Evidence for enhanced chromospheric Ca II H and K emission in stars with close-in extrasolar planets^⋆

¹ Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic
e-mail: terak@physics.muni.cz
² Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovak Republic
e-mail: budaj@ta3.sk

Received: 11 October 2011
Accepted: 13 February 2012

Abstract

Context. The planet-star interaction is manifested in many ways. It has been found that a close-in exoplanet causes small but measurable variability in the cores of a few lines in the spectra of several stars, which corresponds to the orbital period of the exoplanet. Stars with and without exoplanets may have different properties.

Aims. The main goal of our study is to search for the influence that exoplanets might have on atmospheres of their host stars. Unlike the previous studies, we do not study changes in the spectrum of a host star or differences between stars with and without exoplanets. We aim to study a large number of stars with exoplanets and the current level of their chromospheric activity and to look for a possible correlation with the exoplanetary properties.

Methods. To analyse the chromospheric activity of stars, we exploited our own^⋆⋆ and publicly available archival spectra^⋆⋆⋆, measured the equivalent widths of the cores of Ca II H and K lines, and used them to trace their activity. Subsequently, we searched for their dependence on the orbital parameters and the mass of the exoplanet.

Results. We found statistically significant evidence that the equivalent width of the Ca II K line emission and log R'_HK activity parameter of the host star varies with the semi-major axis and mass of the exoplanet. Stars with T_eff ≤ 5500 K having exoplanets with semi-major axis a ≤ 0.15 AU (P_orb ≤ 20 days) have a broad range of Ca II K emissions and much stronger emission in general than stars at similar temperatures but with higher values of semi-major axes. The Ca II K emission of cold stars (T_eff ≤ 5500 K) with close-in exoplanets (a ≤ 0.15 AU) is also more pronounced for more massive exoplanets.

Conclusions. The overall level of the chromospheric activity of stars may be affected by their close-in exoplanets, and stars with massive close-in exoplanets may be more active.