Table of contents

We present an improved version of the POTENT method for reconstructing the cosmological velocity and mass density fields from radial peculiar velocities, test it with mock catalogs, and apply it to the Mark III Catalog of Galaxy Peculiar Velocities. The method is improved in several ways: (1) the inhomogeneous Malmquist bias is reduced by grouping and corrected statistically in either forward or inverse analyses of inferred distances, (2) the smoothing into a radial velocity field is optimized such that window and sampling biases are reduced, (3) the density field is derived from the velocity field using an improved weakly nonlinear approximation in Eulerian space, and (4) the computational errors are made negligible compared to the other errors. The method is carefully tested and optimized using realistic mock catalogs based on an N-body simulation that mimics our cosmological neighborhood, and the remaining systematic and random errors are evaluated quantitatively. The Mark III catalog, with ~3300 grouped galaxies, allows a reliable reconstruction with fixed Gaussian smoothing of 10-12 h^-1 Mpc out to ~60 h^-1 Mpc and beyond in some directions. We present maps of the three-dimensional velocity and mass-density fields and the corresponding errors. The typical systematic and random errors in the density fluctuations inside 40 h^-1 Mpc are ±0.13 and ±0.18 (for Ω = 1). In its gross features, the recovered mass distribution resembles the galaxy distribution in redshift surveys and the mass distribution in a similar POTENT analysis of a complementary velocity catalog (SFI), including such features as the Great Attractor, Perseus-Pisces, and the large void in between. The reconstruction inside ~40 h^-1 Mpc is not affected much by a revised calibration of the distance indicators (VM2, tailored to match the velocities from the IRAS 1.2 Jy redshift survey). The volume-weighted bulk velocity within the sphere of radius 50 h^-1 Mpc about the Local Group is V₅₀ = 370 ± 110 km s^-1 (including systematic errors) and is shown to be mostly generated by external mass fluctuations. With the VM2 calibration, V₅₀ is in a similar direction and reduced to 305 ± 110 km s^-1.

We present results from a dynamical study of the high-redshift, massive, X-ray-luminous galaxy cluster MS 1054-03. We significantly increase the number of confirmed cluster members by adding 20 to an existing set of 12; using the confirmed members, we estimate MS 1054-03's redshift, velocity dispersion, and mass. We find that z = 0.8329 ± 0.0017, σ = 1170 ± 150 km s^-1, and the central mass is approximately 1.9 ± 0.5 × 10¹⁵h^-1M_☉ (within R = 1 h^-1 Mpc; H₀ = 100 h km s^-1 Mpc^-1, q₀ = 0.5). MS 1054-03 is one of a handful of high-redshift (z > 0.5) clusters known that also has X-ray and weak-lensing observations; we find our dynamical mass agrees with mass estimates from both studies. The confirmation of MS 1054-03 as a massive cluster at z ~ 0.8 is consistent with an open (Ω_M ~ 0.3) or flat, Λ-dominated (Ω_M + Ω_Λ = 1) universe. In addition, we compare MS 1054-03's velocity dispersion and X-ray temperature to a sample of low- and intermediate-redshift galaxy clusters to test for evolution in the σ-T_X relation; we find no evidence for evolution in this relation to z ~ 0.8.

It is believed that the biasing of galaxies plays an important role in understanding the large-scale structure of the universe. In general, the biasing of galaxy formation could be stochastic. Furthermore, future galaxy surveys might allow us to explore the time evolution of the galaxy distribution. In this paper the analytic study of the galaxy-mass density relation and its time evolution is presented within the framework of stochastic biasing. In the weakly nonlinear regime, we derive a general formula for the galaxy-mass density relation as a conditional mean using the Edgeworth expansion. The resulting expression contains the joint moments of the total mass and galaxy distributions. Using the perturbation theory, we investigate the time evolution of the joint moments and examine the influence of the initial stochasticity on the galaxy-mass density relation. The analysis shows that the galaxy-mass density relation could be well approximated by the linear relation. Compared with the skewness of the galaxy distribution, we find that the estimation of the higher order moments using the conditional mean could be affected by the stochasticity. Therefore, the galaxy-mass density relation as a conditional mean should be used with caution as a tool for estimating the skewness and the kurtosis.

Imaging of the Sunyaev-Zeldovich (SZ) effect in galaxy clusters combined with cluster plasma X-ray diagnostics can measure the cosmic distance scale to high redshift. However, projecting the inverse Compton scattering and X-ray emission along the cluster line of sight introduces systematic errors in the Hubble constant, H₀, because the true shape of the cluster is not known. In this paper, I present a study of the systematic errors in the value of H₀, as determined by the X-ray and SZ properties of theoretical samples of triaxial isothermal "beta" model clusters, caused by projection effects and observer orientation relative to the model clusters' principal axes. I calculate three estimates for H₀ for each cluster based on their large and small apparent angular core radii and their arithmetic mean. I demonstrate that the estimates for H₀ for a sample of 25 clusters have limited systematic error: the 99.7% confidence intervals for the mean estimated H₀ analyzing the clusters using either their large or mean angular core radius are within ≃14% of the "true" (assumed) value of H₀ (and enclose it), for a triaxial beta model cluster sample possessing a distribution of apparent X-ray cluster ellipticities consistent with that of observed X-ray clusters. This limit on the systematic error in H₀ caused by cluster shape assumes that each sample beta model cluster has a fixed shape; deviations from constant shape within the clusters may introduce additional uncertainty or bias into this result.

We estimate the deformation of the cosmic microwave background radiation by the hot region ("cocoon") around a radio galaxy. A simple model is adopted for cocoon evolution while the jet is on, and a model of evolution is constructed after the jet is off. It is found that at low redshift the phase after the jet is off is longer than the lifetime of the jets. The Compton y-parameter generated by cocoons is calculated with a Press-Schechter number density evolution. The resultant value of y is of the same order as the COBE constraint. The Sunyaev-Zeldovich effect due to cocoons could therefore be a significant foreground source of small angular scale anisotropies in the cosmic microwave background radiation.

We present three-epoch polarimetric images of the quasar 3C 454.3 obtained with the Very Long Baseline Array at 22 and 43 GHz. Polarized intensity images at 22 GHz show a sudden change in the polarization structure of a bright eastern component (which we call the "core," although it may be neither at the upstream end of the jet nor completely stationary) over a 41 day interval, coincident with the ejection of a new component from the core, as resolved in the corresponding 43 GHz images. This polarization outburst is also present at 43 GHz in both the core and the new component. This may represent a rapid change in the electric vector position angle of the ejected component from being orthogonal to that of the core to being almost parallel to it. About 7 months later, the new component, moving superluminally at 2.9 ± 0.4 h^-1c (q₀ = 0.5) relative to the core and 3.9 ± 0.4 h^-1c relative to a bright stationary component about 0.6 mas west of the core—very low compared with previous measurements—is found at 43 GHz to exhibit a further rotation of 90° in the orientation of its polarization. Opacity effects may account for the first rotation, but changes in the magnetic field of the component and/or that of the underlying jet in the inner milliarcsecond structure of 3C 454.3 are needed to account for the second. Polarized intensity images of the quasar 0420-014, used as a calibrator, are also presented. The polarization position angle of the core rotated between late 1994 and late 1996.

According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of different sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity (V_circ) distribution function (VDF) of galaxy satellites in the Local Group. We find that within the volumes of radius of 570 kpc (400 h^-1 kpc assuming the Hubble constant h = 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as n(> V_circ) ≈ 55 ± 11(V_circ/10 km s^-1)^-1.4±0.4h³ Mpc^-3 for V_circ in the range ≈10-70 km s^-1. The observed VDF is compared with results of high-resolution cosmological simulations. We find that the VDF in models is very different from the observed one: n(> V_circ) ≈ 1200(V_circ/10 km s^-1)^-2.75h³ Mpc^-3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s^-1 and mass greater than 3 × 10⁸M_☉ within a 570 kpc radius. This number is significantly higher than the approximately dozen satellites actually observed around our Galaxy. The difference is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict ~300 satellites inside a 1.5 Mpc radius, while only ~40 satellites are observed in the Local Group. The observed and predicted VDFs cross at ≈50 km s^-1, indicating that the predicted abundance of satellites with V_circ ≳ 50 km s^-1 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identification of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background.

The time-dependent synchrotron emission from relativistic jets and the relation between the synchrotron and the inverse Compton scattering of external radiation emission is considered within the framework of the radiative front model. The timescale and profile of the optically thin emission are shown to be determined, in this model, by the shock formation radius, the thickness of expelled fluid slab, and the variation of the front's parameters owing to its transverse expansion. For a range of reasonable conditions, a variety of flare shapes can be produced, varying from roughly symmetric flares with exponential rises and decays, as often seen in blazars, to highly asymmetric ones with a fast rise and a much slower, power-law decay, as seen in the afterglows of gamma-ray bursts. The onset, duration, and fluence of low-frequency (below the initial turnover frequency) and hard gamma-ray (above the initial gamma-spheric energy) outbursts are limited by opacity effects; the emission at these energies is quite generally delayed and, in the case of outbursts of sufficiently short lengths, severely attenuated. The observational consequences are discussed. One distinctive prediction of this model is that in a single, powerful source, the upper cutoff of the gamma-ray spectrum should be correlated with the timescale of the outburst and with the amplitude of variations at long wavelengths (typically radio to millimeter).

