Hot Jupiters and the evolution of stellar angular momentum

INAF – Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy e-mail: nuccio.lanza@oact.inaf.it

Received: 30 June 2009
Accepted: 17 December 2009

Abstract

Context. Giant planets orbiting main-sequence stars closer than 0.1 AU are called hot Jupiters. They interact with their stars affecting their angular momentum.

Aims. Recent observations provide evidence of excess angular momentum in stars with hot Jupiters in comparison to stars with distant and less massive planets. This has been attributed to tidal interaction, but needs to be investigated in more detail considering other possible explanations because in several cases the tidal synchronization timescales are much longer than the ages of the stars.

Methods. We select stars harbouring transiting hot Jupiters to study their rotation and find that those with an effective temperature T_eff 6000 K and a rotation period P_rot 10 days are synchronized with the orbital motion of their planets or have a rotation period approximately twice that of the planetary orbital period. Stars with T_eff 6000 K or P_rot 10 days show a general trend toward synchronization with increasing effective temperature or decreasing orbital period. We propose a model for the angular momentum evolution of stars with hot Jupiters to interpret these observations. It is based on the hypothesis that a close-in giant planet affects the coronal field of its host star leading to a topology with predominantly closed field lines. An analytic linear force-free model has been adopted to compute the radial extension of the corona and its angular momentum loss rate. The corona is more tightly confined in F-type stars and in G- and K-type stars with a rotation period shorter than ~10 days. The angular momentum loss is produced by coronal eruptions similar to solar coronal mass ejections.

Results. The model predicts that F-type stars with hot Jupiters, T_eff 6000 K and an initial rotation period 10 days suffer no or very little angular momentum loss during their main-sequence lifetime. This can explain their rotation as a remnant of their pre-main-sequence evolution. On the other hand, F-type stars with P_rot > 10 days and G- and K-type stars experience a significant angular momentum loss during their main-sequence lifetime, but at a generally slower pace than similar stars without close-in massive planets. Considering a spread in their ages, this can explain the observed rotation period distribution of planet-harbouring stars.

Conclusions. Our model can be tested observationally and has relevant consequences for the relationship between stellar rotation and close-in giant planets, as well as for the application of gyrochronology to estimate the age of planet-hosting stars.