Roche lobe effects on the atmospheric loss from “Hot Jupiters”

¹ Institute for Computational Modelling, Russian Academy of Sciences, and Siberian Federal University, Krasnoyarsk 36, Russian Federation e-mail: erkaev@icm.krasn.ru
² Polar Geophysical Institute, Russian Academy of Sciences, Khalturina 15, 183010 Murmansk, Russian Federation e-mail: kulikov@pgi.ru
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria e-mail: [helfried.biernat; helmut.lammer; daniel.langmayr]@oeaw.ac.at
⁴ Université de Lyon, 69003 Lyon , France; École Normale Supérieure de Lyon, 46 allée d'Italie, 69007 Lyon, France; CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon; Université Lyon 1, 69622 Villeurbanne, France e-mail: franck.selsis@ens-lyon.fr
⁵ Institute for Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: gerald.jaritz@stud.uni-graz.at

Received: 13 December 2006
Accepted: 7 June 2007

Abstract

Context.A study of the mass loss enhancement for very close “Hot Jupiters” due to the gravitational field of the host star is presented.

Aims.The influence of the proximity to a planet of the Roche lobe boundary on the critical temperature for blow-off conditions for estimating the increase of the mass loss rate through hydrodynamic blow-off for close-in exoplanets is investigated.

Methods.We consider the gravitational potential for a star and a planet along the line that joins their mass centers and the energy balance equation for an evaporating planetary atmosphere including the effect of the stellar tidal force on atmospheric escape.

Results.By studying the effect of the Roche lobe on the atmospheric loss from short-periodic gas giants we derived reasonably accurate approximate formulas to estimate atmospheric loss enhancement due to the action of tidal forces on a “Hot Jupiter” and to calculate the critical temperature for the onset of “geometrical blow-off”, which are valid for any physical values of the Roche lobe radial distance. Using these formulas, we found that the stellar tidal forces can enhance the hydrodynamic evaporation rate from TreS-1 and OGLE-TR-56b by about 2 fold, while for HD 209458b we found an enhancement of about 50%. For similar exoplanets which are closer to their host star than OGLE-TR-56b, the mass loss enhancement can be even larger. Moreover, we showed that the effect of the Roche lobe allows “Hot Jupiters” to reach blow-off conditions at temperatures which are less than expected due to the stellar X-ray and EUV heating.