We report on centimeter VLA and VLBI observations of the giant, low-power radio galaxy B2 1144+35. These observations are sensitive to structures on scales of from less than 1 pc to greater than 1 Mpc. Diffuse steep-spectrum lobes on the megaparsec scale are consistent with an age of ~10⁸ yr. On the parsec scale, a complex jet component is seen to move away from the center of activity with an apparent velocity of 2.7 h^-1₅₀c. It shows a central spine-shear layer morphology. A faint parsec-scale counterjet is detected, and an intrinsic jet velocity of 0.95c and an angle to the line of sight of 25° are derived, consistent with an intrinsically symmetric ejection. The central spine in the parsec-scale jet is expected to move at a higher velocity, and a Lorentz factor of γ ~ 15 has been estimated near the core. The age of this inner VLBI structure is ~300 yr. Assuming a constant angle to the line of sight, the jet velocity is found to decrease from 0.95c at 20 mas (32 pc on the plane of the sky) to 0.02c at 15" (24 kpc on the plane of the sky). These findings lend credence to the claim that (1) even the jets of low-power radio galaxies start out relativistic and (2) these jets are decelerated to subrelativistic velocities by the time they reach kiloparsec scales.

This paper discusses the optical spectroscopic properties of the IRAS 1 Jy sample (f₆₀ > 1 Jy) of ultraluminous infrared galaxies (ULIGs: L_IR > 10¹²L_☉; H₀ = 75 km s^-1 Mpc^-1 and q₀ = 0). One hundred eight of the 118 1 Jy ULIGs have been observed at Δλ = 8.3 Å resolution over the wavelength range ~4500-8900 Å. These data are combined with large, previously published sets of optical spectroscopic data of lower luminosity infrared galaxies to look for systematic trends with infrared luminosity over the luminosity range L_IR ≈ 10^10.5-10¹³L_☉. As found in previous studies, the fraction of Seyfert galaxies among luminous infrared galaxies increases abruptly above L_IR ≈ 10^12.3L_☉—about 50% of the galaxies with L_IR > 10^12.3L_☉ present Seyfert characteristics. Many of the optical and infrared spectroscopic properties of the Seyfert galaxies are consistent with the presence of a genuine active galactic nucleus (AGN). About 30% of these galaxies are Seyfert 1 galaxies with broad-line regions similar to those of optical quasars. Published near-infrared spectroscopy also suggests that many of the Seyfert 2 galaxies (especially those with warm IRAS 25-60 μm colors) are in fact obscured Seyfert 1 galaxies with broad (≳2000 km s^-1) recombination lines at 2 μm, where dust obscuration is less important. The percentage of Seyfert 1 ULIGs increases with infrared luminosity, contrary to the predictions of the standard unification model for Seyfert galaxies. Comparisons of the broad-line luminosities of optical and obscured Seyfert 1 ULIGs with those of optically selected quasars of comparable bolometric luminosity suggest that the dominant energy source in most of these ULIGs is the same as in optical quasars, namely mass accretion onto a supermassive black hole, rather than a starburst. These results are consistent with recently published ISO, ASCA, and VLBI data. On the other hand, there is no unambiguous optical or near-infrared spectroscopic evidence for AGNs in ULIGs optically classified as H II region galaxies (~30% of the whole sample) or as LINERs (~40%). The apparent lack of energetically important AGNs in these objects supports the results from recent mid-infrared spectroscopy with ISO. Photoionization by hot stars from recent starbursts appears to be the dominant source of ionization in the objects with H II region-like spectra, while both hot stars and shocks may contribute to the ionization in ULIGs with LINER-like spectra. The weaker Hβ and Mg I b stellar absorption features, larger Hα emission equivalent widths, and bluer optical continuum colors in objects with higher infrared luminosities suggest that the starbursts took place more recently (≲few times 10⁷ yr) and/or are more important (~10% of the galaxy mass) in ULIGs than in their lower luminosity counterparts. As found in optically selected starbursts, the emission-line gas in ULIGs is dustier than the stellar populations that are producing the optical continuum. The color excess derived from the Balmer-line ratio does not significantly depend on the infrared luminosity, optical spectral type, or IRAS 25-60 μm color of the luminous infrared galaxies. These results suggest that the optical method used to determine the color excess in infrared galaxies underestimates the amount of dust in the dustier objects.

This paper reports the latest results from a near-infrared search for hidden broad-line regions (BLRs: ΔV_FWHM ≳ 2000 km s^-1) in ultraluminous infrared galaxies (ULIGs). The new sample contains 39 ULIGs from the 1 Jy sample selected for their lack of BLRs at optical wavelengths. Broad Paα emission is detected for the first time in two sources—F05189-2524 and F13305-1739. Broad Paα emission may also be present in three other sources—F13443+0802SW, F14394+5332, and F16156+0146—but new data are needed to make sure that H₂ 1.8665, 1.8721 μm are not contributing to this excess emission. The [Si VI] feature, a strong indicator of AGN activity, appears to be present in one object—F13305-1739—and perhaps also in Mrk 273 and F13454-2956. In addition, the presence of a hidden BLR is confirmed in the lower luminosity source F11058-1131. The results from this new study are combined with those from our previously published survey to produce a large database on 64 (non-Seyfert 1) ULIGs from the 1 Jy sample. All of the galaxies with strong evidence for a hidden BLR at near-infrared wavelengths present an optical Seyfert 2 spectrum. Overall, at least 50% (and perhaps up to 70%) of the optical Seyfert 2 galaxies in the combined sample present either a BLR or strong [Si VI] emission. In contrast, none of the 41 optically classified LINERs and H II galaxies in the sample shows any obvious signs of an energetically important AGN. Galaxies with "warm" IRAS colors (f₂₅/f₆₀ ≳ 0.2) show a tendency to harbor obscured BLRs in the near-infrared and to have large Paα-to-infrared luminosity ratios. These results support those of our earlier survey and suggest that the screen of dust in most warm Seyfert 2 galaxies is optically thin at 2 μm. When the results from this near-infrared survey are combined with those from a recent optical spectroscopic study of the entire 1 Jy sample of 118 ULIGs, we find that the fraction of all ULIGs with optical or near-infrared signs of genuine AGN activity (either a BLR or [Si VI] emission) is at least 20%-25% but reaches 35%-50% for objects with L_IR > 10^12.3L_☉. Comparisons of the dereddened emission-line luminosities of the optical or obscured BLRs detected in the ULIGs of the 1 Jy sample with those of optical quasars indicate that the obscured AGNs/quasars in ULIGs is the main source of energy in at least 15%-25% of all ULIGs in the 1 Jy sample. This fraction is closer to 30%-50% among ULIGs with L_IR > 10^12.3L_☉. These results are compatible with those from recent mid-infrared spectroscopic surveys carried out with ISO.

We use hard X-ray data for an "optimal" sample of Seyfert 2 galaxies to derive the distribution of the gaseous absorbing column densities among obscured active nuclei in the local universe. Of all Seyfert 2 galaxies in the sample, 75% are heavily obscured (N_H > 10²³ cm^-2), and about half are Compton thick (N_H > 10²⁴ cm^-2). Intermediate type 1.8-1.9 Seyfert galaxies are characterized by an average N_H much lower than "strict" Seyfert 2 galaxies. No correlation is found between N_H and the intrinsic luminosity of the nuclear source. This N_H distribution has important consequences for the synthesis of the cosmic X-ray background. In addition, the large fraction of Compton-thick objects implies that most of the obscuring gas is located within a radius of a few 10 pc from the nucleus.

The Owens Valley millimeter array has been used to map the CO 1-0 emission in the inner 25 of the grand-design spiral galaxy M51 at 2''-3'' resolution. These new images reveal the molecular spiral arms with unprecedented clarity; the emission in the two major arms (northeast and southwest) originates from supermassive cloud complexes, giant molecular associations (GMAs), which are for the first time resolved both along and perpendicular to the arms. The overall morphology of the CO emission is symmetric in reflection about the nucleus, with major complexes occurring opposite each other in the two major arms. On the other hand, the CO line flux in the area of the southwest arm closest to the nucleus is approximately twice as bright as that from the analogous location in the northeast arm. Streaming motions can be studied in detail and appear with great clarity along the major and minor axes of M51. The streaming velocities are very large, 60-150 km s^-1. Our maps offer, for the first time, sufficient resolution to resolve the structure in the molecular streaming motions. Both the radial and tangential velocity components show steep gradients, in qualitative accordance with predictions of the density-wave models of Roberts & Stewart. Our data thus support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the southwest arm, only one shows reduced shear. We find an unusual structure, an expansion in the northeast molecular arm at 25'' radius (1.2 kpc) southeast of the center. This broadening occurs right after the end of the northeast arm at the inner Lindblad resonance (ILR). Multiple-peak spectra, velocity twists, and structures with apparently high velocity dispersion are associated with this feature. Bifurcations in the molecular spiral arm structure, at a radius of ≈3.3 kpc (73''), may be evidence of a secondary compression of the gas caused by, and occurring near, the 4/1 ultraharmonic resonance. Several molecular spurs protrude from the main spiral arms, in particular on the western side of the map. Inside the radius of the ILR, we detect narrow (≈5'') molecular spiral arms, possibly related to the K-band arms found in the same region. We find evidence of noncircular motions in the inner 20'' of M51. The magnitude of these deviations is 20-30 km s^-1, and they are consistent with gas on elliptical orbits in a bar.

