Transit Spectroscopy of Extrasolar Planet HD209458b: The Radiative Transfer Model

Details Transit Spectroscopy of Extrasolar Planet HD209458b: The Radiative Transfer Model Transit Spectroscopy of Extrasolar Planet HD209458b: The Radiative Transfer Model

Nov 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004dps....36.3501r&link_type=abstract

American Astronomical Society, DPS meeting #36, #35.01; Bulletin of the American Astronomical Society, Vol. 36, p.1151

Astronomy and Astrophysics

Astronomy

Scientific paper

We have developed a new code that calculates the modulation of a star's spectrum as a planet transits. We are applying this model to data from the VLT, Palomar, Keck, and IRTF to search for water on HD209458b, the transiting planet with the brightest primary.
Observations of HD209458b's stellar spectrum modulation have yielded the first detections of exoplanetary sodium (Charbonneau et al. 2001), hydrogen, oxygen and carbon (Vidal-Madjar et al. 2003, 2004). Molecules, however, have still avoided detection. Water is predicted to be abundant at all plausible temperatures, but the modulation for most of the observable features is <0.04%. By simultaneously fitting for many excited water features while avoiding telluric water lines, we can significantly increase our signal.
Our model predicts the modulation given line data, system geometry, and thermal and abundance profiles for any transiting planet. We will use this code to compare the observed modulation for HD209458b with that predicted by different planetary theories, do calculations for specific instruments with different resolutions and wavelength ranges, and constrain the abundances of detected species.
We find that integrating the extinction over altitude produces significantly better results than assuming that the planet is an opaque disk whose radius is the altitude of optical depth unity. The latter is a widely used simplification. Our work will allow us to establish or place strong limits on the water abundance in HD209458b's atmosphere. Even a non-detection will be important, as it will require significant modifications to existing theory and/or will justify the need for better space-based instruments.
This work was supported by NASA grant NAG5-13154.

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