star atmosphere during a type I X-ray burst. The convective region is well mixed with ashes of nuclear burning, and its extent determines the rise time of the burst light curve. Using a full nuclear reaction network, we show that the maximum vertical extent of the convective region during photospheric radius expansion (RE) bursts can be sufficiently great that (1) some ashes of burning are ejected by the radiation-driven wind during the RE phase and (2) some ashes of burning are exposed at the neutron star surface following the RE phase. We find that ashes with mass numbers in the range A ~ 30-60 are mixed in with the ejected material. We calculate the expected column density of ejected and surface ashes in hydrogen-like states and determine the equivalent widths of the resulting photoionization edges from both the wind and the neutron star surface. We find that these can exceed 100 eV and are potentially detectable. A detection would probe the nuclear burning processes and might enable a measurement of the gravitational redshift of the neutron star. In addition, we find that in bursts with pure helium burning layers, protons from (α, p) reactions cause a rapid onset of the ¹²C(p, γ)¹³N(α, p)¹⁶O reaction sequence. The sequence bypasses the relatively slow ¹²C(α, γ)¹⁶O reaction and leads to a sudden surge in energy production that is directly observable as a rapid (~millisecond) increase in flux during burst rise.

We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10^-8 ergs cm^-2 s^-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with M_NS = 1.4 M_☉, R_NS = 10 km) to be 6.0 ± 0.5 kpc, or for M_NS = 2 M_☉ at most 7.1 kpc.

We have obtained FUSE and HST STIS time-resolved spectroscopy of the Polar AM Herculis during a deep low state. The spectra are entirely dominated by the emission of the white dwarf. Both the far-ultraviolet (FUV) flux and the spectral shape vary substantially over the orbital period, with maximum flux occurring at the same phase as during the high state. The variations are due to the presence of a hot spot on the white dwarf, which we model quantitatively. The white dwarf parameters can be determined from a spectral fit to the faint-phase data, when the hot spot is self-eclipsed. Adopting the distance of 79 pc determined by Thorstensen, we find an effective temperature of 19,800 ± 700 K and a mass of M_WD = 0.78 M_☉. The hot spot has a lower temperature than during the high state, ~34,000-40,000 K, but covers a similar area, ~10% of the white dwarf surface. Low-state FUSE and STIS spectra taken during four different epochs in 2002-2003 show no variation of the FUV flux level or spectral shape, implying that the white dwarf temperature and the hot spot temperature, size, and location do not depend on the amount of time the system has spent in the low state. Possible explanations are ongoing accretion at a low level or deep heating; both alternatives have some weaknesses, which we discuss. No photospheric metal absorption lines are detected in the FUSE and STIS spectra, suggesting that the average metal abundances in the white dwarf atmosphere are lower than ~10^-3 times their solar values.

Spectropolarimetric observations are presented for 21 AGB stars, 13 proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectra cover the wavelength range from ~4200 to 8400 Å with 16 Å resolution. Among the AGB stars, 8 of 14 M giants, five of six carbon stars, and zero of one S star showed intrinsic polarization. At least 9 of 13 PPNs exhibited intrinsic polarization, while the R CrB-type stars show intrinsic polarization during fading episodes. There is a statistical correlation between mean polarization, ⟨P⟩, and IR color, K - [12], among the AGB stars such that redder stars tend to be more polarized. The PPN sample is significantly redder and more polarized, on average, than the AGB stars. This increase in ⟨P⟩ with increased reddening is consistent with an evolutionary sequence in which AGB stars undergo increasing mass loss, with growing asymmetries in the dust distribution as they evolve up and then off the AGB into the short-lived PPN phase. A related trend is found between polarization and mass-loss rate in gas, _gas. The detectability of polarization increases with mass-loss rate, and probably all AGB stars losing mass at >10^-6M_☉ yr^-1 have detectable polarization. Multiple observations of three polarized AGB stars show that in some cases ⟨P⟩ increases with m_V, and in others it decreases. If polarization arises from scattering of starlight off an aysmmetric distribution of grains, then the distribution varies with time. Polarized features are detected in the TiO bands of three M-type Mira variables, in the CN bands of the carbon stars R Lep and V384 Per, and in the Swan bands of C₂ in R CrB and two PPNs. Polarization effects in the molecular bands appear to be more common and the effects are larger in O-rich than C-rich objects.

We present the results of time-resolved spectroscopy of 13 O-type stars in the Cas OB6 stellar association. We conducted a survey for radial velocity variability in search of binary systems, which are expected to be plentiful in young OB associations. Here we report the discovery of two new single-lined binaries, and we present new orbital elements for three double-lined binaries (including one in the multiple-star system HD 17505). One of the double-lined systems is the eclipsing binary system DN Cas, and we present a preliminary light-curve analysis that yields the system inclination, masses, and radii. We compare the spectra of the single stars and the individual components of the binary stars with model synthetic spectra to estimate the stellar effective temperatures, gravities, and projected rotational velocities. We also make fits of the spectral energy distributions to derive E(B - V), R = A_V/E(B - V), and angular diameter. A distance of 1.9 kpc yields radii that are consistent with evolutionary models. We find that 7 of 14 systems with spectroscopic data are probable binaries, consistent with the high binary frequency found for other massive stars in clusters and associations.

