Letter to the Editor

The effect of evaporation on the evolution of close-in giant planets

¹ CRAL (UMR 5574 CNRS), École Normale Supérieure, 69364 Lyon Cedex 07, France e-mail: [ibaraffe; chabrier; fallard]@ens-lyon.fr
² Centro de Astrobiología (INTA-CSIC), Ctra. de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: selsis@obs.u-bordeaux1.fr
³ Department of Physics, Wichita State University, Wichita, KS 67260-0032, USA e-mail: travis.barman@wichita.edu
⁴ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: yeti@hs.uni-hamburg.de
⁵ Space Research Institute, Austrian Academy of Sciences, Schmieldstrasse 6, 8042 Graz, Austria e-mail: helmut.lammer@oeaw.ac.at

Corresponding author: I. Baraffe, ibaraffe@ens-lyon.fr

Received: 16 January 2004
Accepted: 4 April 2004

Abstract

We include the effect of evaporation in our evolutionary calculations of close-in giant planets, based on a recent model for thermal evaporation taking into account the XUV flux of the parent star (Lammer et al. [CITE]). Our analysis leads to the existence of a critical mass for a given orbital distance  below which the evaporation timescale becomes shorter than the thermal timescale of the planet. For planets with initial masses below m_crit, evaporation leads to a rapid expansion of the outer layers and of the total planetary radius, speeding up the evaporation process. Consequently, the planet does not survive as long as estimated by a simple application of mass loss rates without following consistently its evolution. We find out that the transit planet HD 209458b might be in such a dramatic phase, although with an extremely small probability. As a consequence, we predict that, after a certain time, only planets above a value  should be present at an orbital distance a of a star. For planets with initial masses above m_crit, evaporation does not affect the evolution of the radius with time.