A Combined Subaru/VLT/MMT 1--5 Micron Study of Planets Orbiting HR 8799: Implications for Atmospheric Properties, Masses, and Formation

Abstract: We present new 1--1.25 micron (z and J band) Subaru/IRCS and 2 micron (K band) VLT/NaCo data for HR 8799 and a rereduction of the 3--5 micron MMT/Clio data first presented by Hinz et al. (2010). Our VLT/NaCo data yields a detection of a fourth planet at a projected separation of ~ 15 AU -- "HR 8799e". We also report new, albeit weak detections of HR 8799b at 1.03 microns and 3.3 microns. Empirical comparisons to field brown dwarfs show that at least HR 8799b and HR8799c, and possibly HR 8799d, have near-to-mid IR colors/magnitudes significantly discrepant from the L/T dwarf sequence. Standard cloud deck atmosphere models appropriate for brown dwarfs provide only (marginally) statistically meaningful fits to HR 8799b and c for unphysically small radii. Models with thicker cloud layers not present in brown dwarfs reproduce the planets' SEDs far more accurately and without the need for rescaling the planets' radii. Our preliminary modeling suggests that HR 8799b has log(g) = 4--4.5, Teff = 900K, while HR 8799c, d, and (by inference) e have log(g) = 4--4.5, Teff = 1000--1200K. Combining results from planet evolution models and new dynamical stability limits implies that the masses of HR 8799b, c, d, and e are 6--7 Mj, 7--10 Mj, 7--10 Mj and 7--10 Mj. 'Patchy" cloud prescriptions may provide even better fits to the data and may lower the estimated surface gravities and masses. Finally, contrary to some recent claims, forming the HR 8799 planets by core accretion is still plausible, although such systems are likely rare.

A Study of HR 8799 Exoplanets: Atmospheric Properties, Mass Estimates, and Dynamic Stability

The paper "A Combined Subaru/VLT/MMT 1–5 µm Study of Planets Orbiting HR 8799" presents a comprehensive analysis of the atmospheric properties, masses, and potential formation mechanisms of the exoplanets orbiting HR 8799, the first directly imaged multiplanetary system. Using J, H, K$_s$, [3.3], and L' bands photometric data from Subaru, VLT, and MMT, the study provides new detections and re-evaluations of HR 8799’s planetary companions b, c, and d, and reports on the independent detection of a fourth planet, HR 8799e.

Atmospheric Modeling and Analysis

The paper investigates the spectral energy distributions (SEDs) of HR 8799’s planets using atmospheric models of varying cloud properties. Comparison with field brown dwarfs indicates that HR 8799b, c, and possibly d do not conform to the traditional L/T dwarf sequences, especially at Y and J-band colors. This discrepancy suggests a fundamental difference in atmospheric properties, highlighting the potential for thicker clouds compared to brown dwarfs.

Atmospheric model fitting demonstrates that standard cloud deck models inadequately represent the HR 8799 planets' atmospheres, requiring unphysically small radii for accurate fits. Instead, models with a thicker cloud layer, such as the Model A thick cloud prescription, provide significantly better fits without altering the planets' radii. These models suggest log(g) values of 4–4.5 and effective temperatures (T$_{eff}$) of 900–1200K.

Mass Estimates and Dynamic Stability

Combining results from atmospheric models and dynamical simulations, the paper estimates the masses of HR 8799b, c, d, and e to be 6–7 M$_J$, 7–10 M$_J$, 7–10 M$_J$, and 7–10 M$_J$ respectively. These estimates align with constraints provided by dynamical stability, confirming that the masses are below the deuterium-burning limit. Notably, these masses imply that HR 8799's planets bridge the gap between solar system gas giants and low-mass brown dwarf companions.

Implications on Planet Formation

The study critically evaluates the planet formation mechanisms for HR 8799's planets, challenging claims that their formation via core accretion is implausible. The identified mass and age range does not preclude core accretion, though these systems likely represent rare cases due to the specific conditions required for massive gas giants to form at significant separations from their host stars. The paper suggests that detailed simulations accounting for planet-planet scattering in the presence of gas could support the core accretion scenario more convincingly.

Future Developments

This study emphasizes the need for refined atmospheric models, including intermediate cloud models, to better interpret the physical characteristics of planetary-mass objects. Further, it encourages observations targeting key wavelengths like 2.3 µm and 3.0 µm, to challenge and refine these models. The upcoming significant exoplanet observational surveys, such as those by GPI and SPHERE, will provide more data to enrich our understanding of wide-orbit exoplanets like those in the HR 8799 system.

Overall, this research underscores the diversity of exoplanetary atmospheres and raises vital questions on the compositional and formation dichotomy between wide-orbit gas giants and brown dwarfs, propelling further theoretical and observational studies into the nascent field of directly imaged exoplanets.