We present a three-dimensional non-LTE Monte Carlo radiative transfer code that we use to study the temperature and ionization structure of Keplerian disks around classical Be stars. The method we employ is largely similar to the Monte Carlo transition probability method developed by Lucy. Here we present a simplification of his method that avoids the use of the macroatom concept. Our investigations of the temperature structure of Be star disks show that the disk temperature behavior is a hybrid between the behavior of young stellar object (YSO) disks and hot star winds. The optically thick inner parts of Be star disks have temperatures that are similar to YSO disks, while the optically thin outer parts are like stellar winds. Thus, the temperature at the disk midplane initially drops, reaching a minimum at 3-5 stellar radii, after which it rises back to the optically thin radiative equilibrium temperature at large distances. On the other hand, the optically thin upper layers of the disk are approximately isothermal—a behavior that is analogous to the hot upper layers of YSO disks. Interestingly, unlike the case of YSO disks, we find that disk flaring has little effect on the temperature structure of Be star disks. We also find that the disks are fully ionized, as expected, but that there is an ionization minimum in the vicinity of the temperature minimum. The deficit of photoionization at this location makes it the most likely site for the low ionization state lines (e.g., Fe II) that produce the shell features observed in Be stars. Finally, we find that despite the complex temperature structure, the infrared excess is well approximated by an equivalent isothermal disk model whose temperature is about 60% of the stellar temperature. This is largely because at long wavelengths, the effective photosphere of the disk is located in its isothermal regions.

We present a method for measuring the physical parameters of the coldest T-type brown dwarfs using low-resolution near-infrared spectra. By comparing H₂O and H₂-sensitive spectral ratios between empirical data and theoretical atmosphere models, and calibrating these ratios to measurements for the well-characterized 2-5 Gyr companion brown dwarf Gliese 570D, we derive estimates of the effective temperatures and surface gravities for 13 mid- and late-type field T dwarfs. We also deduce the first quantitative estimate of subsolar metallicity for the peculiar T dwarf 2MASS 0937+2931. Derived temperatures are consistent with prior estimates based on parallax and bolometric luminosity measurements, and examination of possible systematic effects indicate that the results are robust. Two recently discovered late-type T dwarfs, 2MASS 0939-2448 and 2MASS 1114-2618, both appear to be ≳50 K cooler than the latest type T dwarf, 2MASS 0415-0935, and are potentially the coldest and least luminous brown dwarfs currently known. We find that, in general, higher surface gravity T dwarfs have lower effective temperatures and luminosities for a given spectral type, explaining previously observed scatter in the T_eff/spectral type relation for these objects. Masses, radii, and ages are estimated for the T dwarfs in our sample using the evolutionary models of Burrows et al.; we also determine masses and radii independently for eight T dwarfs with measured luminosities. These two determinations are largely consistent, lending support to the validity of evolutionary models at late ages. Our method is well suited to large samples of faint brown dwarfs and can ultimately be used to directly measure the substellar mass function and formation history in the Galaxy.

NICMOS images of the nearby late-type L dwarf 2MASS J22521073-1730134 (DENIS-P J225210.73-173013.4) show that it is a close double, separation 013. The companion is 1.0 mag fainter than the primary in the F110W passband and 1.55 mag fainter in the F170M images. The latter passband is centered on the 1.6 μm methane band. The small separation and the absence of an optical counterpart suggest that the two sources are associated; the relatively blue infrared color suggests that 2MASS J22521073-1730134B is a T-type binary companion of the late-L primary. This hypothesis is supported by infrared spectroscopy, which shows weak methane absorption in both the H and K passbands. We estimate a distance of 13.6 pc to the system and a projected linear separation of 1.75 AU. We consider the potential for measuring dynamical masses of the two components.

While following up L dwarf candidates selected photometrically from the Two Micron All Sky Survey, we uncovered an unusual object designated 2MASS J01415823-4633574. Its optical spectrum exhibits very strong bands of vanadium oxide but abnormally weak absorptions by titanium oxide, potassium, and sodium. Morphologically, such spectroscopic characteristics fall intermediate between old field early-L dwarfs [log(g) ≈ 5] and very late M giants [log(g) ≈ 0], leading us to favor low gravity as the explanation for the unique spectral signatures of this L dwarf. Such a low gravity can be explained only if this L dwarf is much lower in mass than a typical old field L dwarf of similar temperature and is still contracting to its final radius. These conditions imply a very young age. Further evidence of youth is found in the near-infrared spectrum, including a triangular-shaped H-band continuum, reminiscent of young brown dwarf candidates discovered in the Orion Nebula Cluster. Using the above information along with comparisons to brown dwarf atmospheric and interior models, our current best estimate is that this L dwarf has an age of 1-50 Myr and a mass of 6-25M_J. Although the lack of a lithium detection (pseudo-equivalent width <1 Å) might appear to contradict other evidence of youth, we suggest that lithium becomes weaker at lower gravity like all other alkali lines and thus needs to be carefully considered before being used as a diagnostic of age or mass for objects in this regime. The location of 2MASS 0141-4633 on the sky coupled with a distance estimate of ~35 pc and the above age estimate suggests that this object may be a brown dwarf member of either the 30 Myr old Tucana/Horologium association or the ~12 Myr old β Pic moving group. Distance as determined through trigonometric parallax (underway) and a measure of the total space motion are needed to test this hypothesis.