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution, and starbursts are believed to affect the structure and evolution of dwarf galaxies strongly. We have therefore embarked on a systematic study of 12 of the nearest dwarf galaxies thought to be undergoing bursts of star formation. These were selected primarily by their morphological type (blue "amorphous" galaxies). We show that these blue amorphous galaxies are not physically distinguishable from dwarfs selected as starbursting by other methods, such as blue compact dwarfs (BCDs) and H II galaxies. All these classes exhibit surface brightness profiles that are exponential in the outer regions (r ≳ 1.5r_e) but often have a predominantly central blue excess, suggesting a young burst in an older, redder galaxy. Typically, the starbursting "cores" are young (~10⁷-10⁸ yr) events compared to the older (~10⁹-10¹⁰ yr) underlying galaxy (the "envelope"). The ratio of the core to envelope in blue light ranges from essentially zero to about 2. These starbursts are therefore modest events involving only a few percent of the stellar mass. The envelopes have surface brightnesses that are much higher than typical dwarf irregular (dI) galaxies, so it is unlikely that there is a straightforward evolutionary relation between typical dIs and dwarf starburst galaxies. Instead we suggest that amorphous galaxies may repeatedly cycle through starburst and quiescent phases, corresponding to the galaxies with strong and weak/absent cores, respectively. Once amorphous galaxies use up the available gas (either through star formation or galactic winds) so that star formation is shut off, the faded remnants would strongly resemble dwarf elliptical galaxies. However, in the current cosmological epoch, this is evidently a slow process that is the aftermath of a series of many weak, recurring bursts. Present-day dE's must have experienced more rapid and intense evolution than this in the distant past.

The morphology and kinematics of the luminous blue starburst galaxy NGC 7673 are explored using the WIYN (Wisconsin-Indiana-Yale-NOAO) 3.5 m telescope. Signs of a past kinematic disturbance are detected in the outer galaxy; the most notable feature is a luminous ripple located 155 from the center of NGC 7673. Subarcsecond imaging in B and R filters also reveals red dust lanes and blue star clusters that delineate spiral arms in the bright inner disk, and narrowband Hα imaging shows that the luminous star clusters are associated with giant H II regions. The Hα kinematics measured with echelle imaging spectroscopy using the WIYN DensePak fiber array imply that these H II regions are confined to a smoothly rotating disk. The velocity dispersion in ionized gas in the disk is σ ~ 24 km s^-1, which sets an upper boundary on the dispersion of young stellar populations. Broad emission components with σ ~ 63 km s^-1 found in some regions are likely produced by mechanical power supplied by massive, young stars; a violent starburst is occurring in a kinematically calm disk. Although the asymmetric outer features point to a merger or interaction as the starburst trigger, the inner disk structure constrains the strength of the event to the scale of a minor merger or weak interaction that occurred at least an outer disk dynamical timescale in the past.

Scanning and pointed Rossi X-Ray Timing Explorer (RXTE) observations of the nearby Seyfert 2 galaxy NGC 6300 reveal that it is a source of hard X-ray continuum and large equivalent-width Fe Kα emission. These properties are characteristic of Compton reflection-dominated Seyfert 2 galaxies. The continuum can be modeled as Compton reflection; subsolar iron abundance is required, and a high inclination is preferred. However, the poor energy resolution of RXTE means that this description is not unique, and the continuum can also be modeled using a "dual absorber," i.e., a sum of absorbed power laws. Observations with higher energy resolution detectors will cleanly discriminate between these two models. Optical observations support the Compton reflection-dominated interpretation as L_X/L is low. NGC 6300 is notable because with F_2-10 ≈ 6.4 × 10^-12 ergs cm^-2 s^-1, it is the second brightest such object known.

It is known that merging galaxies with luminous starbursts and high far-infrared luminosities tend to have higher R_1-0 = ¹²CO (J = 1-0)/¹³CO (J = 1-0) integrated line intensity ratios (R_1-0 ≃ 20-50) than normal spiral galaxies (R_1-0 ≃ 5-15). Comparing far-infrared luminosities [L(FIR)] with those of ¹²CO (J = 1-0) and ¹³CO (J = 1-0) for a sample of normal and starburst galaxies, Taniguchi & Ohyama found that the observed high R_1-0 values for the luminous starburst mergers are attributed to their lower (by a factor of 3 on average) ¹³CO line intensities. They suggested the following two possibilities: in the luminous starburst mergers (1) ¹³CO is underabundant with respect to ¹²CO, or (2) exitation and/or optical depth effects are responsible for the change in R_1-0. In this paper, we investigate the second possibility, using higher transition data of both ¹²CO and ¹³CO emission lines. Applying the same method proposed by Taniguchi & Ohyama to both ¹²CO (J = 2-1) and ¹³CO (J = 2-1), we find that ¹³CO (J = 2-1) is also depressed with respect to ¹²CO (J = 2-1). This suggests that the ¹³CO gas may be underabundant in the high-R_1-0 starburst mergers, although we cannot rule out the possibility that excitation and optical depth effects are still affecting R_2-1, for example, as a result of the large velocity widths in the CO emission lines. Additional observations of both ¹²CO and ¹³CO lines at J ≥ 3 are required to better constrain the conditions of the molecular gas in luminous starburst galaxies.

We examine dust-to-gas ratio as a function of metallicity for dwarf galaxies (dwarf irregular galaxies [dIrrs] and blue compact dwarf galaxies [BCDGs]). Using a one-zone model and adopting the instantaneous recycling approximation, we prepare a set of basic equations that describes processes of dust formation and destruction in a galaxy. Four terms are included for the processes: dust formation from heavy elements ejected by stellar mass loss, dust destruction in supernova remnants, dust destruction in star-forming regions, and accretion of heavy elements onto preexisting dust grains. Solving the equations, we compare the result with observational data of nearby dIrrs and BCDGs. The solution is consistent with the data within the reasonable ranges of model parameters constrained by previous examinations. This means that the model is successful in understanding the dust amount of nearby galaxies. We also show that the accretion rate of heavy element onto preexisting dust grains is less effective than the condensation of heavy elements.

With the Akeno Giant Air Shower Array, 581 cosmic rays above 10¹⁹ eV, 47 above 4 × 10¹⁹ eV, and seven above 10²⁰ eV were observed until 1998 August. The arrival direction distribution of these extremely high energy cosmic rays has been studied. While no significant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 × 10¹⁹ eV, there are one triplet and three doublets within a separation angle of 25, and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1%. The triplet is especially observed against expected 0.05 events. The cos(θ_GC) distribution expected from the dark matter halo model fits the data as well as an isotropic distribution above 2 × 10¹⁹ and 4 × 10¹⁹ eV, but the fit with the dark matter halo model is poorer than the isotropic distribution above 10¹⁹ eV. The arrival direction distribution of seven 10²⁰ eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of the seven are members of doublets above about 4 × 10¹⁹ eV.

The molecular components of three giant H II regions (NGC 5461, NGC 5462, and NGC 5471) in the galaxy M101 are investigated with new observations from single-dish telescopes (James Clerk Maxwell Telescope and the NRAO 12 m) and from the Owens Valley Radio Observatory millimeter array. Of the three H II regions, only NGC 5461 had previously been detected in CO emission. We calculate preliminary values for the molecular mass of the GMCs in NGC 5461 by assuming a CO-to-H₂ factor (X factor) and then comparing these values with the virial masses. We found that the appropriate X factor is 5 times smaller than the X factor in the Milky Way despite the lower metallicity of M101. We conclude that the data in this paper demonstrate for the first time that the value of X may decrease in regions with intense star formation. The molecular mass for the association of clouds in NGC 5461 is approximately 3 × 10⁷M_☉ and is accompanied by 1-2 times as much atomic mass. The observed CO emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, but it was not possible to detect molecular gas toward NGC 5471 with the James Clerk Maxwell Telescope. An even larger ratio of atomic to molecular gas in NGC 5471 was observed, which might be attributed to efficient conversion of molecular to atomic gas. The masses of the individual clouds in NGC 5461, which are gravitationally bound, cover a range of (2-8) × 10⁵M_☉, comparable with the masses of Galactic giant molecular clouds. Higher star-forming efficiencies, and not massive clouds, appear to be the prerequisite for the formation of the large number of stars whose radiation is required to produce the giant H II regions in M101.

