Searching for helium in the exosphere of HD 209458b^*

¹ Laboratoire d'Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France
² LESIA, Observatoire de Meudon, Pl. J. Janssen, 92195 Meudon Cedex, France
³ LUTH, Observatoire de Meudon, Pl. J. Janssen, 92195 Meudon Cedex, France
⁴ Observatoire de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland

Corresponding author: C. Moutou, Claire.Moutou@oamp.fr

Received: 16 December 2002
Accepted: 16 April 2003

Abstract

Atmospheric models of the extrasolar, close-in giant planet HD 209458b predict strong absorption features from alkali metals (Seager & Sasselov [CITE]; Brown 2001). This was confirmed by the discovery of NaI by HST observations (Charbonneau et al. [CITE]). In this study we focus on the search for the helium absorption feature at 10 830 Å, also predicted to be among the strongest ones. Helium is a major component of the planet's exosphere, for which models are yet not as robust as atmosphere models. One full transit was observed with the VLT/ISAAC instrument. We do not report the detection of the HeI feature. The data set is strongly affected by instrumental fringing, at a level up to 5% in the extracted spectra. After filtering, a residual noise of the order of 0.2% remains. An upper limit of the HeI line was derived, which further constrains future models of the HD 209458b planet exosphere. This upper limit, in terms of the feature depth, is 0.5% at 3σ for a 3 Å bandwidth. Prospects are proposed to lower the detectability limit; the ultimate detectability limit with ISAAC in the absence of electronic fringing and in ideal atmospheric conditions could be as low as a line depth of 0.1% (3 Å width, 3σ).