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Issue A&A
Volume 471, Number 3, September I 2007
Page(s) L51 - L54
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078283



A&A 471, L51-L54 (2007)
DOI: 10.1051/0004-6361:20078283

Letter

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

M. Gillon1, 2, B.-O. Demory1, T. Barman3, X. Bonfils4, T. Mazeh5, F. Pont1, S. Udry1, M. Mayor1, and D. Queloz1

1  Observatoire de l'Université de Genève, 1290 Sauverny, Switzerland
    e-mail: michael.gillon@obs.unige.ch
2  Institut d'Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium
3  Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
4  Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
5  School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel

(Received 16 July 2007 / Accepted 21 July 2007)

Abstract
We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 $\mu$m band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 $\mu$m IRAC band allow us to derive (assuming M = 0.44 $\pm$ 0.04 $M_\odot$ for the primary) a precise value for the planetary radius (4.19 +0.21-0.16 $R_\oplus$), the stellar radius (0.463 +0.022-0.017 $R_\odot$), the orbital inclination (85.90° $^{+0.19^\circ}_{-0.18^\circ}$) 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.


Key words: techniques: photometric -- eclipses -- stars: individual: GJ 436 -- planetary systems -- infrared: general



© ESO 2007

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