Living with a Red Dwarf: Rotation and X-ray and Ultraviolet Properties of the Halo Population Kapteyn's Star

TL;DR

This study characterizes the rotation, X-ray, and ultraviolet emissions of Kapteyn's Star, an old, high-velocity red dwarf, to understand its activity and implications for orbiting planets' atmospheres and habitability.

Contribution

It provides new UV, X-ray, and optical data for Kapteyn's Star, calibrates activity-rotation-age relations, and assesses the star's impact on planetary atmospheres, especially in the context of potential habitable planets.

Findings

Kapteyn's Star exhibits low rotation and activity consistent with its old age.

Reconstructed the star's past XUV emissions impacting orbiting planets.

Confirmed the star's X-ray and Lyman-alpha emissions are suitable for activity studies.

Abstract

As part of Villanova's Living with a Red Dwarf program, we have obtained UV, X-ray and optical data of the Population II red dwarf -- Kapteyn's Star. Kapteyn's Star is noteworthy for its large proper motions and high RV of ~+245 km s^-1. As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating Activity/Irradiance-Rotation-Age relations, and an important test bed for stellar dynamos and the resulting X-ray -- UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth candidates, one of which (Kapteyn b) is orbiting within the habitable zone (Anglada-Escude et al. 2014a, 2015). However, Robertson et al. (2015) questioned the planet's existence since its orbital period may be an artifact of activity, related to the star's rotation period. Because of its large Doppler-shift, measures of the important, chromospheric H I Lyman-alpha 1215.67A emission line can be reliably made, because it is mostly displaced from ISM and geo-coronal sources. Lyman-alpha emission dominates the FUV region of cool stars. Our measures can help determine the X-ray--UV effects on planets hosted by Kapteyn's Star, and planets hosted by other old red dwarfs. Stellar X-ray and Lyman-alpha emissions have strong influences on the heating and ionization of upper planetary atmospheres and can (with stellar winds and flares) erode or even eliminate planetary atmospheres. Using our program stars, we have reconstructed the past exposures of Kapteyn's Star's planets to coronal -- chromospheric XUV emissions over time.