First Kepler results on compact pulsators I. Survey target selection and the first pulsators

TL;DR

This study uses Kepler data to identify and analyze pulsations in compact stellar objects, revealing new hybrid pulsators and providing detailed classifications and properties of these stars.

Contribution

First Kepler survey to classify and analyze pulsations in compact stars, discovering hybrid pulsators and expanding understanding of pulsation behaviors.

Findings

Identified nine compact pulsators with various pulsation types

Discovered hybrid pulsators showing both short and long period modes

Confirmed low fraction of strong pulsators among hotter stars

Abstract

We present results from the first two quarters of a survey to search for pulsations in compact stellar objects with the Kepler spacecraft. The survey sample and the various methods applied in its compilation are described, and spectroscopic observations are presented to separate the objects into accurate classes. From the Kepler photometry we clearly identify nine compact pulsators, and a number of interesting binary stars. Of the pulsators, one shows the strong, rapid pulsations typical for a V361 Hya type sdB variable (sdBV), seven show long-period pulsations characteristic of V1093 Her type sdBVs, and one shows low-amplitude pulsations with both short and long periods. We derive effective temperatures and surface gravities for all the subdwarf B stars in the sample and demonstrate that below the boundary region where hybrid sdB pulsators are found, all our targets are pulsating. For the stars hotter than this boundary temperature a low fraction of strong pulsators (<10 per cent) is confirmed. Interestingly, the short-period pulsator also shows a low-amplitude mode in the long-period region, and several of the V1093 Her pulsators show low amplitude modes in the short-period region, indicating that hybrid behaviour may be common in these stars, also outside the boundary temperature region where hybrid pulsators have hitherto been found.