The phase-induced amplitude apodization coronagraph (PIAAC) uses a lossless achromatic apodization of the telescope pupil to produce a coronagraphic image without compromising the throughput and angular resolution of the telescope. Whereas the principle of the PIAAC concept was discussed in a previous paper, the purpose of this work is to provide an exhaustive analysis of the expected performances of a PIAAC on a 4 m diameter telescope in space. Results presented here are based on realistic simulations of extrasolar terrestrial planets (ETPs) orbiting F, G, K, and M stars within 30 pc of the solar system and take into account the probability distributions of planet phase and angular separation. We show that a quasi-complete detection survey of 100 stars (with six observations per star) would require about 2 days of "open shutter" observing time in the ideal system considered in this work (4 m telescope, 100% throughput). A classical apodizer would require exposure times about 100 times longer than PIAAC on a Sun-Earth system at 10 pc. Small pointing errors and non-monochromatic observing require slight oversizing of the focal plane mask with little impact on the system performance.

We present data obtained with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope (Spitzer) for a sample of 74 young (t < 30 Myr old) Sun-like (0.7 < M_*/M_☉ < 1.5) stars. These are a subset of the observations that comprise the Spitzer Legacy science program entitled the Formation and Evolution of Planetary Systems (FEPS). Using IRAC, we study the fraction of young stars that exhibit 3.6-8.0 μm infrared emission in excess of that expected from the stellar photosphere, as a function of age from 3 to 30 Myr. The most straightforward interpretation of such excess emission is the presence of hot (300-1000 K) dust in the inner regions (<3 AU) of a circumstellar disk. Five out of the 74 young stars show a strong infrared excess, four of which have estimated ages of 3-10 Myr. While we detect excesses from five optically thick disks and photospheric emission from the remainder of our sample, we do not detect any excess emission from optically thin disks at these wavelengths. We compare our results with accretion disk fractions detected in previous studies and use the ensemble results to place additional constraints on the dissipation timescales for optically thick, primordial disks.

We calculate the degree of linear polarization of radiation from stars having planets that may not be spatially resolved. We assume single scattering by water and silicate particulates in the planetary atmosphere. The dilution of the reflected polarized radiation of the planet by the unpolarized stellar radiation and the effect of the oblateness of the planet, as well as its elliptical orbit, are included. We employ a chemical equilibrium model to estimate the number density of water and silicate condensates and calculate the degree of linear polarization at the R band of starlight as a function of (1) mean size of condensates, (2) planetary oblateness, (3) inclination angle, (4) phase angle, (5) orbital eccentricity e, and (6) epoch of periastron passage. We show that the polarization profile alters significantly at all inclination angles when an elliptical orbit is considered, and the degree of polarization peaks at the epoch of periastron passage. We predict that a detectable amount of linear polarization may arise if the planetary atmosphere is optically thin, the mean size of the condensates is not greater than a few microns, and the oblateness of the planet is as high as that of Jupiter.

This paper considers the distribution of dust that originates in the breakup of planetesimals that are trapped in resonance with a planet. It is shown that there are three distinct grain populations with different spatial distributions: (I) large grains have the same clumpy resonant distribution as the planetesimals; (II) moderate-sized grains are no longer in resonance and have an axisymmetric distribution; and (III) small grains are blown out of the system by radiation pressure and so have a density distribution that falls off as τ ∝ 1/r. Population III can be further divided into two subclasses: (IIIa) grains produced from population I that exhibit trailing spiral structure that emanates from the resonant clumps and (IIIb) grains produced from population II that have an axisymmetric distribution. Since observations in different wavebands are sensitive to different dust sizes, multiwavelength imaging of debris disks can be used to test models that explain the submillimeter structure of debris disks as due to resonant trapping of planetesimals. For example, a collisional cascade without blowout grains would appear clumpy in the submillimeter (which samples population I) and smooth at mid- to far-IR wavelengths (which sample population II). The wavelength of transition from clumpy to smooth structure is indicative of the mass of the perturbing planet. The size distribution of Vega's disk is modeled showing that the large quantities of population III grains detected recently by Spitzer must originate in the destruction of the grains seen in the submillimeter images. Thus, at high resolution and sensitivity the far- and mid-IR structure of Vega's disk is predicted to include spiral structure emanating from the submillimeter clumps.