Predictions are made of the effect of variations in the [Fe/H] metallicity on the zero point of the Cepheid period-luminosity relation in bolometric, B, V, and I magnitudes. Theoretical evolutionary tracks in the H-R diagram, computed by three independent groups at Geneva, Padua, and Basel, are combined with the positions of the blue and red edges of the instability strip in the relevant H-R diagrams to give the predicted P-L relations for [Fe/H] metallicities of 0.0, -0.4, -0.7, -1.3, and -1.7. The predictions are based on the pulsation equation, P(,L,T_e,Z,Y), calculated at the points where the tracks of a given mass for each metallicity intersect the instability strip in the H-R diagrams.

     New model atmospheres and synthetic spectra are computed giving sets of grids of the bolometric corrections and B - V, V - R, R - I, and V - I colors for temperatures between 7500 and 5000 K, gravities between log g = 3.0 and log g = 0.75, and metallicities between [Fe/H] = 0.0 and [Fe/H] = -1.7. Interpolation in the grids at the relevant temperatures, gravities, and metallicities of the Cepheid instability strip give theoretical P-L relations on the Cape Cousins BVI photometric system at the blue and red edges of the strip. The metallicity dependence of the P-L relations, read at P = 10 days, are dM/d[Fe/H] = 0.00 mag dex^-1 in M(bol), +0.03 mag dex^-1 in B, -0.08 mag dex^-1 in V, and -0.10 mag dex^-1 in I in the sense that lower metallicities mean brighter magnitudes in B and fainter ones in V and I. Similar dependencies are found at P = 31.6 days.

     Confirmation that the zero points of the Cepheid P-L relations are not steeper functions of [Fe/H] than the theoretical values derived here is found by comparing the distance moduli of the LMC, the SMC, and IC 1613 based on the P-L relation for Galactic Cepheids (〈[Fe/H]〉 = 0) with the distance moduli determined for these galaxies using RR Lyrae variables with the steep absolute magnitude-metallicity calibration of M_V(RR) = 0.30[Fe/H] + 0.94. Applying this bright calibration, based on the Oosterhoff-Arp-Preston (OAP) metallicity effect, to the RR Lyrae variables in the LMC, the SMC, and IC 1613 gives individual distance moduli that agree to within 0.10 mag with the Cepheid distance moduli for [Fe/H] =0 for each galaxy, confirming that no metallicity dependence of the Cepheid P-L relation can be detected at this level with the present observational data if the bright RR Lyrae M_V ([Fe/H]) calibration is used.

     Using the stated calibrations with the Cepheid data from the literature gives the distance modulus of the LMC to be (m - M)₀ = 18.57. The modulus for the SMC, corrected for the mild metallicity effect derived here, reconciles the deviant individual B, V, and I moduli to within ±0.01 mag, giving (m - M)₀ = 18.94 for the SMC.

Using the available data for nearby stars from the Hipparcos catalog, we derive the velocity ellipsoid of dwarf O-B5.5 stars belonging to the Gould Belt (GB). The resulting vertex deviation for the whole sample of 252 stars is l_v ~ -64° ± 20°, and this value is modified to l_v = 22° ± 8° when the members of the Pleiades moving group are removed from the sample. This implies the existence of, at least, two different kinematic groups defining the GB system. We also model the evolution of a supershell in the solar neighborhood and obtain a fit to the shape and kinematics of the gas in the GB. Assuming that the expanding shell is also forming stars, we obtain the corresponding velocity fields for the shell and its newly formed stars. The average vertex deviation value resulting from these models for the new stars (assuming different velocity dispersion values and cutoff distances) is l_v ≃ 20° and is consistent with the observed value when the Pleiades moving group members are excluded from the GB.

Two Galactic star-forming regions, one in a very early phase of evolution and another evolved one, associated with the IRAS sources 00338+6312 and 03595+5110 (RAFGL 5111), respectively, have been studied in detail. These sources have been mapped simultaneously in two far-infrared bands (λ_eff = 143 and 185 μm), with ~15 angular resolution, using the Tata Institute of Fundamental Research (TIFR) 100 cm balloon-borne telescope. The HIRES-processed IRAS maps at 12, 25, 60, and 100 μm have been used for comparison. Whereas IRAS 00338+6312 is resolved only in the TIFR bands, RAFGL 5111 is very well resolved in both the TIFR bands as well as in at least three IRAS bands. The neighboring fainter source IRAS 04004+5114 has also been resolved in the TIFR bands. Taking advantage of the identical beams in the two TIFR bands at 143 and 185 μm, dust color temperature, T(143/185), and optical depth, τ₁₅₀, maps have been generated for RAFGL 5111. These maps show interesting structural details. Radiative-transfer modeling in spherical geometry has been carried out for individual sources to extract information about the cloud size, the type of the embedded source, the radial density distribution, the optical depth, the gas-to-dust ratio, and the dust grain composition. The best-fit models are in good agreement with the observed spectral energy distribution (SED), radio continuum data, and so on. Another scheme of radiative transfer through the interstellar dust-gas cloud including the heavier elements has been used to predict ionic nebular line emission, which is in reasonable agreement with the measurements for RAFGL 5111. An important conclusion from the present study is that, for all three sources (IRAS 00338+6312, 03595+5110, and 04004+5114, a faint source in the neighborhood of RAFGL 5111), the best fit to the observed SED is obtained for a uniform density [n(r) ~ r⁰] cloud.

We report a kinematic study of the symbiotic star system R Aqr derived from [N II] λ6584 emission observations with a Fabry-Perot imaging spectrometer. The [N II] spatial structure of the R Aqr jet, first observed circa 1977, and surrounding hourglass-shaped nebulosity, due to an explosion ~660 yr ago, are derived from 41 velocity planes spaced at ~12 km s^-1 intervals. Fabry-Perot imagery shows that the elliptical nebulosity making up the waist of the hourglass shell is consistent with a circular ring expanding radially at 55 km s^-1 as seen at an inclination angle i ~ 70^°. With respect to the position of R Aqr, at optical and radio wavelengths the jet is made up of a northeast component and a less intense, more amorphous southwest component. The [N II] Fabry-Perot data cube demonstrates that the two jet components possess quite different intensity-velocity structures which represent collimated flow in opposite directions. We offer an idealized schematic model for the R Aqr jet motion which results in a small-scale helical structure forming around a larger scale helical path. The implications of such a jet model are discussed. We present a movie showing a side-by-side comparison of the spatial structure of the model and the data as a function of the 41 velocity planes.

Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.

We present a theoretical formalism for determining the structure of molecular clouds and the precollapse conditions in star-forming regions. The model consists of a pressure-bounded, self-gravitating sphere of an ideal gas that is supported by several distinct pressures. Since each pressure component is assumed to obey a polytropic law P_i(r) ∝ ρ^γ_pi, we refer to these models as "multipressure polytropes." We treat the case without rotation. The time evolution of one of these polytropes depends additionally on the adiabatic index γ_i of each component, which is modified to account for the effects of any thermal coupling to the environment of the cloud. We derive structure equations as well as perturbation equations for performing a linear stability analysis. Special attention is given to representing properly the significant pressure components in molecular clouds: thermal motions, static magnetic fields, and turbulence. The fundamental approximation in our treatment is that the effects of turbulent motions in supporting a cloud against gravity can be approximated by a polytropic pressure component. In particular, we approximate the turbulent motions as a superposition of Alfvén waves. We generalize the standard treatment of the stability of polytropes to allow for the flow of entropy in response to a perturbation, as expected for the entropy associated with wave pressure. In contrast to the pressure components within stars, the pressure components within interstellar clouds are "soft," with polytropic indices γ_pi ≤ 4/3 and (except for Alfvén waves) adiabatic indices γ_i ≤ 4/3. This paper focuses on the characteristics of adiabatic polytropes with a single pressure component that are near the brink of gravitational instability as a function of γ_pi and γ_i for γ_pi ≤ 4/3. The properties of such polytropes are generally governed by the conditions at the surface. We obtain upper limits for the mass and size of polytropes in terms of the density and sound speed at the surface. The mean-to-surface density and pressure drops are limited to less than a factor 4 for γ_p ≤ 1, regardless of the value of γ. The central-to-surface density and pressure drops in isentropic clouds (γ_i = γ_pi) are also limited, but they can become quite large (as observed) in nonisentropic clouds, which have γ_i > γ_pi. We find that the motions associated with Alfvén waves are somewhat less effective in supporting clouds than are the kinetic motions in an isothermal gas.

