FIRST LIGHT LBT AO IMAGES OF HR 8799 bcde AT 1.6 AND 3.3 μm: NEW DISCREPANCIES BETWEEN YOUNG PLANETS AND OLD BROWN DWARFS^*

Efforts are underway to characterize the first generation of directly imaged extrasolar planets. A principal focus has been the HR 8799 planetary system (Marois et al. 2008, 2010), which with four planets is currently unique as a directly imaged multiple-planet system. Studying these planets simultaneously is particularly powerful given their connected formation histories and appearances.

HR 8799 is a young, A5V star with a λ Boo deficiency of heavy metals and three distinct circumstellar dust structures (Marois et al. 2008; Cowley et al. 1969; Gray & Kaye 1999; Su et al. 2009). There is some disagreement about the age of the system. Traditional age-dating methods, such as galactic space motion and Hertzsprung–Russell diagram position, suggest that HR 8799 has an age of 20–160 Myr (Moór et al. 2006; Marois et al. 2008; Hinz et al. 2010; Zuckerman et al. 2011), while astroseismology estimates are more consistent with ∼1 Gyr, which would make the planets significantly more massive brown dwarfs (Moya et al. 2010). Interestingly, the dynamical stability of the planets themselves places upper limits on the masses of the planets (Fabrycky & Murray-Clay 2010; Moro-Martín et al. 2010; Sudol & Haghighipour 2012), which directly converts to a young system age, based on the planets' photometry and evolutionary models (Burrows et al. 1997; Chabrier et al. 2000).

An important implication of the relative youth and low masses of the HR 8799 planets is that their appearances and atmospheric properties might be different than field brown dwarfs, which can have the same effective temperatures as giant planets while being older and more massive. Brown dwarf spectra have been used as proxies for giant planet spectra to plan direct imaging surveys and to interpret early discoveries. However, initial results show that there are several key differences between the atmospheres of giant exoplanets and brown dwarfs.

For field brown dwarfs, the L→T spectral-type transition occurs at ∼1200–1400 K, where dust clouds settle/condense below the photosphere (Saumon & Marley 2008), and CO is converted to CH₄ (Burrows et al. 2003; Geballe et al. 2002). For the HR 8799 planets, clouds are suspended in the photosphere at lower effective temperatures (900–1200 K) than is typical for brown dwarfs (Currie et al. 2011; Madhusudhan et al. 2011; Barman et al. 2011a). Additionally, there appears to be more CO than CH₄ relative to equilibrium chemistry models, implying that convection is mixing hot material into the photosphere faster than the CO↔CH₄ reaction can re-equilibrate (Hinz et al. 2010; Barman et al. 2011a). Similar but more extreme results have been found for 2MASS 1207 b, a 5–7 M_jup companion to a 25 M_jup TW Hya brown dwarf primary (Chauvin et al. 2004; Skemer et al. 2011; Barman et al. 2011b). Evidently, the HR 8799 planets and 2MASS 1207 b look similar to L-type brown dwarfs, despite having effective temperatures more consistent with T-type brown dwarfs.

Multiwavelength photometry and spectroscopy are the keys to understanding the differences between brown dwarfs and giant planets. In particular, working over a broad wavelength range is critical for understanding clouds, chemistry, and the radiative budget of extrasolar planets. The challenge of working over a broad wavelength range is that adaptive optics (AO) systems perform better at longer wavelengths where atmospheric turbulence is less severe. But background radiation increases at longer wavelengths, and most AO systems have numerous warm optics, which can make working at long wavelengths impractical (Lloyd-Hart 2000). The sweet spot for most AO systems has typically been the near-infrared (∼1–2.5 μm), although for extrasolar planets, there is a benefit to working at L' (3.8 μm), where the planet–star contrast improves (Heinze et al. 2008).

The Large Binocular Telescope (LBT) AO system can increase the wavelength range over which we study extrasolar planets. With its 672-actuator deformable secondary mirror (installed on one side of the telescope at the time of our observations), the system produces unprecedented image quality and contrast at short wavelengths. And because it is a deformable secondary AO system, it has a minimal number of warm optics, so that background noise remains low at long wavelengths.

