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Studying the atmosphere of the exoplanet HAT-P-7b via secondary eclipse measurements with EPOXI, Spitzer and Kepler
The highly irradiated transiting exoplanet, HAT-P-7b, currently provides one
of the best opportunities for studying planetary emission in the optical and
infrared wavelengths. We observe six near-consecutive secondary eclipses of
HAT-P-7b at optical wavelengths with the EPOXI spacecraft. We place an upper
limit on the relative eclipse depth of 0.055% (95% confidence). We also analyze
Spitzer observations of the same target in the infrared, obtaining secondary
eclipse depths of 0.098+/-0.017%, 0.159+/-0.022%, 0.245+/-0.031% and
0.225+/-0.052% in the 3.6, 4.5, 5.8 and 8.0 micron IRAC bands respectively. We
combine these measurements with the recently published Kepler secondary eclipse
measurement, and generate atmospheric models for the day-side of the planet
that are consistent with both the optical and infrared measurements. The data
are best fit by models with a temperature inversion, as expected from the high
incident flux. The models predict a low optical albedo of ~< 0.13, with
subsolar abundances of Na, K, TiO and VO. We also find that the best fitting
models predict that 10% of the absorbed stellar flux is redistributed to the
night side of the planet, which is qualitatively consistent with the
inefficient day-night redistribution apparent in the Kepler phase curve. Models
without thermal inversions fit the data only at the 1.25 sigma level, and also
require an overabundance of methane, which is not expected in the very hot
atmosphere of HAT-P-7b. We also analyze the eight transits of HAT-P-7b present
in the EPOXI dataset and improve the constraints on the system parameters,
finding a period of P = 2.2047308+/-0.0000025 days, a stellar radius of R* =
1.824+/-0.089Rsun, a planetary radius of Rp = 1.342+/-0.068RJup and an
inclination of i = 85.7+3.5-2.2 deg.Comment: 21 pages, 8 figures, accepted by the Astrophysical Journa