A COMBINED VERY LARGE TELESCOPE AND GEMINI STUDY OF THE ATMOSPHERE OF THE DIRECTLY IMAGED PLANET, β PICTORIS b

Abstract

We analyze new/archival VLT/NaCo and Gemini/NICI high-contrast imaging of the\nyoung, self-luminous planet $\\beta$ Pictoris b in seven near-to-mid IR\nphotometric filters, using advanced image processing methods to achieve high\nsignal-to-noise, high precision measurements. While $\\beta$ Pic b's near-IR\ncolors mimick that of a standard, cloudy early-to-mid L dwarf, it is\noverluminous in the mid-infrared compared to the field L/T dwarf sequence. Few\nsubstellar/planet-mass objects -- i.e. $\\kappa$ And b and 1RXJ 1609B -- match\n$\\beta$ Pic b's $JHK_{s}L^\\prime$ photometry, and its 3.1 $\\mu m$ and 5 $\\mu m$\nphotometry are particularly difficult to reproduce. Atmosphere models adopting\ncloud prescriptions and large ($\\sim$ 60 $\\mu m$) dust grains fail to reproduce\nthe $\\beta$ Pic b spectrum. However, models incorporating thick clouds similar\nto those found for HR 8799 bcde but also with small (a few microns) modal\nparticle sizes yield fits consistent with the data within uncertainties.\nAssuming solar abundance models, thick clouds, and small dust particles ($<a>$\n= 4 $\\mu m$) we derive atmosphere parameters of log(g) = 3.8 $\\pm$ 0.2 and\n$T_{eff}$ = 1575--1650 $K$, an inferred mass of 7$^{+4}_{-3}$ $M_{J}$, and a\nluminosity of log(L/L$_{\\odot}$) $\\sim$ -3.80 $\\pm$ 0.02. The best-estimated\nplanet radius, $\\approx$ 1.65 $\\pm$ 0.06 $R_{J}$, is near the upper end of\nallowable planet radii for hot-start models given the host star's age and\nlikely reflects challenges with constructing accurate atmospheric models.\nAlternatively, these radii are comfortably consistent with hot-start model\npredictions if $\\beta$ Pic b is younger than $\\approx$ 7 Myr, consistent with a\nlate formation, well after its host star's birth $\\sim$ 12$^{+8}_{-4}$ Myr ago.\n