In this paper, we present LBT First Light Adaptive Optics (FLAO) images of HR 8799 at H band (1.65 μm) and 3.3 μm, detecting the four planets (b–e) at both wavelengths. Both images are superior to previous attempts at these wavelengths due to the high performance of the LBT's AO system. Our images are the first detections of HR 8799 e at H band and 3.3 μm and the first unambiguous detections of HR 8799 b and d at 3.3 μm. In Section 2, we give a basic description of the instrumental setup for the FLAO system, the near-infrared imager, PISCES, and the mid-infrared imager, L- and M-band Infrared Camera (LMIRCam), which is a component of the Large Binocular Telescope Interferometer (LBTI). Additionally, we describe our data reduction procedure, which is an implementation of the Locally Optimized Combination of Images (LOCI) algorithm. In Section 3 we estimate photometry for the four planets, based on our LOCI reductions. In Section 4 we use our H-band image to search for additional companions interior to HR 8799 e, taking advantage of the unprecedented contrast afforded by the LBT AO system. In Section 5 we present thick-cloud, non-equilibrium chemistry model atmospheres and mixed-cloud atmospheres in an effort to explain the appearances of the HR 8799 planets. We conclude in Section 6 and make suggestions for future work characterizing HR 8799 and other directly imaged exoplanets. A companion paper, Esposito et al. (2012), describes the instrumental setup for the AO system and PISCES in detail, provides an independent analysis of the H-band data (along with new K_s-band data), and presents astrometry and a new orbital analysis of the system.

2.1. PISCES H Band

We observed HR 8799 at H band (λ = 1.66 μm; FWHM = 0.29 μm) with PISCES (McCarthy et al. 2001) on UT 2011 October 16, during Science Verification Time for the LBT FLAO system. The LBT's FLAO system (PI: Simone Esposito) is a 672-actuator deformable secondary AO system that makes use of an innovative pyramid wavefront sensor, producing high-Strehl-ratio, low-background images over a broad wavelength range (Esposito et al. 2010, 2011). At the time of our observations, one AO system was installed on the right telescope, so for the observations presented in this paper, only one 8.4 m mirror is used. PISCES (PI: Don McCarthy) is a 1–2.5 μm imager with a Hawaii 1024 × 1024 HgCdTe array, which at the LBT (single 8.25 m aperture) critically samples a diffraction-limited point-spread function (PSF) at H band (with a plate scale of 00193 pixel⁻¹). For the observations in this paper, PISCES was installed on the front bent-Gregorian focus of the LBT's right side to take advantage of the LBT's new AO system, while facility infrared cameras are being delivered and commissioned.

During our H-band observations of HR 8799, conditions were photometric and the natural seeing, as measured by a differential image motion monitor (DIMM) on the telescope structure, was as good as ∼09. We obtained 901 images with 2 s integrations, over the course of 2 hr (90° of sky rotation) with the telescope rotator turned off angular differential imaging (ADI; Marois et al. 2006). PISCES' readout time is 6 s, which means our observations were inefficient. However, high-contrast observations are usually limited by PSF stability rather than photon noise, so the long readouts had a negligible effect on our final results. The 2 s integrations saturated out to a radius of ∼015. We were not able to obtain unsaturated H-band images of the star for photometry and astrometry with PISCES' shortest integration time, 0.8 s.

Images were processed to remove cross-talk and persistence as described in McCarthy et al. (2001) using the corquad correction software.¹³ We then dark subtracted, flat fielded, and distortion corrected the images. Finally, images were aligned by maximizing their cross-correlation. We processed the aligned images using the LOCI algorithm (Lafrenière et al. 2007), which has been shown to produce higher contrast images than other algorithms, such as ADI. In the terminology of Lafrenière et al. (2007), we used N_δ = 1, N_A = 300, g = 1, and a 1 pixel subtraction region.¹⁴ The values for N_A and g are adopted from Lafrenière et al. (2007), and we use N_δ = 1 instead of N_δ = 0.5 to suppress self-subtraction, although we note that changing N_δ to 0.5 has a negligible effect on our photometry. Our implementation of the LOCI algorithm also includes an FWHM-sized mask around every subtraction region to further suppress self-subtraction.

After reducing all 901 images with LOCI, we evaluated the noise of the images (standard deviation of pixel count) within an annulus from 03 to 05. The noise dramatically increased after the 500th image, corresponding to an increase in natural seeing. We reran LOCI using just the first 500 images (1000 s, 61° of sky rotation), marginally improving our final image, which is shown in Figure 1. HR 8799 b, c, d, and e are all clearly visible.

