Stellar activity and planetary atmosphere evolution in tight binary star systems^★

Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: colin.johnstone@univie.ac.at

Received: 10 February 2018
Accepted: 11 April 2019

Abstract

Context. In tight binary star systems, tidal interactions can significantly influence the rotational and orbital evolution of both stars, and therefore their activity evolution. This can have strong effects on the atmospheric evolution of planets that are orbiting the two stars.

Aims. In this paper, we aim to study the evolution of stellar rotation and of X-ray and ultraviolet (XUV) radiation in tight binary systems consisting of two solar mass stars and use our results to study planetary atmosphere evolution in the habitable zones of these systems.

Methods. We have applied a rotation model developed for single stars to binary systems, taking into account the effects of tidal interactions on the rotational and orbital evolution of both stars. We used empirical rotation-activity relations to predict XUV evolution tracks for the stars, which we used to model hydrodynamic escape of hydrogen dominated atmospheres.

Results. When significant, tidal interactions increase the total amount of XUV energy emitted, and in the most extreme cases by up to factor of ~50. We find that in the systems that we study, habitable zone planets with masses of 1 M_⊕ can lose huge hydrogen atmospheres due to the extended high levels of XUV emission, and the time that is needed to lose these atmospheres depends on the binary orbital separation. For some orbital separations, and when the stars are born as rapid rotators, it is also possible for tidal interactions to protect atmospheres from erosion by quickly spinning down the stars. For very small orbital separations, the loss of orbital angular momentum by stellar winds causes the two stars to merge. We suggest that the merging of the two stars could cause previously frozen planets to become habitable due to the habitable zone boundaries moving outwards.