Context: Atomic hydrogen escaping from the planet
Aims: Here, we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets.
Methods: Eight HST orbits were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail.
Results: Transit depths of (9.6±7.0)% and (5.3±10.0)% were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0±5.7)%, in good agreement with previous studies.
Conclusions: The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate.
Astronomy and Astrophysics
- Pub Date:
- June 2008
- stars: individual: HD 209458;
- planets and satellites: general;
- ultraviolet: stars;
- Accepted for publication in Astronomy &