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The LOCI algorithm assumes accurate knowledge of the star's position. However, the stellar core saturated even with PISCES' fastest readout. We were able to estimate the stellar centroid to within 0.5 pixels (001) based on the circular symmetry of the PSF. We then reran LOCI with a set of different stellar centroid positions composing a 1 × 1 pixel box and found no significant astrometric or photometric discrepancies between the results.

2.2. LBTI/LMIRCam 3.3 μm

Since LOCI self-subtracts, we calibrated our photometry by subtracting fake planets from the raw data at the positions of the detected planets and rerunning the full LOCI pipeline. Best-fit photometry and error bars were determined by adjusting the fluxes of the fake planets to determine the range of values resulting in reasonable subtractions. For the PISCES H-band data, the star was saturated, so we calculated photometry for planets c, d, and e with respect to HR 8799 b and converted to absolute magnitudes by adopting the magnitude of HR 8799 b from Metchev et al. (2009). For the LMIRCam 3.3 μm data, we calculated photometry for all four planets with respect to unsaturated images of the star, obtained immediately after the saturated images used to detect the planets. We converted these to absolute photometry using HR 8799's absolute magnitude at 3.3 μm from Hinz et al. (2010). Errors on the LMIRCam absolute calibration are primarily the result of changing Strehl ratios and telluric absorption variation throughout our observations. To test the magnitude of the Strehl-ratio variation, we performed r = λ/D aperture photometry on the unsaturated data and found a standard deviation of only ∼2%. To test the magnitude of the telluric absorption variation, we compared the Airy pattern of the saturated and unsaturated images and found them to be consistent within ∼5%. Combining these error terms, we adopt an absolute calibration error of 0.06 mag for the LMIRCam 3.3 μm data. H-band and 3.3 μm photometry of HR 8799 is presented in Table 1.

Our H-band photometry for HR 8799 c and d is consistent with the results of Marois et al. (2008) and Metchev et al. (2009), and we detect HR 8799 e for the first time at H band, finding it to be ∼0.3 mag brighter than the next brightest planet (HR 8799 c). Our 3.3 μm data are somewhat inconsistent with previous photometry from Hinz et al. (2010) and Currie et al. (2011), which are independent reductions of observations taken with MMT/Clio (note that the Clio and LMIRCam 3.3 μm filters are identical). Hinz et al. (2010) reported detections of HR 8799 c and d but not b, and Currie et al. (2011) reported detections of HR 8799 b and c but not d. The most substantial disparity is for HR 8799 b, for which Hinz et al. (2010) reported an absolute magnitude upper limit of 14.82, while Currie et al. (2011) reported a detection of 13.96 ± 0.28. Our detection of 13.22 ± 0.11 is closer to the result of Currie et al. (2011) but is still brighter by a significant amount. We reanalyzed the final reduced images from Hinz et al. (2010) and find the upper limit reported by Hinz et al. (2010) to be erroneous (likely a typographic error). Our photometry for HR 8799 c is also brighter than observed by Hinz et al. (2010) and Currie et al. (2011), and our photometry of HR 8799 d is brighter than observed by Hinz et al. (2010). We present a comparison of the MMT/Clio image and our new LBT/LMIRCam image in Figure 2. The fact that our photometry is somewhat inconsistent with the values of Hinz et al. (2010) and Currie et al. (2011) can likely be explained by the non-photometric conditions reported in Hinz et al. (2010), varying AO performance (caused by the non-photometric conditions) and overly optimistic systematic and/or measurement error analyses considering the quality of the Clio data. Variability is unlikely to be a factor, given that it has not been reported in any other photometric band and the amplitude of variability needed to rectify the disparity is quite large.

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While HR 8799 is known to have four giant planets at wide separations, the inner system might have one or more companions that have not yet been discovered. Additional companions would challenge formation models, which already have a hard time explaining the outer four planets (Dodson-Robinson et al. 2009; Kratter et al. 2010). A fifth planet would also complicate dynamical stability analyses, which require mean-motion resonances and planet masses that are on the low end of the range predicted by evolutionary models and independent age estimates (Fabrycky & Murray-Clay 2010; Moro-Martín et al. 2010; Sudol & Haghighipour 2012). Hinkley et al. (2011) used non-redundant masking to rule out the presence of massive inner companions from ∼001 to 05, but only for objects significantly more massive than the four known planets. Here we use our H-band image to search for planetary mass companions interior to HR 8799 e.