H⁺₃ has been detected using infrared absorption spectroscopy along the lines of sight to six infrared sources in dense molecular clouds: AFGL 2136, W33A, Mon R2 IRS 3, AFGL 961E, AFGL 2591, and AFGL 490. Upper limits to the column densities of H⁺₃ are reported for an additional nine sources. The column densities of CO toward Mon R2 IRS 3 and AFGL 961E have been determined from observations of the first-overtone lines of CO. For the six sources toward which H⁺₃ was detected, a simple model of H⁺₃ chemistry has been used together with column densities of H₂ derived from infrared CO measurements to estimate column lengths, mean number densities, and temperatures of molecular clouds. The derived column lengths are on the order of a parsec, the number densities are 10⁴-10⁵ cm^-3, and the temperatures are ~25-50 K.

We have used the NRAO Very Long Baseline Array (VLBA) to image OH (1720 MHz) masers in the supernova remnants (SNR) W28 and W44 at a resolution of 40 mas. We also used MERLIN to observe the same OH (1720 MHz) masers in W44 at a resolution of 290 × 165 mas. All the masers are resolved by these VLBA and MERLIN observations. The measured sizes range from 50 to 180 mas and yield brightness temperature estimates from 0.3 to 20 × 10⁸ K. We investigate whether these measured angular sizes are intrinsic and hence originate as a result of the physical conditions in the supernova remnant shock, or whether they are scatter-broadened sizes produced by the turbulent ionized gas along the line of sight. While the current data on the temporal and angular broadening of pulsars, masers, and extragalactic sources toward W44 and W28 can be understood in terms of scattering, we cannot rule out that these large sizes are intrinsic. Recent theoretical modeling by Lockett, Gauthier, & Elitzur suggests that the physical parameters in the shocked region are indicative of densities and OH abundances that lead to estimates of sizes as large as what we have measured. If the sizes and structure are intrinsic, then the OH (1720 MHz) masers may be more like the OH (1612 MHz) masers in circumstellar shells than OH masers associated with H II regions. At two locations in W28 we observe the classical S-shapes in the Stokes V profiles caused by Zeeman splitting and use it to infer magnetic fields of order 2 mG.

Spectra of interstellar CO₂ ice absorption features at a resolving power of λ/Δλ ≈ 1500-2000 are presented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO₂ near 4.27 μm in all sources; the bending mode near 15.2 μm is also detected in 12 of them. The selected sources include massive protostars (Elias 29 [in ρ Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NGC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16); they thus probe a diverse range of environments. Column densities of interstellar CO₂ ice relative to H₂O ice fall in the range 10%-23%: this ratio displays remarkably little variation for such a physically diverse sample. Comparison of the observed profiles with laboratory data for CO₂-bearing ice mixtures indicates that CO₂ generally exists in at least two phases, one polar (H₂O dominant) and one nonpolar (CO₂ dominant). The observed CO₂ profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for CO₂ and implications for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing, rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO₂-bearing ices.

We present narrowband imagery and long-slit intermediate-resolution spectroscopy of the multiple-shell planetary nebulae NGC 2438 and NGC 5882. A comparative study of the chemical abundances of the main nebulae and their surrounding halos shows a moderate N/O enrichment of the central regions. The oxygen abundances appear constant throughout the nebulae, but there are some indications of a moderate helium enrichment in the NGC 5882 main nebula. This study, along with with data published in the literature for six other multiple-shell planetary nebulae (M1-46, NGC 6543, NGC 6720, NGC6751, NGC 6826, and NGC 7662), provides insight into the chemical enrichment and physical conditions of halos. All the nebulae in this sample have chemical abundances typical of type II planetary nebulae, thus indicating a small chemical enrichment. In addition, we do not find evidence to support a strong chemical enrichment of the main nebulae relative to the halos. The conclusion is that the third dredge-up, in these nebulae, is of small or null effect. Finally, we find a clear evidence that the gas in the halos is ionized by stellar UV radiation leaking through the material of the main nebula.

We present a fully sampled map of the Helix Nebula in the 1.3 mm CO J = 2-1 line, made using the Caltech Submillimeter Observatory. The angular resolution is 31'', and the velocity resolution is 1.5 km s^-1. The CO emission is found to extend over a region ~1000'' × 800'' and delineates the well-known double ring or "helix" seen in optical images of the ionized gas. Our observations provide the first complete view of the global kinematics of the envelope and reveal the three-dimensional structure of the gas. The helix is seen to be an expanding, equatorial ring around the ionized nebula, with arcs extending to higher latitudes and exhibiting remarkable point symmetries about the central star. The molecular gas is fragmented into many small condensations with narrow line widths (~1 km s^-1). The close relationship found between the structure of the envelope and the ionized nebula underscores the important role of neutral gas in determining the morphology of planetary nebulae. The point symmetries provide evidence for early shaping of the envelope by bipolar outflows or jets.

We have investigated 52 Rossi X-Ray Timing Explorer pointed observations of GRO J1655-40 spanning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of quasi-periodic oscillations (QPOs): three with relatively stable central frequencies near 300, 9, and 0.1 Hz, and a fourth QPO that varied over the range 14-28 Hz. The 300 and 0.1 Hz QPOs appear only when the power-law component dominates the X-ray spectrum and the estimated unabsorbed X-ray luminosity is above ~ 0.2L_Edd. At lower luminosity (L_X ~ 0.1L_Edd), the thermal component dominates the spectrum; the disk appears somewhat cooler (~1.3 keV), and its inner radius appears larger. In this state only two of the QPOs are observed: the broad and spectrally "soft" 9 Hz QPO and the narrow, "hard" QPO that varies from 14 to 28 Hz as the hard flux decreases. At still lower luminosities (L_X < 0.1L_Edd), the power-law component contributes less than 30% of the total luminosity, the inner disk appears both larger and cooler, the 9 Hz QPO vanishes, and only a very weak (rms 0.3%) and narrow QPO at 28 Hz remains. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS 1915+105. We discuss four models of these high-frequency QPOs that depend on effects due to general relativity. The models suggest that these rapid QPOs may eventually provide a measure of the mass and rotation of the accreting black hole. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency is not uniquely consistent with any of the natural timescales associated with the disk. The mechanism for the 14-28 Hz QPOs appears to be linked to the power-law component, similar to the 1-10 Hz QPOs in GRS 1915+105. Thus these low-frequency QPOs have the potential to lead us to the origin of the energetic electrons that radiate the power-law spectral component. Finally, we show data for GRO J1655-40 and GRS 1915+105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the respective spectral parameters allows us to speculate that the black hole mass in GRS 1915+105 is very large, possibly ~100 M_☉.

We present a series of two-dimensional core-collapse supernova simulations for a range of progenitor masses and different input physics. These models predict a range of supernova energies and compact remnant masses. In particular, we study two mechanisms for black hole formation: prompt collapse and delayed collapse owing to fallback. For massive progenitors (greater than 20 M_☉), after a hydrodynamic time for the helium core (a few minutes to a few hours), fallback drives the compact object beyond the maximum neutron star mass, causing it to collapse into a black hole. With the current accuracy of the models, progenitors more massive than 40 M_☉ form black holes directly with no supernova explosion (if rotating, these black holes may be the progenitors of gamma-ray bursts). We calculate the mass distribution of black holes formed and compare these predictions to the observations, which represent a small biased subset of the black hole population. Uncertainties in these estimates are discussed.

The ^180mTa (γ,γ') ¹⁸⁰Ta reaction was induced by gamma rays from a strong ⁶⁰Co source. Population of the ground state was observed (effective cross section 0.12 ± 0.02 μbarns) with an integrated cross section of 0.13 ± 0.04 μbarns MeV through mediating levels assumed near 1 MeV. Transition multipolarities and astrophysical consequences are discussed.

The Transient Gamma Ray Spectrometer (TGRS) on board the Wind spacecraft has spent most of the interval 1995-1997 in a high-altitude orbit where γ-ray backgrounds are low. Its high-resolution Ge spectrometer is thus able to detect weak lines that are slightly offset from stronger background features. One such line is predicted from nucleosynthesis in classical novae, where β decays on a timescale of a few hours in an expanding envelope produce positrons that annihilate to generate a line that is blueshifted by a few keV away from the background annihilation line at 511 keV. The broad TGRS field of view contained five known Galactic novae during 1995 January-1997 June, and we have searched the spectra taken around the times of these events for the blueshifted nova annihilation line. Although no definite detections were made, the method is shown to be sensitive enough to detect novae occurring on ONeMg-rich white dwarfs out to about 2.5 kpc.

Medium band (H' and K') images of the episodic dust-forming, long-period (~13 yr) WC7 + OB binary WR 137 were obtained with the Near-Infrared Camera and Multiobject Spectrometer 2 camera of the Hubble Space Telescope during periastron passage in 1997-1998. We have resolved IR-emitting dust in the close environment of this system. The dust emission occurs in a few clumps within about 05 of the star, as well as in a jetlike structure with a total extension of ~025. The dust is likely either created or enhanced in the zone of gas shocked by wind-wind collision. We estimate the total mass of the resolved dust features during the 1997-1998 outburst to be ~ 2 × 10^-7M_☉ (~ 0.1M_⊕) within a factor of 3.

