Characterization of the hot Neptune GJ 436 b with Spitzer and ground-based observations^*

¹ Observatoire de Genève, Université de Genève, 1290 Sauverny, Switzerland e-mail: brice-olivier.demory@obs.unige.ch
² Observatoire François-Xavier Bagnoud – OFXB, 3961 Saint-Luc, Switzerland
³ Institut d'Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium
⁴ Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
⁵ Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁶ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
⁷ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bd. Arago, 75014 Paris, France
⁸ Laboratoire d'Astrophysique de Grenoble, Observatoire de Grenoble, UMR5571 de l'Université J.Fourier et du CNRS, BP 53, 38041 Grenoble, France

Received: 25 July 2007
Accepted: 11 September 2007

Abstract

We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ 436 b. The observations were obtained using the 8-μm band of the InfraRed Array Camera (IRAC). The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration. The observed eclipse depth of 0.58 mmag allows us to estimate a blackbody brightness temperature of T_p = 717 ± 35 K at 8 μm. We compare this infrared flux measurement to a model of the planetary thermal emission, and show that this model reproduces properly the observed flux decrement. The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet, while also increasing its precision (e = 0.14 ± 0.01). Additional new spectroscopic and photometric observations allow us to estimate the rotational period of the star and to assess the potential presence of another planet.