Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b

M. Gillon (1 , 2) , B.-O. Demory (1) , T. Barman (3) , X. Bonfils (4) , T. Mazeh (5) , F. Pont (1) , S. Udry (1)

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M. Mayor (1) D. Queloz (1) ((1) Observatoire de l'Universite de Geneve Sauverny Switzerland (2) Institut d'astrophysique et de geophysique Universite de Liege Liege Belgium (3) Lowell Observatory Flagstaff USA (4) Observatorio Astronomico de Lisboa Lisboa Portugal (5) School of Physics Astronomy Raymond Beverly Sackler Faculty of Exact Sciences Tel Aviv University Tel Aviv Israel)

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classification 🌌 astro-ph

keywords radiusinfraredneptunetransitassumingbandiracmicrons

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We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 microns band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 microns IRAC band allow us to derive (assuming M = 0.44 +- 0.04 Msun for the primary) a precise value for the planetary radius (4.19 +0.21-0.16 Rearth), the stellar radius (0.463 +0.022-0.017 Rsun), the orbital inclination (85.90 +0.19-0.18 degrees) and transit timing (2454280.78186 +0.00015-0.00008 HJD). Assuming current planet models, an internal structure similar to that of Neptune with a small H/He envelope is necessary to account for the measured radius of GJ 436b.

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Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b

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