EDP Sciences
Free access
Volume 405, Number 1, July I 2003
Page(s) 341 - 348
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20030656

A&A 405, 341-348 (2003)
DOI: 10.1051/0004-6361:20030656

Searching for helium in the exosphere of HD 209458b

C. Moutou1, A. Coustenis2, J. Schneider3, D. Queloz4 and M. Mayor4

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

(Received 16 December 2002 / Accepted 16 April 2003)

Atmospheric models of the extrasolar, close-in giant planet HD 209458b predict strong absorption features from alkali metals (Seager & Sasselov 2000; Brown 2001). This was confirmed by the discovery of NaI by HST observations (Charbonneau et al. 2002). 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 3 $\sigma$ 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, 3 $\sigma$).

Key words: stars: planetary systems -- infrared: stars -- planets and satellites: general

Offprint request: C. Moutou, Claire.Moutou@oamp.fr

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