We evaluate our ability to detect a close-in companion by making a contrast curve of our residual H-band image (after the four planets have been removed).¹⁵ We smooth the image with a Gaussian that is the same size as our diffraction-limited PSF and calculate the standard deviation in 1 pixel annuli. Counts are converted to photometry using the peak-flux (central pixel) of planet "b" from the smoothed image. We also correct for self-subtraction, which is measured by inserting fake planets into the raw data at various radii. Our 5σ contrast curve is shown in Figure 3. HR 8799 b–e are shown as diamonds. The vertical dashed line denotes the separation of the 2:1 mean-motion resonance with HR 8799 e (assuming a face-on circular orbit for simplicity). If there is a massive interior planet, it is likely to be in a stable resonance, as has been found for pairs of the outer four planets. We find no fifth planet at or exterior to the 2:1 resonance with HR 8799 e, with limits down to the approximate brightnesses (and by extension, masses) of the inner three planets. As a check, we insert a fake "e"-like planet into our raw images at a separation of 0235, the approximate position of the 2:1 resonance with "e." Our reduced image (Figure 4) shows that we would have detected "planet f" anywhere exterior to the 2:1 resonance, and that there are no residuals brighter than "f" in the image. Note that we do not repeat this analysis with the 3.3 μm image because it is not as sensitive to additional companions as the H-band image.

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Multiwavelength photometry and spectroscopy have improved our understanding of the physical properties of the HR 8799 planets, which are not well fit by the same models that have been used to interpret the properties of field brown dwarfs. Marois et al. (2008) first noted that the HR 8799 planet spectral energy distributions (SEDs) were best fit by model atmospheres with T = 1400–1700 K, while their luminosities were more consistent with T = 800–1000 K. The disparity was driven by the planets' faint/red appearance with respect to models and a lack of methane absorption in narrowband 1.59/1.68 CH₄ photometry. Subsequent 3.88–4.10 μm spectroscopy of HR 8799 c was inconsistent with both COND (Baraffe et al. 2003) and DUSTY (Chabrier et al. 2000) atmospheric models, which the authors interpreted as evidence for non-equilibrium chemistry (Janson et al. 2010). Three-color photometry in the L and M bands was also inconsistent with equilibrium chemistry atmospheric models and, in particular, showed a relative lack of methane absorption at 3.3 μm (Hinz et al. 2010). H and K spectroscopy of HR 8799 b also showed a methane deficiency (Bowler et al. 2010; Barman et al. 2011a). Currie et al. (2011) and Madhusudhan et al. (2011) were able to parameterize thick-cloud models to fit multiwavelength photometry for HR 8799 bcd. However, the models were not able to reproduce the 3.3 μm photometry or the subsequent spectroscopy of Barman et al. (2011a), who fit both the photometry and spectroscopy of HR 8799 b with models that incorporated clouds and non-equilibrium chemistry.

The combined result of these studies is that HR 8799 b, c, and d have non-equilibrium CO↔CH₄ chemistry and cloudy atmospheres at low effective temperatures where, in field brown dwarfs, the clouds are thought to have settled below the photosphere. Two color–magnitude diagrams, shown in Figure 5, demonstrate these effects. On the left is an H − K versus H color–magnitude diagram, which shows the M→L→T sequence of field brown dwarfs. L dwarfs are characterized by cloudy atmospheres, while T dwarfs are characterized by cloud-free atmospheres. The intermediate region is the L→T transition, where objects are thought to have patchy clouds or clouds that have partially descended below the photosphere. The HR 8799 planets appear to be an extension of the field L-dwarf sequence, i.e., they have clouds at an effective temperature where the field brown dwarfs are transitioning to cloudless. The right-hand side of Figure 5 shows a 3.3 μm−L' versus L' color–magnitude diagram, which includes a sequence of chemical equilibrium, thick-cloud models from Madhusudhan et al. (2011). The HR 8799 planets are all much brighter at 3.3 μm than predicted by the models, implying a relative absence of CH₄, which is a strong absorber at 3.3 μm.