We report the results of a spectroscopic search for debris disks surrounding 41 nearby solar-type stars, including eight planet-bearing stars, using the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. With the accurate relative photometry of the IRS between 7 and 34 μm we are able to look for excesses as small as ~2% of photospheric levels, with particular sensitivity to weak spectral features. For stars with no excess, the 3 σ upper limit in a band at 30-34 μm corresponds to ~75 times the brightness of our zodiacal dust cloud. Comparable limits at 8.5-13 μm correspond to ~1400 times the brightness of our zodiacal dust cloud. These limits correspond to material located within the <1 to ~5 AU region that, in our solar system, originates predominantly from debris associated with the asteroid belt. We find excess emission longward of ~25 μm from five stars, of which four also show excess emission at 70 μm. This emitting dust must be located in a region starting around 5-10 AU. One star has 70 μm emission but no IRS excess. In this case, the emitting region must begin outside 10 AU; this star has a known radial velocity planet. Only two stars of the five show emission shortward of 25 μm, where spectral features reveal the presence of a population of small, hot dust grains emitting in the 7-20 μm band. One of these stars, HD 72905, is quite young (300 Myr), while the other, HD 69830, is older than 2 Gyr. The data presented here strengthen the results of previous studies to show that excesses at 25 μm and shorter are rare: only 1 out of 40 stars older than 1 Gyr or ~2.5% shows an excess. Asteroid belts 10-30 times more massive than our own appear are rare among mature, solar-type stars.

The core solar wind protons are observed to be heated perpendicularly to the magnetic field. This is taken to be a signature of the cyclotron damping of the turbulent fluctuations, which are thought to be responsible for the heating. At the same time, it is commonly accepted that the turbulent cascade produces mostly highly oblique (quasi-two-dimensional) fluctuations, which cannot be immediately cyclotron resonant with the ions because of their low frequencies and small parallel wavenumbers. To address this problem, we propose a new, indirect mechanism for damping the quasi-two-dimensional fluctuations. The mechanism involves a plasma instability, which excites ion cyclotron resonant waves. As the cascade proceeds to higher wavenumbers, it generates increasingly high velocity shear associated with the turbulent fluctuations. The shear eventually becomes unstable to waves near harmonics of the ion cyclotron frequency. Once the frequency of the waves is upshifted, they can heat ions perpendicularly, extracting the energy from the quasi-two-dimensional fluctuations. The dissipation rates of quasi-two-dimensional fluctuations are incorporated into a model of the energy transfer in the turbulent cascade. Our analysis of the observed spectra shows that the spectral break separating the inertial and dissipation ranges of the turbulence, where the dissipation sets in, corresponds to the same shear under a wide range of plasma conditions, in agreement with the prediction of the theory. The observed turbulence spectra often have power-law dissipation ranges with an average spectral index of -3. We demonstrate that this fact is simply a consequence of a marginal state of the instability in the dissipation range.

Worldwide neutron monitor observations of relativistic solar protons on 1989 October 22 have proven puzzling, with an initial spike at some stations followed by a second peak, which is difficult to understand in terms of transport along a standard Archimedean spiral magnetic field or a second injection near the Sun. Here we analyze data from polar monitors, which measure the directional distribution of solar energetic particles (mainly protons) at rigidities of ~1-3 GV. This event has the unusual properties that the particle density dips after the initial spike, followed by a hump with bidirectional flows and then a very slow decay. The spectral index, determined using bare neutron counters, varies dramatically, with energy dispersion features. The density and anisotropy data are simultaneously fit by simulating the particle transport for various magnetic field configurations and determining the best-fit injection function near the Sun. The data are not well fit for an Archimedean spiral field, a magnetic bottleneck beyond Earth, or particle injection along one leg of a closed magnetic loop. A model with simultaneous injection along both legs of a closed loop provides a better explanation: particles moving along the near leg make up the spike, those coming from the far leg make up the hump, both legs contribute to the bidirectional streaming, and trapping in the loop accounts for the slow decay of the particle density. Refined fits indicate a very low spectral index of turbulence, q < 1, a parallel mean free path of 1.2-2.0 AU, a loop length of 4.7 ± 0.3 AU, and escape of relativistic protons from the loop on a timescale of 3 hr. The weak scattering is consistent with reports of weak fluctuations in magnetic loops, while the low q-value may indicate a smaller correlation length as well.

Sixty-nine ground level events (GLEs) caused by relativistic solar protons have been observed from 1942 to 2005. GLEs are characteristically associated with intense solar flares [having peak ~9 GHz flux densities S_P(9 GHz) > 10³ sfu] and fast (>1000 km s^-1) coronal mass ejections (CMEs). The small GLEs on 1979 August 21 and 1981 May 10 provide an exception to these rules of thumb. In comparison with other GLEs, they were associated with significantly weaker flares [S_P(9 GHz) < 30 sfu vs. a median value of ~8000 sfu for all GLEs] and slower CMEs (plane-of-sky speeds ~800 km s^-1 vs. a median of ~1600 km s^-1). The sunspot groups in which these two events originated ranked near the bottom of GLE-parent regions in terms of sunspot area (~100 millionths of a solar hemisphere [msh] vs. a median of ~850 msh). What enabled these two otherwise commonplace solar eruptions to accelerate protons to GeV energies? In both cases, intense, long-duration, metric type II bursts were observed. In addition, both of these GLEs occurred when the background ~10 MeV proton intensity at 1 AU was >1000 times the normal background because of preceding SEP events originating in active regions that were located in each case ~100° east of the active region responsible for the GLE. We suggest that the relativistic solar protons observed in these two events resulted from CME-driven shock acceleration of an elevated coronal seed population, reflecting the enhanced background proton intensity at 1 AU. For this scenario, the timing onset of the relativistic protons in the two events indicates that the shocks had access to the energetic seed particles within ~2-5 R_☉ of the solar surface. While an elevated ~10 MeV proton background at Earth is a favorable/common condition for GLE occurrence, it is not a requirement.