We report the analysis of 1 hour's worth of RXTE data in which it was possible to detect individual pulses from the Crab pulsar. These data demonstrate that the Crab pulsar X-ray pulses are steady at the level of 7%, and we suggest that the emitting region is likely to come from a region where the magnetic field is "stiff."

We present phase-resolved spectra of the Be/X-ray pulsar system X Persei/4U 0352+309 obtained with the Tübingen far-UV echelle spectrometer of the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer SPAS II (ORFEUS-SPAS II). Data were obtained in 1996 November and folded on the ~837.5 s pulsar period derived from contemporaneous X-ray observations. Contrary to optical results by several groups, we find no evidence for periodic modulation in our far-UV line profile data. The limits we can place on the presence of any periodic signal are ~1% of the continuum level in the Si IV λ1394 line and ~1.6% in N V λ1238. We have also combined these ORFEUS spectra with newly reprocessed IUE data to discover radial velocity variations in X Per that are consistent with a period of the order of 20-30 yr and a binary separation of ~20 AU. Even with a wide separation for this system, we argue that certain types of reported rapid variability as well as long-term X-ray enhancements could be caused by the residue of expanding Be disks falling onto the neutron star. The UV resonance lines appear to be typical for a classical Be star, showing variability on timescales of 1 day and also a few hours.

With a newly derived equation of state (EOS) of dense matter, we construct zero-temperature compact-star models in hydrostatic equilibrium, for central densities 1.0 ≤ ρ_c/ρ_N ≤ 10.0 (ρ_N = 2.575 × 10¹⁴ g cm^-3 is the nuclear saturation density). Based on Skyrme's concept of baryons as solitons (of finite extent) in the meson field, the new EOS represents a fluid of Skyrmions coupled to a dilaton field (associated with the glueball of quantum chromodynamics) and a vector meson field (coupled to the baryon number). We find stable configurations to exist for ρ_c/ρ_N ≤ 5.0, and they are mostly fluid (the Skyrmion fluid); we thus name them "Skyrmion stars." The outer region of the star (the crust, for densities below the nuclear saturation density) is constructed using the EOS of Baym, Pethick, and Sutherland and accounts on average for 15% of the total mass of the star. Their masses and radii are 0.5 ≤ M/M_☉ ≤ 2.95 and 11.0 km ≤ R ≤ 15.3 km, respectively. The new EOS describes a fluid of Skyrmions with a unique behavior at high densities. The Skyrmions shrink as the density increases, allowing for a high compression of matter near the core of the star and thus greater gravitational binding energy. The heaviest stars, which can then withstand greater centrifugal forces, are expected to rotate the fastest in our model. Much of this interesting behavior is inherent in the glueball potential, with its negative contribution to the pressure acting to bind the system; the Skyrmion responds in a nonlinear fashion by shrinking (a result of Skyrmions having structure). Skyrmion stars are fundamentally different from quark stars; the quark degrees of freedom are integrated out, leaving only meson degrees of freedom. Furthermore, unlike boson/soliton stars where the soliton describes the global structure of the star, Skyrmion stars can be looked at as being made of fermionic soliton objects.

We discuss a series of observations of the black hole candidate GX 339-4 in low-luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data sets and eight separate Rossi X-Ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 MHz and 8.3-9.1 GHz radio observations. All of these observations have (3-9 keV) flux ≲10^-9 ergs s^-1 cm^-2. The ASCA data show evidence for an ≈6.4 keV Fe line with equivalent width ≈40 eV, as well as evidence for a soft excess that is well modeled by a power law plus a multicolor blackbody spectrum with peak temperature ≈150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths ≈20-140 eV. Reflection models show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. "Sphere+disk" Comptonization models and advection-dominated accretion flow (ADAF) models also provide reasonable descriptions of the RXTE data. The former models yield coronal temperatures in the range 20-50 keV and optical depths of τ ≈ 3. The model fits to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio flux is synchrotron emission from an extended outflow of size greater than (10⁷GM/c²).

Models for the characteristically soft X-ray spectrum of the compact X-ray source V1408 Aql (=4U 1957+11) have ranged from optically thick Comptonization to multicolor accretion disk models. We critically examine the X-ray spectrum of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data, archival Röntgensatellit data, and recent Rossi X-Ray Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray spectral models to these data, we favor an interpretation of the X-ray spectrum as being due to an accretion disk viewed at large inclination angles. Evidence for this hypothesis includes long-term (117, 235, 352 day) periodicities seen by the RXTE All Sky Monitor, which we interpret as being due to a warped precessing disk, and a 1 keV feature in the ASCA data, which we interpret as being the blend of L fluorescence features from a disk atmosphere or wind. We also present a timing analysis of the RXTE data and find upper limits of 4% for the rms variability between f = 10^-3 and 16 Hz. The situation of whether the compact object is a black hole or neutron star is still ambiguous; however, it now seems more likely that an X-ray-emitting, warped accretion disk is an important component of this system.

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low-mass red giant (RG) in which strong winds from the accreting WD play a key role increasing the WD mass to the Chandrasekhar mass limit. However, the occurrence frequency of SNe Ia through this channel is still controversial. Here we propose two new evolutionary processes that make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD+RG close binary can form from a wide binary even with an initial separation as large as a_i ≲ 40,000 R_☉. Such a binary consists of a low-mass main-sequence (MS) star and an asymptotic giant branch (AGB) star that is undergoing a superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Then the AGB star undergoes to form a common envelope (CE) evolution. After the CE evolution, the binary becomes a pair consisting of a carbon-oxygen WD and an MS star. When the MS star evolves to an RG, a WD+RG system is formed. Therefore, the WD+RG binary can form from much wider binaries than our earlier estimate, which was constrained by a_i ≲ 1500 R_☉. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of the RG to the WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG-to-WD mass ratio exceeds 1.15. If this mass-stripping effect is strong enough, though its efficiency is subject to uncertainties, the symbiotic channel can produce SNe Ia for a much (10 or more times) wider range of the binary parameters than our earlier estimate predicted. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy. The immediate progenitor binaries in this symbiotic channel to SNe Ia may be observed as symbiotic stars, luminous supersoft X-ray sources, or recurrent novae, such as T CrB or RS Oph, depending on the wind status.

We investigate the expected space velocities and misalignment angles of the eccentric binary pulsars for a range of possible progenitor systems and natal kick models. In particular, the effect of the orbital separation at the instant of supernova is discussed. The results of our simulations, combined with recent proper-motion and misalignment angle measurements, lead us to conclude that, although the known systems generally provide evidence for asymmetric supernovae, none of the proposed natal kick velocity distributions are adequate. Instead, we find that while kicks are required to explain the space velocities of J0045-7319 and B1259-63, a slow kick velocity distribution with σ = 0 km s ^-1 and μ = 50 km s^-1 predicts space velocities that demonstrate the best agreement with observation. The expected misalignment angle distributions for B1534+12, J0045-7319, and B1259-63 favor larger kicks but do not preclude a slow kick velocity distribution. Such a model is, however, inconsistent with the large kicks usually invoked to explain the velocities of single pulsars. We discuss possible means to reconcile the space velocities of eccentric binary pulsars and the velocity distribution of the pulsar population. These include constraining the progenitor systems of binary pulsars to preclude tight post-common-envelope systems and erroneous pulsar velocity measurements due to an inaccurate distance model.

We introduce a new method of searching for and characterizing extrasolar planets. We show that by monitoring the center-of-light motion of microlensing alerts using the next generation of high-precision astrometric instruments the probability of detecting a planet orbiting the lens is high. We show that adding astrometric information to the photometric microlensing light curve greatly helps in determining the planetary mass and projected separation. We introduce astrometric maps as a new way for calculating astrometric motion and planet detection probabilities. Finite source effects are important for low-mass planets, but even Earth-mass planets can give detectable signals.

We investigate the dependence of convective instability in a vertical photospheric magnetic flux tube, often referred to as convective collapse, over a broad range of parameters. The temporal evolution of the instability is examined by performing one-dimensional magnetohydrodynamic numerical simulations, adopting the thin flux tube approximation. Radiative energy transport is taken into account using Newton's law of cooling. To initiate the instability, the flux tubes are perturbed by a small downflow. The parameters of the instability are the initial photospheric tube radius a₀₀ and the ratio of gas pressure to magnetic pressure β₀₀. It is shown that the parameter plane is divided into convectively stable and unstable regions by a critical curve which is represented as a₀₀ ~ 3 × 10²/β₀₀ (km). A region of shock formation is found within the most unstable portion of the parameter range. Since a strong downflow due to the instability bounces off high-density plasma in the deeper layers, the resulting upflow leads to a shock wave. It is also shown that an initially unstable weak flux tube evolves into a static intense flux tube. We next investigate the parameter dependence of the collapsed state. In the unstable region without shock formation, the field strength of the collapsed tube at τ = 1 increases with a₀₀ and β₀₀. On the other hand, the field strength in the shock-forming region decreases with increasing β₀₀ because the shock lifts denser matter from lower layers to upper layers. Thus the field amplification due to convective collapse is suppressed by shock formation.