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In the following sections, we present model atmospheres of the HR 8799 planets in an attempt to reproduce their photometry. A summary of the available photometry is presented in Table 2, colors are presented in Table 3, and a listing of all the models used in this paper is presented in Table 4. For all model comparisons in this paper, we convolve the model planet atmosphere and a model of Vega (Cohen et al. 1992) with filter profiles to produce predicted magnitudes in the different filters, which are then compared to the measured photometry. The filter profiles have all been multiplied by a model telluric atmosphere.¹⁶ For most filters, this step has a negligible effect, because the telluric atmosphere has a flat transmission profile, but Earth's atmosphere has a large transmission slope in the 3.3 μm filter that changes the effective wavelength of the filter and, in turn, changes the model magnitudes by ∼0.1 mag. For the sake of plotting the model fits, we convert the filter magnitudes to Jy by convolving the Vega model with filter profiles to produce zero-point fluxes. For HR 8799 b, we also include the H and K spectroscopy from Barman et al. (2011a). The HK spectrum comparison is made by smoothing the planet model atmosphere by the published spectrum's resolution (0.01 μm) and directly comparing to the observed spectrum. The absolute fluxes of the H and K spectroscopy are tied to the H and K photometry, so for our comparison we allow the overall brightness of the two spectra to vary and only fit the shapes of the spectra.

Table 2. Photometry of the HR 8799 Planets

Planet	z	J	H	$\rm {CH}_{4}\rm {s}$	$\rm {CH}_{4}\rm {l}$	Ks	3.3	L'	M
	(1.03 μm)	(1.25 μm)	(1.63 μm)	(1.59 μm)	(1.68 μm)	(2.15 μm)	(3.3 μm)	(3.8 μm)	(4.7 μm)
HR 8799 b	18.24 ± 0.29	16.30 ± 0.16	15.08 ± 0.13	15.18 ± 0.17	14.89 ± 0.18	14.05 ± 0.08	13.2 ± 0.11	12.66 ± 0.11	13.07 ± 0.30
HR 8799 c		14.65 ± 0.17	14.18 ± 0.14	14.25 ± 0.19	13.90 ± 0.19	13.13 ± 0.08	12.2 ± 0.11	11.74 ± 0.09	12.05 ± 0.14
HR 8799 d		15.26 ± 0.43	14.23 ± 0.2	14.03 ± 0.30	14.57 ± 0.23	13.11 ± 0.12	12.0 ± 0.11	11.56 ± 0.16	11.67 ± 0.35
HR 8799 e			13.88 ± 0.2			12.93 ± 0.22	12.1 ± 0.21	11.61 ± 0.12
Reference (b,c,d,e)	2	3,3,3	4,1,1,1	3,3,3	3,3,3	3,3,3,5	1,1,1,1	3,3,3,5	6,6,6

References. (1) This work; (2) Currie et al. 2011; (3) Marois et al. 2008; (4) Metchev et al. 2009; (5) Marois et al. 2010; (6) Galicher et al. 2011.

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5.1. HR 8799 b

HR 8799 b is the most challenging atmosphere to explain because it is the coolest of the four planets and is thus the largest outlier in the color–magnitude diagrams shown in Figure 5. Additionally, the HR 8799 b atmosphere is the most constrained of the four planets due to the H and K spectroscopy of Barman et al. (2011a). Our new 3.3 μm photometry of HR 8799 b is significantly brighter (by ∼100%) than the previously published value by Currie et al. (2011). As described in Section 3, the Currie et al. (2011) 3.3 μm photometry is likely erroneous, due to non-photometric conditions and/ory overly optimistic error bars. There is the possibility that HR 8799 b is extremely variable at 3.3 μm, but we consider that scenario unlikely due to the many times HR 8799 has been observed at other wavelengths where there has been no evidence of such large variability.

Before proceeding with our modeling, we examine Barman et al.'s (2011a) recent hypothesis regarding the SED of HR 8799 b. Barman et al. (2011a) were able to reproduce all existing photometry and spectroscopy of HR 8799 b by using a non-equilibrium CO↔CH₄ cloudy atmosphere with T_eff = 1100 K and log (g) = 3.5. However, their model is inconsistent with interior evolutionary models, which predict a larger object radius. Barman et al. (2011a) also present model atmospheres that obey the predictions of the evolutionary tracks, with a best-fit model that has T = 896 K and Z = 1.0 metallicity. This model demonstrates that it might be possible to explain HR 8799 b's appearance with a combination of clouds, non-equilibrium chemistry, and higher than solar metallicity. However, the model does not fit all of the existing photometry, and our new 3.3 μm photometry makes the fit significantly worse.