The effects of turbulence on the mixing of gases and dust in the outer solar nebula are examined using three-dimensional MHD calculations in the shearing-box approximation with vertical stratification. The turbulence is driven by the magnetorotational instability. The magnetic and hydrodynamic stresses in the turbulence correspond to an accretion time at the midplane about equal to the lifetimes of T Tauri disks, while accretion in the surface layers is 30 times faster. The mixing resulting from the turbulence is also fastest in the surface layers. The mixing rate is similar to the rate of radial exchange of orbital angular momentum, so that the Schmidt number is near unity. The vertical spreading of a trace species is well matched by solutions of a damped wave equation when the flow is horizontally averaged. The damped wave description can be used to inexpensively treat mixing in one-dimensional chemical models. However, even in calculations reaching a statistical steady state, the concentration at any given time varies substantially over horizontal planes, due to fluctuations in the rate and direction of the transport. In addition to mixing species that are formed under widely varying conditions, the turbulence intermittently forces the nebula away from local chemical equilibrium. The different transport rates in the surface layers and interior may affect estimates of the grain evolution and molecular abundances during the formation of the solar system.

Relict Ca-Al-rich inclusions (CAIs) in chondrules crystallized before their host chondrules and were subsequently partly melted together with chondrule precursors during chondrule formation. Like most CAIs, relict CAIs are ¹⁶O enriched (Δ¹⁷O < -20‰) compared to their host chondrules (Δ¹⁷O > -9‰). Hibonite in a relict CAI from the ungrouped carbonaceous chondrite Adelaide has a large excess of radiogenic ²⁶Mg (²⁶Mg*) from the decay of ²⁶Al, corresponding to an initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)_I] of (3.7 ± 0.5) × 10^-5; in contrast, melilite in this CAI and plagioclase in the host chondrule show no evidence for ²⁶Mg* [(²⁶Al/²⁷Al)_I of <5 × 10^-6]. Grossite in a relict CAI from the CH carbonaceous chondrite PAT 91546 has little ²⁶Mg*, corresponding to a (²⁶Al/²⁷Al)_I of (1.7 ± 1.3) × 10^-6. Three other relict CAIs and their host chondrules from the ungrouped carbonaceous chondrite Acfer 094, CH chondrite Acfer 182, and H3.4 ordinary chondrite Sharps do not have detectable ²⁶Mg* [(²⁶Al/²⁷Al)_I < 1 × 10^-5, <(4-6) × 10^-6, and <1.3 × 10^-5, respectively]. Isotopic data combined with mineralogical observations suggest that relict CAIs formed in an ¹⁶O-rich gaseous reservoir before their host chondrules, which originated in an ¹⁶O-poor gas. The Adelaide CAI was incorporated into its host chondrule after ²⁶Al had mostly decayed, at least 2 Myr after the CAI formed, and this event reset ²⁶Al-²⁶Mg systematics.

We present measurements at optical wavelengths of the spectral reflectance, rotational light curve, and solar phase curve of 2003 EL61. With apparent visual magnitude 17.5 at 51 AU from the Sun, this newly discovered member of the classical Kuiper Belt is now the third brightest KBO after Pluto and 2005 FY9. Our observations reveal an unambiguous, double-peaked rotational light curve with period 3.9154 ± 0.0002 hr and peak-to-peak amplitude 0.28 ± 0.04 mag. This is the fastest rotation period reliably determined for any body in the solar system larger than 100 km. Assuming the body has relaxed over time to the shape taken by a homogenous fluid body, our observations tightly constrain the shape and density. Given the mass we recently determined for 2003 EL61 from the orbit of a small satellite, we also constrain the size and albedo. We find a total length of 1960-2500 km, a mean density of 2600-3340 kg m^-3, and a visual albedo greater than 0.6. We also measure a neutral reflectance at visible wavelengths and a linear phase curve with slope varying from 0.09 mag deg^-1 in the B band to 0.13 mag deg^-1 in the I band. The absolute V-band magnitude is 0.444 ± 0.021.

