Rayleigh scattering in the transit spectrum of HD 189733b (original) (raw)

A&A 481, L83-L86 (2008)

Letter to the Editor

1 CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France e-mail: lecaveli@iap.fr
2 UPMC Univ. Paris 6, UMR 7095, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
3 Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 11 January 2008
Accepted: 18 February 2008

Abstract

The transit spectrum of the exoplanet HD 189733b has recently been obtained between 0.55 and 1.05 _μ_m. Here we present an analysis of this spectrum. We develop first-order equations to interpret absorption spectra. In the case of HD 189733b, we show that the observed slope of the absorption as a function of wavelength is characteristic of extinction proportional to the inverse of the fourth power of the wavelength ( $\propto$ $\lambda^{-4}$ ). Assuming an extinction dominated by Rayleigh scattering, we derive an atmospheric temperature of 1340 ± 150 K. If molecular hydrogen is responsible for the Rayleigh scattering, the atmospheric pressure at the planetary characteristic radius of 0.1564 stellar radius must be 410 ± 30 mbar. However the preferred scenario is scattering by condensate particles. Using the Mie approximation, we find that the particles must have a low value for the imaginary part of the refraction index. We identify MgSiO3 as a possible abundant condensate whose particle size must be between ~10-2 and ~10-1 _μ_m. For this condensate, assuming solar abundance, the pressure at 0.1564 stellar radius is found to be between a few microbars and few millibars, and the temperature is found to be in the range 1340–1540 K, and both depend on the particle size.

Key words: stars: planetary systems / scattering / techniques: spectroscopic / stars: individual: HD 189733b