5.1.1. Non-equilibrium CO↔CH₄ Chemistry

We start our modeling by using the parameterized thick-cloud atmospheres of Madhusudhan et al. (2011) and adjusting the CO and CH₄ mixing ratios. These models assume evolutionary track radii but make no attempt to self-consistently explain the CO and CH₄ mixing ratios, which Barman et al. (2011a) produce with turbulent mixing. In Figure 6, we plot the best-fit chemical equilibrium model of HR 8799 b from Madhusudhan et al. (2011), as well as two models that have suppressed CH₄ and enhanced CO with respect to the equilibrium models (by factors of 10 and 100, respectively). Suppressing CH₄ and enhancing CO has a negligible effect on the near-infrared (zJHK) photometry, but it greatly affects the HK spectroscopy, favoring the 100×CO, 0.01×CH₄ model. Non-equilibrium chemistry dramatically affects the 3–5 μm SED, where a lack of CH₄ makes the object brighter in the 3.3 μm and L' filters and excess CO makes the object fainter in the M-band filter. The 100×CO, 0.01×CH₄ model fits the 3.3 μm photometry and comes closest to fitting the M-band photometry (further increasing the CO mixing ratio would improve this fit). However, the lack of CH₄ strongly increases the predicted flux in the L' filter, making it incompatible with HR 8799 b's observed L' photometry. Based on this analysis, it seems unlikely that a cool (850 K) atmosphere with non-equilibrium chemistry could explain HR 8799 b's 3.3 μm−L' color. However, there remains the possibility that more complex radial profiles of non-equilibrium chemistry and clouds could explain all of the data.

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5.1.2. Mixed-cloud Atmospheres

Based on Figure 5, HR 8799 b has colors reflective of a ∼1300 K atmosphere, but a luminosity consistent with an ∼850 K atmosphere. In lieu of resolving the difference by assuming a small (unphysical) radius, we consider the possibility that the planet emits non-isotropically, with bright and dark sections, such that the bright sections dominate the shape of the SED. Bright and dark regions have been observed on Jupiter in the mid-infrared (Westphal 1969), and several studies seeking to explain the L→T transition have used hybrid cloudy/cloud-free models (e.g., Burgasser et al. 2002). For our purposes, mixtures of standard cloudy and cloud-free models are unlikely to explain the appearance of HR 8799 b because its H − K color is redder than the L-dwarf sequence, implying that it is more cloudy, not less cloudy, than the other L dwarfs.

An alternative is that HR 8799 b has a mixture of clouds, such that the whole planet is cloudy, but with regions that have thicker clouds where the planet appears darker. A coarse approximation of this phenomenon is to linearly combine cloudy models of different effective temperatures (analogously to how Burgasser et al. 2002 and others have linearly combined cloudy and cloud-free models of different effective temperatures). This method is somewhat non-physical due to the fact that the two atmospheres have different temperature–pressure profiles (Marley et al. 2010). Linearly combining models with a shared temperature–pressure profile but different cloud structures would be more correct but is beyond the scope of this paper. In Figure 7, we present an example of a hybrid atmosphere that is 93% T_eff = 700 K, "A"-type cloudy and 7% T_eff = 1400 K, "AE"-type cloudy (A and AE cloud profiles are described in Madhusudhan et al. (2011), and further model details are presented in Burrows et al. 2006). The hybrid model adequately fits all of the photometry except for M band, which can be explained with further increased CO absorption. The HK model spectrum is muted compared to the data, but its bulk shape is generally correct (i.e., it does not show strong CH₄ absorption, as would be expected for a cool object). We note that this model is meant to be representative, but that a true mixed-cloud atmosphere should be calculated self-consistently.

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5.2. HR 8799 c, d, and e

We repeat our analysis of HR 8799 b (Section 5.1) for HR 8799 c and d, again starting with the best-fit, thick-cloud models from Madhusudhan et al. (2011). A comparison of non-equilibrium CO↔CH₄ models is shown in Figure 8, and example mixed-cloud atmospheres are shown in Figure 9. Our conclusions for HR 8799 c and d are similar to our conclusions for HR 8799 b: we are unable to fit the 3.3 μm−L' colors of HR 8799 c and d with cloudy/non-equilibrium chemistry models, and mixed-cloud atmospheres do a reasonable job fitting all of the data. We purposely construct the mixed-cloud atmospheres from the same two model atmospheres used to make the mixture for HR 8799 b (see Section 5.1.2). In this scenario, we find that HR 8799 c and d have higher fractions of the warm atmosphere than HR 8799 b, explaining their higher luminosity.