The Lyα forest at z ≳ 5.5 shows strong scatter in the mean transmission even when smoothed over extremely large spatial scales, ≳50 Mpc h^-1. This has been interpreted as a signature of strongly fluctuating radiation fields or patchy reionization. To test this claim, we calculate the scatter arising solely from density fluctuations, assuming a uniform ionizing background, via analytic arguments and simulations. This scatter alone is comparable to that observed. It rises steeply with redshift and is of order unity by z ~ 6, even on ~50 Mpc h^-1 scales. This arises because (1) at z ~ 6, transmission spectra, which are sensitive mainly to rare voids, are highly biased (with a linear bias factor b ≥ 4) tracers of underlying density fluctuations and (2) small-scale transverse modes are aliased to long-wavelength line-of-sight modes. Inferring patchy reionization from quasar spectra is therefore subtle and requires much more detailed modeling. Similarly, we expect density fluctuations alone to produce order unity transmission fluctuations in the z ~ 3 He II Lyα forest on the scales over which these measurements are typically made.

We present the discovery of strong mid-infrared emission lines of molecular hydrogen of apparently high-velocity dispersion (~870 km s^-1) originating from a group-wide shock wave in Stephan's Quintet. These Spitzer Space Telescope observations reveal emission lines of molecular hydrogen and little else. This is the first time an almost pure H₂ line spectrum has been seen in an extragalactic object. Along with the absence of PAH-dust features and very low excitation ionized gas tracers, the spectra resemble shocked gas seen in Galactic supernova remnants, but on a vast scale. The molecular emission extends over 24 kpc along the X-ray-emitting shock front, but it has 10 times the surface luminosity as the soft X-rays and about one-third the surface luminosity of the IR continuum. We suggest that the powerful H₂ emission is generated by the shock wave caused when a high-velocity intruder galaxy collides with filaments of gas in the galaxy group. Our observations suggest a close connection between galaxy-scale shock waves and strong broad H₂ emission lines, like those seen in the spectra of ultraluminous infrared galaxies where high-speed collisions between galaxy disks are common.

We present high signal-to-noise ratio Spitzer Infrared Spectrograph observations of 17 Virgo early-type galaxies. The galaxies were selected from those that define the color-magnitude relation of the cluster, with the aim of detecting the silicate emission of their dusty, mass-losing evolved stars. To flux calibrate these extended sources, we have devised a new procedure that allows us to obtain the intrinsic spectral energy distribution and to disentangle resolved and unresolved emission within the same object. We have found that 13 objects of the sample (76%) are passively evolving galaxies with a pronounced broad silicate feature that is spatially extended and likely of stellar origin, in agreement with model predictions. The other four objects (24%) are characterized by different levels of activity. In NGC 4486 (M87), the line emission and the broad silicate emission are evidently unresolved, and, given also the typical shape of the continuum, they likely originate in the nuclear torus. NGC 4636 shows emission lines superposed on extended (i.e., stellar) silicate emission, thus pushing the percentage of galaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central unresolved region. A more detailed analysis of our sample, with updated models, will be presented in a forthcoming paper.

Analysis of the wavelength dependence of extinction in the ultraviolet is shown to provide additional insight into the physical properties of the carrier of the interstellar absorption feature at 217.5 nm (5.7 eV). In particular, it is found that this band has a cutoff at both long and short wavelengths, supporting the assignment of this feature to a plasmon band. The photon energies corresponding to these cutoffs are highly diagnostic of the carrier of this feature and suggest that the absorber is a seven-ring aromatic molecule. Comparison with laboratory data, together with theoretical calculations of the plasmon resonance in small dehydrogenated aromatic-ring structures, supports the assignment of this feature to the π-π* plasmon resonance in dehydrogenated coronene molecules, C₂₄H_x and its cations, where x ≤ 3. It is shown that this assignment is consistent with available observational data on the 217.5 nm feature, including such properties as a constant central wavelength, variable width, its relation to E(1250-V)/E(B-V), and the carbon abundance in the interstellar medium. The π and π* energy levels in the 217.5 nm carrier, derived from an analysis of interstellar extinction, have been used to construct its energy-level diagram, enabling comparison with other observational data

Diamond nanocrystals (size ~100 nm) emit bright luminescence at 600-800 nm when exposed to green and yellow photons. The photoluminescence, arising from excitation of the nitrogen-vacancy defect centers created by proton-beam irradiation and thermal annealing, closely resembles the extended red emission (ERE) bands observed in reflection nebulae and planetary nebulae. The central wavelength of the emission is ~700 nm, and it blueshifts to ~660 nm as the excitation wavelength decreases from 535 to 470 nm as the result of a combined excitation of two different detect centers [(N-V)^- and (N-V)⁰]. Our observations lend support to the suggestion that nanodiamond is a possible carrier for the ERE band.

A recently published sample of 21 detached eclipsing binaries in the Small Magellanic Cloud provides a valuable test of the binary mass function for massive stars. We show that 50% of detached binaries have companions with very similar masses, q = M₂/M₁ > 0.87, where M₁ and M₂ denote the masses of the two binary components, M₁ ≥ M₂. A Salpeter relative mass function for the secondary is very strongly excluded, and the data are consistent with a flat mass function containing 55% of the systems and a "twin" population with q > 0.95 containing the remainder. We survey the existing literature on binary mass ratios and conclude that a significant twin population (of order 20%-25%) exists in binaries that are likely to interact across a broad range of stellar masses and metallicity. Interactions involving twins have distinctly different properties from those involving stars of unequal mass; the secondaries will tend to be evolved, and common-envelope evolution is qualitatively different. The implications of such a population for both binary interactions and star formation are substantial, and we present some examples. We argue that twin systems may provide a natural stellar population to explain the recently proposed prompt channel for Type Ia supernovae, and the presence of a twin population dramatically reduces the maximum inferred merger rate between neutron stars (NSs) and black holes relative to the NS-NS merger rate. Twins may also be important for understanding the tendency of white dwarf and NS binaries to be nearly equal in mass, and inclusion of twins in population studies will boost the blue straggler production rate.

