A COMBINED SUBARU/VLT/MMT 1-5 μm 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\nband) VLT/NaCo data for HR 8799 and a rereduction of the 3--5 micron MMT/Clio\ndata first presented by Hinz et al. (2010). Our VLT/NaCo data yields a\ndetection of a fourth planet at a projected separation of ~ 15 AU -- "HR\n8799e". We also report new, albeit weak detections of HR 8799b at 1.03 microns\nand 3.3 microns. Empirical comparisons to field brown dwarfs show that at least\nHR 8799b and HR8799c, and possibly HR 8799d, have near-to-mid IR\ncolors/magnitudes significantly discrepant from the L/T dwarf sequence.\nStandard cloud deck atmosphere models appropriate for brown dwarfs provide only\n(marginally) statistically meaningful fits to HR 8799b and c for unphysically\nsmall radii. Models with thicker cloud layers not present in brown dwarfs\nreproduce the planets' SEDs far more accurately and without the need for\nrescaling the planets' radii. Our preliminary modeling suggests that HR 8799b\nhas log(g) = 4--4.5, Teff = 900K, while HR 8799c, d, and (by inference) e have\nlog(g) = 4--4.5, Teff = 1000--1200K. Combining results from planet evolution\nmodels and new dynamical stability limits implies that the masses of HR 8799b,\nc, d, and e are 6--7 Mj, 7--10 Mj, 7--10 Mj and 7--10 Mj. 'Patchy" cloud\nprescriptions may provide even better fits to the data and may lower the\nestimated surface gravities and masses. Finally, contrary to some recent\nclaims, forming the HR 8799 planets by core accretion is still plausible,\nalthough such systems are likely rare.\n