Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b

¹ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
² Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, Université de Franche-Comté, Besançon, France e-mail: olivier.mousis@obs-besancon.fr
³ The HOLMES collaboration
⁴ Department of Physics and Astronomy, University College London, London, UK
⁵ Institut d'astrophysique de Paris, CNRS-UMR 7095, Université Pierre & Marie Curie, France

Received: 21 August 2009
Accepted: 23 September 2009

Abstract

Context. Oxygen (O) and carbon (C) have been inferred recently to be subsolar in abundance from spectra of the atmosphere of the transiting hot Jupiter HD 189733b. Yet, the mass and radius of the planet coupled with structure models indicate a strongly supersolar abundance of heavy elements in the interior of this object.

Aims. Here we explore the discrepancy between the large amount of heavy elements suspected in the planet's interior and the paucity of volatiles measured in its atmosphere.

Methods. We describe the formation sequence of the icy planetesimals formed beyond the snow line of the protoplanetary disk and calculate the composition of ices ultimately accreted in the envelope of HD 189733b on its migration pathway. This allows us to reproduce the observed volatile abundances by adjusting the mass of ices vaporized in the envelope.

Results. The predicted elemental mixing ratios should be 0.15–0.3 times solar in the envelope of HD 189733b if they are fitted to the recent O and C determinations. However, our fit to the minimum mass of heavy elements predicted by internal structure models gives elemental abundances that are 1.2–2.4 times oversolar in the envelope of HD 189733b.

Conclusions. We propose that the most likely cause of this discrepancy is irradiation from the central star leading to development of a radiative zone in the planet's outer envelope which would induce gravitational settling of elements. Hence, all strongly irradiated extrasolar planets should present subsolar abundances of volatiles. We finally predict that the abundances of nitrogen (N), sulfur (S) and phosphorus (P) are of ~2.810^-5, 5.310^-6 and 1.810^-7 relative to H₂, respectively in the outer part of HD 189733b's atmosphere.