Rapidly Rotating, X-ray Bright Stars in the Kepler Field

TL;DR

This study analyzes twenty X-ray bright stars in the Kepler field, revealing rapid rotation, high chromospheric activity, and possible FK Com variables, suggesting they are evolved, merged binary stars with short-lived but highly active stages.

Contribution

It provides the first detailed Kepler light curves and spectroscopy of these stars, identifying their nature as likely FK Com variables resulting from binary mergers.

Findings

Most stars are (sub)giants with rapid rotation and high activity.

Evidence suggests these stars are evolved FK Com variables.

The stars exhibit flaring behavior and chromospheric activity levels much higher than the Sun.

Abstract

We present Kepler light curves and optical spectroscopy of twenty X-ray bright stars located in the Kepler field of view. The stars, spectral type FK, show evidence for rapid rotation including chromospheric activity 100 times or more above the Sun at maximum and flaring behavior in their light curves. Eighteen of our objects appear to be (sub)giants and may belong to the class of FK Com variables, that is evolved rapidly spinning single stars with no excretion disk and high levels of chromospheric activity. Such stars are rare and are likely the result of W UMa binary mergers, a process believed to produce the FK Com class of variable and their descendants. The FK Com stage, including the presence of an excretion disk, is short-lived but leads to longer-lived stages consisting of single, rapidly rotating evolved (sub)giants with high levels of stellar activity.