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HR 8799 e has less data than the outer three planets, but the existing data are consistent with the photometry of HR 8799 c and d. With the addition of our new H-band and 3.3 μm photometry, HR 8799 e has now been studied at four wavelengths, and we can proceed with atmospheric modeling, based on lessons learned from HR 8799 b, c, and d. We begin by using the thick-cloud models from Madhusudhan et al. (2011) to look for a good fit of the H, K, and L' photometry. We find that a 1000 K, log(g) = 4.0 model fits well (shown in Figure 10). Based on the small number of data points, degeneracies between different cloud properties, surface gravity, and effective temperature are not explored, but it is reasonable to assume that the cloud properties and surface gravity will be similar to the other HR 8799 planets. Figures 11 and 12 show non-equilibrium chemistry models and mixed-cloud models. As was true for the outer three planets, we are unable to fit the available photometry with non-equilibrium chemistry models, and a mixed-cloud model can be made to fit.

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5.3. The HR 8799 Planets in Aggregate

We have directly imaged the HR 8799 planetary system, detecting all four planets at H band and 3.3 μm with the LBT's FLAO system. The images are of unprecedented quality, allowing us to rule out the presence of a massive (HR 8799 cde-like) planet exterior to HR 8799 e's 2:1 inner resonance (H band 5σ contrast of 11.6 mag at 0235). We detect HR 8799 e at H band, for the first time, and find that it is approximately as bright as HR 8799 c and d. Combined with K_s and L' data (Marois et al. 2010) and our new 3.3 μm data, this indicates that HR 8799 e has similar atmospheric properties to HR 8799 c and d. The planets are all brighter than expected at 3.3 μm, where equilibrium chemistry models predict that CH₄ opacity should make the planets faint.

We model the HR 8799 planets with thick-cloudy atmospheres (Madhusudhan et al. 2011) and allow the CO and CH₄ mixing ratios to vary arbitrarily. The models that fit our 3.3 μm data (which have very little CH₄) substantially overpredict the planets' fluxes at L' (3.8 μm). Hotter atmospheres (>1300 K) have a similar 3.3 μm–3.8 μm color as the HR 8799 planets, and the shape of the Barman et al. (2011a) HK spectrum is also well fit by an atmosphere that is significantly hotter than indicated by the HR 8799 planets' bolometric magnitudes. As a result, we consider the possibility that small sections of the planets' atmospheres are hot (>1300 K), dominating the shape of the SEDs, while the majority of the planets' atmospheres are cooler and do not produce much flux. The temperature–pressure profile must be the same between the "hot" and "cool" regions, so the physical difference would be that the "cool" regions have increased cloud opacity. Since the SED is consistent with a cloudy atmosphere, the "hot" regions must also be cloudy, so the combined atmosphere is composed of mixed clouds, some of which are thicker than others. Our mixed-cloud models are able to fit all of the HR 8799 data. However, we caution that our models are not fully self-consistent and that more theoretical work is necessary to validate our hypothesis.

The HR 8799 planets have unusual SEDs that are not well fit by the same models that have been used to fit field brown dwarfs. From an observational standpoint, HK spectroscopy and 3.3 μm−L' colors have been particularly powerful in ruling out model atmospheres. HK spectroscopy of HR 8799 cde, and other directly imaged planets in general, is critical to our understanding of clouds and non-equilibrium chemistry. We also note that, given the wide range of 3–4 μm SEDs predicted by the models in this paper, it would be very useful to obtain low-resolution spectroscopy of the HR 8799 planets in this range. From a theoretical standpoint, our new 3.3 μm photometry is a challenge even for non-equilibrium chemistry models, which predict bright 3.3 μm photometry. Mixed-cloud models are one possible way to flatten out 3.3 μm–3.8 μm photometry and hide CH₄ opacity.

The authors thank Piero Salinari for his insight, leadership, and persistence, which made the development of the LBT adaptive secondaries possible. We also thank the many dedicated individuals who have worked on LBTI, LMIRCam, PISCES, and the AO system over the years. The authors thank the anonymous referee for his/her helpful report. The Large Binocular Telescope Interferometer is funded by the National Aeronautics and Space Administration as part of its Exoplanet Exploration program. LMIRCam is funded by the National Science Foundation through grant NSF AST-0705296.