Kepler Cycle 1 Observations of Low Mass Stars: New Eclipsing Binaries, Single Star Rotation Rates, and the Nature and Frequency of Starspots

TL;DR

This study analyzes Kepler data for low-mass stars, discovering new eclipsing binaries, measuring rotation rates, and examining starspot activity, revealing high activity levels even in stars with long rotation periods.

Contribution

It provides new detections of eclipsing binaries, measures stellar rotation periods, and investigates starspot distributions in low-mass stars using Kepler data.

Findings

Six new eclipsing binaries identified

79% of low-mass stars show variability

Stars with longer periods can still have high activity levels

Abstract

We have analyzed Kepler light curves for 849 stars with T_eff < 5200 K from our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one of which has an orbital period of 29.91 d, and two of which are probably W UMa variables. In addition, we identify a candidate "warm Jupiter" exoplanet. We further examine a subset of 670 sources for variability. Of these objects, 265 stars clearly show periodic variability that we assign to rotation of the low-mass star. At the photometric precision level provided by Kepler, 251 of our objects showed no evidence for variability. We were unable to determine periods for 154 variable objects. We find that 79% of stars with T_eff < 5200 K are variable. The rotation periods we derive for the periodic variables span the range 0.31 < P_rot < 126.5 d. A considerable number of stars with rotation periods similar to the solar value show activity levels that are 100 times higher than the Sun. This is consistent with results for solar-like field stars. As has been found in previous studies, stars with shorter rotation periods generally exhibit larger modulations. This trend flattens beyond P_rot = 25 d, demonstrating that even long period binaries may still have components with high levels of activity and investigating whether the masses and radii of the stellar components in these systems are consistent with stellar models could remain problematic. Surprisingly, our modeling of the light curves suggests that the active regions on these cool stars are either preferentially located near the rotational poles, or that there are two spot groups located at lower latitudes, but in opposing hemispheres.