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Application to the HR~8799 Planets Draft version May 13, 2011 Preprint typeset using LATEX style emulateapj v. 2/16/10 MODEL ATMOSPHERES FOR MASSIVE GAS GIANTS WITH THICK CLOUDS: APPLICATION TO THE HR 8799 PLANETS AND PREDICTIONS FOR FUTURE DETECTIONS Nikku Madhusudhan1, Adam Burrows1, & Thayne Currie2 Draft version May 13, 2011 ABSTRACT We have generated an extensive new suite of massive giant planet atmosphere models and used it to obtain fits to photometric data for the planets HR 8799b, c, and d. We consider a wide range of cloudy and cloud-free models. The cloudy models incorporate different geometrical and optical thicknesses, modal particle sizes, and metallicities. For each planet and set of cloud parameters, we explore grids in gravity and effective temperature, with which we determine constraints on the planet's mass and age. Our new models yield statistically significant fits to the data, and conclusively confirm that the HR 8799 planets have much thicker clouds than those required to explain data for typical L and T dwarfs. Both models with 1) physically thick forsterite clouds and a 60-µm modal particle size and 2) clouds made of 1 µm-sized pure iron droplets and 1% supersaturation fit the data. Current data are insufficient to accurately constrain the microscopic cloud properties, such as composition and particle size. The range of best-estimated masses for HR 8799b, HR 8799c, and HR 8799d conservatively span 2{12 MJ , 7{13 MJ , and 3{11 MJ , respectively and imply coeval ages between ∼20 and ∼150 Myr, consistent with previously reported stellar age. The best-fit temperatures and gravities are slightly lower than values obtained by Currie et al. (2011) using even thicker cloud models. Finally, we use these models to predict the near-to-mid IR colors of soon-to-be imaged planets. Our models predict that planet-mass objects follow a locus in some near-to-mid IR color-magnitude diagrams that is clearly separable from the standard L/T dwarf locus for field brown dwarfs. Subject headings: planetary systems | planets and satellites: general | planets and satellites: indi- vidual (HR 8799b, HR 8799c, HR 8799d) 1. INTRODUCTION ever, transit surveys for planet detection are sensitive to Advances in high-contrast imaging have made it pos- small orbital separations much less than an astronomical sible to image young giant planets at wide orbital sep- unit (AU). Consequently, spectra of giant planets at sep- arations around host stars. Major efforts are currently arations analogous to those in the solar system (a ≥ 5 underway targeting progressively higher contrast levels, AU) can be observed only with high-contrast imaging. e.g., GPI (Macintosh et al. 2006), SPHERE (Beuzit et al. Since currently achievable sensitivities can resolve only 2008), Project 1640 (Hinkley et al. 2008,2011), SEEDS systems that are hot, hence likely young, the planets are (Tamura 2009), and NICI (Liu et al. 2010). Current also likely formed in situ at the observed separations, and instruments are capable of detecting contrast ratios in provide valuable probes for models of planet formation. the near infrared down to ∼ 10−5 (Marois et al. 2010), Recent photometric observations of directly imaged leading to a number of planet detections in recent years. planets are increasingly highlighting the need for a new The initial discovery of a giant planet around the low- class of atmospheric models to explain the data. Inter- mass cool dwarf 2M1207 (Chauvin et al. 2004 ) was fol- pretations of current observations are based on tradi- lowed by more recent discoveries of giant planets around tional models that were used to interpret atmospheric massive stellar hosts HR 8799 (Marois et al. 2008,2010), spectra of brown dwarfs (Burrows et al. 2001, 2006; Formalhaut (Kalas et al. 2008), and β Pic (Lagrange Baraffe et al. 2003; Saumon & Marley, 2008). While et al. 2009). Such detections provide the exciting op- giant planets at large orbital distances, and, hence, with portunity of obtaining planetary spectra (Marois et al. low stellar irradiation, could have been expected to have 2008; Lafreniere et al. 2009; Hinz et al. 2010; Janson had similar spectral characteristics to low temperature et al. 2010; Bowler et al. 2010) and characterizing their brown dwarfs, recent observations are proving otherwise atmospheres, in particular their temperature structures, (Marois et al. 2008 & 2010, Bowler et al. 2010, Currie compositions, and gravities. Furthermore, the planetary et al. 2011). radii, masses, and ages can be derived from the atmo- The HR 8799 system (Marois et al. 2008,2010) with spheric parameters (Burrows et al. 2001; Burrows, 2005). four planetary companions at wide orbital separations is Extrasolar planets detected with high-contrast imaging a case in point, epitomizing the difficulty in modeling di- occupy a unique region in orbital phase space. Planetary