We present a compact, self-consistent formulation for the description of polarized radiation from magnetic-dipole transitions occurring in the magnetized solar corona. This work differs from earlier treatments by Sahal-Bréchot and House in the 1970s, in that the radiative emission coefficients for the four Stokes parameters, I, Q, U, and V, are treated to first order in a Taylor expansion of the line profile in terms of the Larmor frequency of the coronal magnetic field. In so doing, the influence on the scattered radiation of both atomic polarization, induced through anisotropic irradiation, and the Zeeman effect is accounted for in a consistent way. It is found that the well-known magnetograph formula, relating the V profile to the frequency derivative of the I profile, must be corrected in the presence of atomic alignment produced by anisotropic irradiation. This correction is smallest for lines where collisions and cascades dominate over excitation by anisotropic radiation, but it systematically increases with height above the solar limb (up to a theoretical maximum of 100%, in the collisionless regime and in the limit of vanishing longitudinal magnetic field). Although the correction to the magnetograph formula must be calculated separately for each line as a function of heliocentric distance, it is likely to be small for some lines of practical interest, along lines of sight close to the solar limb.

We report here the results of a study of the temperature variation of the network boundary thicknesses in the quiet-Sun transition region. A Fourier-based two-dimensional autocorrelation method has been applied to 240'' × 240'' rasters obtained in several transition-region lines by the CDS spectrometer on SOHO. The quantitative variation of the network boundary width with temperature has been obtained for the first time in a full two-dimensional field. It appears that network boundaries have an almost constant width up to a temperature of about 10^5.4 K and then fan out rapidly at coronal temperatures. This expansion of the transition-region network boundaries with temperature is found to be quantitatively in agreement with earlier theoretical models of the transition region.

We analyze a small flare using imaging data at millimeter, microwave, and soft X-ray wavelengths and microwave and hard X-ray spectral observations. The remarkable aspect of this flare is evidence for the presence of MeV-energy electrons, which are responsible for the nonthermal millimeter emission, at a time when no hard X-rays from lower energy electrons are detected. This occurs during a smoothly varying phase, which is seen at radio wavelengths to last several minutes and is the brightest phase at millimeter wavelengths but is undetected in hard X-rays: it follows a brief spike of emission at flare onset, which has the more usual properties of impulsive events and features nonthermal microwave, millimeter, and hard X-ray emission. We interpret the phase that is brightest at millimeter wavelengths as being due to efficient trapping of a relatively small number of nonthermal electrons, whereas during the hard X-ray emission, trapping is much less efficient, and the decay time is much shorter at all energies, which leads to a larger ratio of hard X-ray flux to radio flux. As in many previous events studied at millimeter wavelengths, there is a discrepancy between the electron energy spectral indices inferred from the milllimeter and hard X-ray data during the impulsive phase when both are detected: again it appears that the energy spectrum at 1 MeV must be significantly flatter than at several hundred keV and below. However, there are problems in reconciling quantitatively the energy spectra for the hard X-ray-emitting and radio-emitting components: based on the most plausible parameters, the radio-emitting electrons should produce most of the hard X-rays. One solution to this contradiction is to invoke a coronal magnetic field stronger than seems likely based on the photospheric magnetic field.

In this Letter we will show that, contrary to what is widely believed, an almost isotropic cosmic microwave background (CMB) temperature does not imply that the universe is "close to a Friedmann-Lemaitre universe." There are two important manifestations of anisotropy in the geometry of the universe: (1) the anisotropy in the overall expansion, and (2) the intrinsic anisotropy of the gravitational field, described by the Weyl curvature tensor; the former usually receives more attention than the latter in the astrophysical literature. Here we consider a class of spatially homogeneous models for which the anisotropy of the CMB temperature is within the current observational limits but whose Weyl curvature is not negligible, i.e., these models are not close to isotropy even though the CMB temperature is almost isotropic.

There is an ongoing debate in cosmology about the value of the length scale at which "homogeneity" in the matter distribution is reached or even if such a scale exists. In the wake of this debate, we intend in this Letter to clarify the meaning of the statement transition to homogeneity and of the concept of correlation length. We show that there are two scales, each associated with the fractal and with the homogeneous regimes of the matter distribution, respectively. The distinction between both scales has deep consequences, for there can be a regime that, despite having small fluctuations around the average density, exhibits large clusters of galaxies.

We report the discovery of an extremely luminous galaxy lying at a redshift of z = 5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22 field using a 105 Å bandpass filter centered at 8185 Å during the course of the Hawaii narrowband survey using the Low-Resolution Imaging Spectrograph (LRIS) on the 10 m Keck II telescope, and it was identified by the equivalent width of the emission [W_λ(observed) = 175 Å, flux = 1.7 × 10^-17 ergs cm^-2 s^-1]. Comparison with broadband colors shows the presence of an extremely strong break (>4.2 at the 2 σ level) between the Z band above the line, where the AB magnitude is 25.5, and the R band below, where the object is no longer visible at a 2 σ upper limit of 27.1 (AB magnitudes). These properties are only consistent with this object's being a high-z Lyα emitter. An 18,000 s spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in its continuum shape, line properties, and observed equivalent width to the z = 5.60 galaxy HDF 4-473.0, as recently described by Weymann et al., but is 2-3 times more luminous in the line and in the red continuum. For H₀ = 65 km s^-1 Mpc^-1 and q₀ = (0.02, 0.5), we would require star formation rates of around (40, 7) M_☉ yr^-1 to produce the UV continuum in the absence of extinction.

Using CCD detectors on board the forthcoming Chandra X-Ray Observatory and X-Ray Multimirror Mission, it is possible to devise a measurement of the absolute density of heavy elements in the hypothetical warm gas that is filling intercluster space. This gas may be the largest reservoir of baryonic matter in the universe, but even its existence has not been proved observationally at low redshifts. The proposed measurement would make use of a unique filament of galaxy clusters spanning over 700 h Mpc (0.1 ≲ z ≲ 0.2) along the line of sight in a small area of the sky in Aquarius. The surface density of Abell clusters there is more than 6 times the sky average. It is likely that the intercluster matter column density is enhanced by a similar factor, making its detection feasible under certain optimistic assumptions about its density and elemental abundances. One can compare photoabsorption depth, mostly in the partially ionized oxygen edges, in the spectra of clusters at different distances along the filament, looking for a systematic increase of depth with the distance. The absorption can be measured by the same detector and through the same Galactic column, hence the differential test. A CCD moderate-energy resolution (ΔE ~ 100 eV) is adequate for detecting an absorption edge at a known redshift.

We present the discovery of an isolated compact group of galaxies that is extremely dense (median projected galaxy separation: 6.9 kpc), that has a very low velocity dispersion (σ_2D = 67 km s^-1), and in which all observed members show emission lines and are morphologically disturbed. These properties, together with the lack of spiral galaxies and the presence of a prominent tidal tail, make this group one of the most evolved compact groups.

Upcoming weak lensing surveys on wide fields will provide the opportunity to reconstruct the structure along the line of sight tomographically by employing photometric redshift information about the source distribution. We define power spectrum statistics (including cross-correlation between redshift bins), quantify the improvement that redshift information can make in cosmological parameter estimation, and discuss ways to optimize the redshift binning. We find that within the adiabatic cold dark matter class of models, crude tomography using two or three redshift bins is sufficient to extract most of the information and improve, by up to an order of magnitude, the measurements of cosmological parameters that determine the growth rate of structure.

The highly inclined galaxy NGC 2841 shows an amorphous linear strip transverse to the nearside minor axis that is bright in the optical but coincident with a dark dust lane at 2.1 μm. Spectra of this strip obtained with the Low-Resolution Imaging Spectrometer on the Keck II telescope are essentially identical to spectra of the bulge, indicating that the emission from the strip is scattered light from the bulge. Fifteen Lick/IDS absorption features (including the G band, the MgH + Mg b band, and Na D) have been measured in the strip spectrum, and their indices are typical of late-type bulge stars. The equivalent widths of these absorption indices are essentially the same in the bulge and strip spectra, as are the shapes of the continua. The spectrum of a typical association of stars in the disk close to the strip is very different from both bulge and strip spectra, showing Balmer lines characteristic of a young population of stars. We estimate that ~2% of the bulge light received by the amorphous strip is scattered in our direction. This is consistent with our previous prediction of this effect, based on V and K' imaging data. Considering the geometry, we suggest the scattering cloud could extend to ~500 pc above the plane, similar to the heights of other dust features recently found in galaxies.

