A Kepler K2 view of subdwarf A-type stars

TL;DR

This study uses Kepler K2 data to analyze the variability of subdwarf A-type stars, revealing diverse pulsations and binary behaviors that inform their evolutionary status.

Contribution

It provides the first comprehensive variability analysis of sdA stars using high-precision Kepler K2 light curves, identifying various types of stellar variability.

Findings

13 out of 25 sdA stars are variable of different kinds

Detection of pulsations, eclipsing binaries, and rotational variables

Establishment of variability thresholds for non-variable stars

Abstract

The spectroscopic class of subdwarf A-type (sdA) stars has come into focus in recent years because of their possible link to extremely low-mass white dwarfs, a rare class of objects resulting from binary evolution. Although most sdA stars are consistent with metal-poor halo main-sequence stars, the formation and evolution of a fraction of these stars are still matters of debate. The identification of photometric variability can help to put further constraints on the evolutionary status of sdA stars, in particular through the analysis of pulsations. Moreover, the binary ratio, which can be deduced from eclipsing binaries and ellipsoidal variables, is important as input for stellar models. In order to search for variability due to either binarity or pulsations in objects of the spectroscopic sdA class, we have extracted all available high precision light curves from the Kepler K2 mission. We have performed a thorough time series analysis on all available light curves, employing three different methods. Frequencies with a signal-to-noise ratio higher than four have been used for further analysis. From the 25 targets, 13 turned out to be variables of different kinds (i.e. classical pulsating stars, ellipsoidal and cataclysmic variables, eclipsing binaries, and rotationally induced variables). For the remaining 12 objects, a variability threshold was determined.