We present the discovery of the first X-ray counterpart to a Rotating RAdio Transient (RRAT) source. RRAT J1819-1458 is a relatively highly magnetized (B ~ 5 × 10¹³ G) member of a new class of unusual pulsar-like objects discovered by their bursting activity at radio wavelengths. A Chandra observation of that position revealed a pointlike source, CXOU J181934.1-145804, with a soft spectrum well fit by an absorbed blackbody with N_H = 7 × 10²¹ cm^-2, temperature kT = 0.12 ± 0.04 keV, and an unabsorbed flux of ~2 × 10^-12 ergs cm^-2 s^-1 between 0.5 and 8 keV. No optical or infrared (IR) counterparts are visible within 1'' of our X-ray position. The positional coincidence, spectral properties, and lack of an optical/IR counterpart make it highly likely that CXOU J181934.1-145804 is a neutron star and is the same object as RRAT J1819-1458. The source showed no variability on any timescale from the pulse period of 4.26 s up to the 5 day window covered by the observations, although our limits (especially for pulsations) are not particularly constraining. The X-ray properties of CXOU J181934.1-145804, while not yet measured to high precision, are similar to those of comparably aged radio pulsars and are consistent with thermal emission from a cooling neutron star.

We report the detection with FUSE of strong, highly blueshifted absorption features appearing in the absorption troughs of practically all major P Cygni profiles in the presumably single Wolf-Rayet star WR 135. These features also appear in the shock-sensitive O VI λλ1032, 1038 doublet, coincident both in time and in velocity space with the rest of the lower ionization species. Choosing between two alternative interpretations (large-scale, coherent structures vs. localized, random shocks), we favor the latter. The absolute value of the velocity as well as the velocity dispersion in the shocked region, the density of the shocked gas, and the timescales of the observed variability allow us to relate the observed shocks to the incidence of numerous overdense clumps (blobs) in the wind of a hot, massive star.

We present Spitzer Space Telescope observations of two TW Hydrae association brown dwarfs, 2MASSW J1207334-393254 and 2MASSW J1139511-315921, in the IRAC and MIPS 24 μm bands. On the basis of their IRAC colors, we have classified them as classical and weak-line T Tauri stars, respectively. For 2MASSW J1207334-393254, we have found that a flat-disk model fits the data very well. This brown dwarf shows the presence of warm (T ≳ 100 K) circumstellar dust close (R ≲ 0.2 AU) to it and does not display any signs of cleansing of dust within several AU of the star. In comparison with other TWA members that show excess in IR, we suggest that there exists a different disk evolution/dust processing mechanism for stellar and substellar objects. The star 2MASSW J1139511-315921 does not show any significant excess in any of the IRAC bands but a small one at 24 μm, which is not significant enough to suggest the presence of warm dust around this star. It shows signs of dust cleansing in the inner several AU, similar to most of the other TWA members.

Recent observations show a strong correlation between stellar mass and accretion rate in young stellar and substellar objects, with the scaling _acc ∝ M holding over more than 4 orders of magnitude in accretion rate. We explore the consequences of this correlation in the context of disk evolution models. We note that such a correlation is not expected to arise from variations in disk angular momentum transport efficiency with stellar mass, and we suggest that it may reflect a systematic trend in disk initial conditions. In this case we find that brown dwarf disks initially have rather larger radii than those around more massive objects. By considering disk evolution, and invoking a simple parameterization for a shutoff in accretion at the end of the disk lifetime, we show that such models predict that the scatter in the stellar mass-accretion rate relationship should increase with increasing stellar mass, in rough agreement with current observations.

⁶⁰Fe decays to ⁶⁰Ni with a half-life of 1.49 × 10⁶ yr, so all of the original ⁶⁰Fe atoms incorporated into the solar system have decayed. Because ⁶⁰Fe is produced only in stars, its initial abundance in the solar system provides a constraint on the stellar contribution of radionuclides to the early solar system and on the nature of the stellar source. Because of its short half-life, ⁶⁰Fe is also a potential high-resolution chronometer of early-solar-system events. The presence of ⁶⁰Fe in primitive meteorites has been confirmed in sulfides, but the initial abundance of ⁶⁰Fe in the solar system has been only loosely constrained because it is uncertain when the sulfides formed. We show that ⁶⁰Fe was present with abundance ratios of ⁶⁰Fe/⁵⁶Fe = (2.2-3.7) × 10^-7 when ferromagnesian chondrules formed. By applying the time difference of 1.5-2.0 million years between formation of ferromagnesian chondrules and Ca-Al-rich inclusions (CAIs), the oldest known solar system solids, a solar system initial ⁶⁰Fe/⁵⁶Fe ratio [(⁶⁰Fe/⁵⁶Fe)₀] of (5-10) × 10^-7 is estimated. This new solidly based (⁶⁰Fe/⁵⁶Fe)₀ ratio is consistent with predictions for nucleosynthesis in a supernova or in an intermediate-mass asymptotic giant branch (AGB) star just before the solar system formation, but is too high for the source to have been a low-mass AGB star. Considering the rarity of encounters between a molecular cloud and an AGB star, our results can be considered strong evidence of a contribution of material from a nearby supernova and of a role for a supernova in the origin of the solar system.