We present direct observations of molecular hydrogen in the disk of the nearby edge-on spiral galaxy NGC 891. With Infrared Space Observatory's Short-Wavelength Spectrometer (SWS) it has been possible, for the first time, to observe the lowest pure rotational lines of H₂ [S(0) at 28.2 μm and S(1) at 17.0 μm] at eight positions throughout the stellar disk of NGC 891. Both lines have been detected at all the surveyed positions out to 11 kpc north of the center of the galaxy. An H₂ rotation curve is derived, and we compare H₂ radial profiles with CO and H I data. The observed line ratios indicate relatively warm (T = 150-230 K) molecular clouds scattered throughout the disk in addition to a massive cooler (T = 80-90 K) component which dominates the signal in the outer regions. For H₂ ortho/para ratios of 2-3, the cool gas has typical edge-on column densities (1–3) × 10²³ cm^-2 (or ~3000 M_☉ pc^-2), in which case it outweighs the H I by a factor of 5-15. This factor matches well the mass required to resolve the problem of the missing matter of spiral galaxies within at least the optical disk. The newly discovered cool H₂ component would be less massive in the case in which its dominant ortho/para ratio is near unity. We address the thermal balance of this component by a comparison with [C II] 158 μm data. When combining the new coolish molecular gas results with recent SCUBA cold dust observations of NGC 891, the total gas-to-dust ratio at r < 12 kpc remains around 200.

The drag force experienced by a gravitational body moving in a straight-line trajectory through a homogeneous isothermal gaseous medium of given sound speed is investigated numerically. For perturbers with constant velocity, linear theory describes successfully the temporal evolution and magnitude of the force. The result obtained recently by E. Ostriker—that for Mach numbers = 1-2 the force is stronger in a gaseous medium than in a collisionless medium, as described by the standard Chandrasekhar formula—is confirmed. The corresponding minimum impact radius r_min for a body described with a Plummer model with core radius R_soft is r_min/R_soft ≈ 2.25. When < 1, the drag force is strongly suppressed, which is consistent with Ostriker's results but in disagreement with the Chandrasekhar formula. However, when the perturber is decelerated by its own wake to < 1, the effective drag force remains initially somewhat larger than the value in the case of constant velocity because it takes some time to get rid of the wake that was generated during its supersonic history.

We present BVRI observations of the optical counterpart to GRB 990510 obtained with the Las Campanas 1.0 m telescope between 15 and 48 hr after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically t^-0.76±0.01 at early times (t ≪ 1 day) and t^-2.40±0.02 at late times, with the break time at t₀ = 1.57 ± 0.03 days. GRB 990510 is the most rapidly fading of the well-documented gamma-ray burst (GRB) afterglows. It is also the first well-observed example of a broadband break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power law with ν^-0.61±0.12. However, when the B-band point is dropped from the fit, the power law becomes ν^-0.46±0.08, indicating a possible deviation from the power law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at z = 1.62. Broadband break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.

Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as the morphology, metallicity, and age of the host galaxies. The influence of these environmental properties on the resulting SNe Ia is studied in this Letter. First, it is shown that the carbon mass fraction X(C) in the C + O white dwarf SN Ia progenitors tends to be smaller for a lower metallicity environment and an older binary system. It is then suggested that the variation of X(C) causes the diversity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of the turbulent flame are necessary to confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low-metallicity environment, the number of bright SNe Ia is reduced, so that the variation in brightness among the SNe Ia is also smaller. Thus, our model can explain why the mean SN Ia brightness and its dispersion depend on the morphology of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z ≳ 1), both the mean brightness of SNe Ia and its variation should be smaller in spiral galaxies than in elliptical galaxies. These variations are within the range observed in nearby SNe Ia. Insofar as the variation in X(C) is the most important cause for the diversity among SNe Ia, the light-curve shape method that is currently used to determine the absolute magnitude of SNe Ia can also be applied to high-redshift SNe Ia.

In previous papers, we presented the discovery of a 12 s X-ray pulsar in the supernova remnant Kes 73, providing the first direct evidence for an ultramagnetized neutron star, a magnetar, with an equivalent dipole field of nearly 20 times the quantum critical magnetic field (m_e²c³/eℏ). Our conclusions were based on two epochs of the measurement of the spin, along with an age estimate of the host supernova remnant. Herein, we present a spin chronology of the pulsar using additional Ginga, ASCA, RXTE, and BeppoSAX data sets spanning over a decade. The timing and spectral analyses confirm our initial results and severely limit an accretion origin for the observed flux. Over the 10 yr baseline, the pulsar is found to undergo a rapid, constant spin-down while maintaining a steady flux and an invariant pulse profile. Within the measurement uncertainties, no systematic departures from a linear spin-down are found—departures as in the case of glitches or simply stochastic fluctuations in the pulse times of arrival (e.g., red timing noise). We suggest that this pulsar is akin to the soft γ-ray repeaters; however, it is remarkably stable and has yet to display similar outbursts. Future γ-ray activity from this object is likely.

In previous programs, we have identified a large number of possible, very low mass members of the young open cluster IC 2391 based on their location in an I versus (R-I)_c color-magnitude diagram. We have now obtained new photometry and intermediate-resolution (Δλ = 2.7 Å) spectra of 19 of these objects (14.9 ≤ I_c ≤ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their VRI photometry, spectral types, radial velocity, and Hα emission strengths. Higher signal-to-noise ratio spectra were obtained for eight of these probable cluster members in order to measure the strength of the doublet Li λ6708 and thus obtain an estimate of the cluster's age. One of these eight stars has a definite lithium detection, and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium-depletion boundary in IC 2391 is at I_c = 16.2. Using recent theoretical model predictions, we derive an age for IC 2391 of 53 ± 5 Myr. While this is considerably older than the age most commonly attributed to this cluster (~35 Myr), this result for IC 2391 is comparable to those recently derived for the Pleiades and α Persei clusters and can be explained by new models for high-mass stars that incorporate a modest amount of convective-core overshooting.

The efficiency of thin disk accretion onto black holes depends on the inner boundary condition, specifically the torque applied to the disk at the last stable orbit. This is usually assumed to vanish. I estimate the torque on a magnetized disk using a steady magnetohydrodynamic inflow model originally developed by Takahashi et al. I find that the efficiency can depart significantly from the classical thin disk value. In some cases > 1, i.e., energy is extracted from the black hole.

We report the discovery of the coolest field dwarf yet known, selected as an unresolved object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 μm is dominated by strong bands of H₂O and CH₄. Its spectrum and colors over this range are very similar to those of Gl 229B, the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gl 229B, suggesting that it lies at a distance of ~10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 M_☉ and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 M_☉ for an age range of 0.3-5 Gyr.

We report the discovery of four field methane ("T"-type) brown dwarfs using Two Micron All-Sky Survey (2MASS) data. One additional methane dwarf, previously discovered by the Sloan Digital Sky Survey, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 μm CH₄ absorption bands characteristic of objects with T_eff ≲ 1300 K as well as broadened H₂O bands at 1.4 and 1.9 μm. Comparing the spectra of these objects with that of Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, 2MASS J1225-27, 2MASS J1047+21, and 2MASS J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs deg^-2, or ~90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs pc^-3, is comparable to that of the first L dwarf sample from Kirkpatrick et al.

We have obtained narrowband images of the young planetary nebula Hubble 12 showing [Fe II] line emission in bipolar bubbles near the core. Bright [Fe II] emission is strong evidence for shocks, suggesting the presence of a high-velocity wind emanating from the central star. We compare our data to previously published images of Hubble 12—finding similar structures in hydrogen recombination and free-free emission—and we propose three possible interpretations of the data: that the bubbles indicate the inner shock of the fast wind, that the [Fe II] emission is evidence of the outer shock of an episodic wind, or that the emission is cooling line radiation from a photodissociation region. We argue that the first two interpretations are more likely, since the [Fe II] emission must be shock excited.

Using the NRAO 140 Foot Telescope, we have confirmed our earlier detection of ³He in the planetary nebula NGC 3242 made with the 100 m radio telescope of the Max-Planck-Institut für Radioastronomie. Both spectrometers show that the observed ³He⁺ hyperfine line has a double-peaked structure. This line shape naturally arises if NGC 3242 is modeled as a dense core surrounded by a very low density expanding shell. The observed ³He⁺ emission suggests an abundance by number of ³He/H = (2–5) × 10^-4, which is more than an order of magnitude higher than typically found in Galactic H II regions.

A raster scan by Solar Ultraviolet Measurements of Emitted Radiation/Solar and Heliospheric Observatory (SOHO) of the full solar disk containing over 2 million individual spectra of the He I line at 584 Å provides the possibility to study small- and large-scale variations of intensity, line shift, and line width. New striking features are the enhanced line width and the blueshifts in the coronal holes as well as the long-known reduced intensity there. The relation of intensity versus line shift and the variation of the line shift in the coronal holes indicate that the blueshifts in the holes are not consistent with a uniform outflow. The observations suggest that optical depth effects might be responsible for the observed blueshifts and line widths. These results motivate new observations by SOHO and ground-based facilities.