rectly imaged planets. In the discovery paper, Marois et spectra can also be observed for transiting planets. How- al. (2008) reported six-channel near-infrared photome- try of HR 8799 b, c, and d, and derived effective tem- 1 Department of Astrophysical Sciences, Princeton Uni- peratures (Teff ), masses (M), and radii (Rp) from the versity, Princeton, NJ 08544; [email protected], observed luminosities and an assumed common age of [email protected] the objects, using already published theoretical cooling 2 NASA Goddard Space Flight Center,Greenbelt, Maryland tracks (Baraffe et al. 2003). On the other hand, deriving 20771; [email protected] 2 the atmospheric parameters from spectral fits to their tween the various model parameters. As anticipated in photometry, instead of cooling tracks, yielded unrealisti- recent literature (Marois et al. 2008, Bowler et al. 2010, cally small radii (see Supplementary Online Material of Currie et al. 2011), the observations of the HR 8799 plan- Marois et al. 2008). This discrepancy was attributed to ets indicate a potentially new class of objects with cloud missing atmospheric physics, including inadequate cloud properties unlike those of any field brown dwarfs known. modeling, suggesting the need for new spectral models. We do not consider the recently discovered fourth com- Several follow-up observations of the HR 8799 system panion HR 8799e (Marois et al. 2010), for which data are have also highlighted the need for new models. Bowler presently inadequate for a detailed atmospheric analysis. et al. (2010) reported spectroscopy of HR 8799b in the We report constraints on the atmospheric properties, 2.12 - 2.23 µm range. While existing models were un- masses, and ages of HR 8799b, c, and d. We use existing able to explain all their observations, the better fits were observations of near-infrared photometry for these ob- obtained with models with thick clouds. Furthermore, jects compiled in Currie et al. (2011). Our best-fit mod- the Teff and Rp they derived from the best fitting spec- els allow us to constrain Teff , log10(g), mass (M), and age tral model were inconsistent with those derived based on (t) for each object. The range of estimated masses for evolutionary models. Hinz et al. (2010) reported pho- HR 8799b, HR 8799c, and HR 8799d conservatively span tometry at 3.3, 3.8, and 4.8 µm, and suggested that mod- 2{12 MJ , 7{13 MJ , and 3{11 MJ , respectively. The best els with non-equilibrium chemistry provided better fits fits for HR 8799b, c, and d yield Teff of ∼850 K, ∼1000 to their data, while noting the differences in fits based K, and ∼900 K, respectively, with log10(g) of 4.3, 4.2, on different model assumptions. Based on VLT spec- and 3.8, respectively. The corresponding estimates of troscopy in the 3.88-4.1 µm range (Janson et al. 2010), the masses are ∼12, ∼11, and ∼6 MJ, respectively, and similar conclusions were drawn for HR 8799c. of the ages are ∼150, ∼65, and ∼20 Myr, indicating that Currie et al. (2011) further explored the HR 8799 plan- they are indeed roughly coeval, if the adopted age of the ets' atmospheres with data from Marois et al.(2008), new star is ∼30-160 Myr. Subaru and VLT data, and re-reduced MMT data first The observations of all the objects can be explained presented in Hinz et al. (2010). Using the model formal- with models involving forsterite clouds, with physically ism of Burrows et al. (2006), they found that standard thick cloud cover, and 60-µm modal grain size. Models L/T dwarf atmosphere models over-predicted the flux without clouds cannot explain the observations for any between 1 and 1.25 µm. They found that \thick cloud" of the objects. The observations are not explained by models, where the cloud density tracks that of the gas canonical model spectra of L and T brown dwarfs (Bur- high in the atmosphere, reproduced the suppressed 1{ rows et al. 2006; Saumon & Marley 2008) at the same 1.25 micron flux and, thus, provided far improved fits Teff , indicating that the present objects might form a to the data. However, they suggested that these mod- new class with thicker (geometrically) clouds than those els may \overcorrect" for the weak cloud coverage in inferred for L and T dwarfs, as also suggested in Cur- the standard models, as their preliminary `partly cloudy' rie et al. (2011). The recent inference of clouds in the models fit the data better. They also acknowledged that late T Dwarf Ross 458C (Burgasser et al. 2010) suggests their grid sampling was too coarse to precisely deter- that this object may be related. While the requirement mine best-fit surface gravities and effective temperatures of clouds at altitude is stringent, the cloud composition from the data. Thus, further refinements of the \thick and particle sizes are less constrained by the data, as is cloud" formalism with a new, expansive, set of atmo- the gas-phase metallicity.
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