The transport of energetic particles in a mean magnetic field and the presence of anisotropic magnetic turbulence are studied numerically, for parameter values relevant to the solar wind. A numerical realization of magnetic turbulence is set up in which we can vary the type of anisotropy by changing the correlation lengths l_x, l_y, l_z. We find that for l_x, l_y ≫ l_z, transport can be non-Gaussian, with superdiffusion along the average magnetic field and subdiffusion perpendicular to it. Decreasing the l_x/l_z ratio down to ≲0.3, Gaussian diffusion is obtained, showing that the transport regime depends on the turbulence anisotropy. Implications for energetic particle propagation in the solar wind and for diffusive shock acceleration are discussed.

Filamentary structures following magnetic field lines pervade the Sun's atmosphere and offer us insight into the solar magnetic field. Radio propagation measurements have shown that the smallest filamentary structures in the solar corona are more than 2 orders of magnitude finer than those seen in solar imaging. Here we use radio Doppler measurements to characterize their transverse density gradient and determine their finest scale in the outer corona at 20-30 R_☉ where open magnetic fields prevail. Filamentary structures overlying active regions have the steepest gradient and finest scale, while those overlying coronal holes have the shallowest gradient and least finest scale. Their organization by the underlying corona implies that these subresolution structures extend radially from the entire Sun, confirming that they trace the coronal magnetic field responsible for the radial expansion of the solar wind. That they are rooted all over the Sun elucidates the association between the magnetic field of the photosphere and that of the corona, as revealed by the similarity between the power spectra of the photospheric field and the coronal density fluctuations. This association along with the persistence of filamentary structures far from the Sun demonstrate that subresolution magnetic fields must play an important role not only in magnetic coupling of the photosphere and corona, but also in coronal heating and solar wind acceleration through the process of small-scale magnetic reconnection. They also explain why current widely used theoretical models that extrapolate photospheric magnetic fields into the corona do not predict the correct source of the solar wind.

We investigate high spatial resolution radio polarization data obtained by the Nobeyama Radioheliograph (NoRH) and high time resolution data observed with the Nobeyama Radio Polarimeters (NoRP) during the well-studied flare/CME event of 2002 April 21. A 17 GHz radio source at the loop top was seen by NoRH to move upward together with the expanding flare loops at a speed of around 10 km s^-1. In the 5 minutes before the source began its upward motion, the Stokes V of the radio signals at 17 GHz showed quasi-periodic reversals between left-circular polarization (LCP) and right-circular polarization (RCP). Following this interval, the polarizations gradually turned to LCP. During this period, the polarization of the corresponding footpoint source maintained the RCP sense. The reversal of Stokes V between RCP and LCP was also detected at lower frequencies (1-2 GHz) by NoRP, without spatial resolution. The observed reversals between RCP and LCP of the radio signals from the top of the flare loop system can be taken as evidence that magnetic energy is released or energetic particles are produced at the magnetic reconnection site in a quasi-periodic fashion.

We address the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays. We studied in the laboratory the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid H₂O₂ with 50 keV H⁺ at 17 K. The changes in composition and release of volatiles during warming to 200 K were monitored by infrared spectroscopy, mass spectrometry, and microbalance techniques. We find evidence for voids in the water component from the infrared bands due to dangling H bonds. The absorption from these bands increases during heating and can be observed at temperatures as high as ~155 K. More O₂ is stored in the radiolyzed film than can be retained by codeposition of O₂ and H₂O. This O₂ remains trapped until ~155 K, where it desorbs in an outburst as water ice crystallizes. Warming of the ice also drastically decreases the intrinsic absorbance of O₂ by annealing defects in the ice. We also observe loss of O₃ in two stages during heating, which correlates with desorption and possibly chemical reactions with radicals stored in the ice, triggered by the temperature increase.

The pure rotational spectrum of AlSH ( ¹A') has been recorded using millimeter and submillimeter direct absorption techniques. Measurements of its deuterium isotopomer have been recorded as well. This study is the first laboratory detection of this molecule, which was created by the reaction of aluminum vapor and H₂S. Nine and seven rotational transitions of AlSH and AlSD were measured, respectively, each consisting of K_a = 0 through K_a = 6 asymmetry components. From these data, rotational constants have been established. Calculation of an r₀ structure indicates that AlSH is bent with an angle near 90°. These measurements will enable searches for AlSH to be carried out in circumstellar gas, where aluminum-bearing molecules, as well as H₂S, are present.