First Kepler results on compact pulsators III: Subdwarf B stars with V1093~Her and hybrid (DW~Lyn) type pulsations

TL;DR

This paper reports the discovery of nonradial pulsations in five hot subdwarf B stars observed by Kepler, revealing new hybrid pulsators with both g- and p-mode oscillations, and providing detailed frequency analysis.

Contribution

First Kepler-based identification of hybrid and long-period pulsators among sdB stars, including the coolest and longest-period examples, with detailed frequency and mode analysis.

Findings

All observed sdB stars cooler than 27,500 K are pulsators.

Discovery of the coolest and longest-period V1093 Her pulsator to date.

Identification of hybrid pulsators with dominant g-mode amplitudes.

Abstract

We present the discovery of nonradial pulsations in five hot subdwarf B (sdB) stars based on 27 days of nearly continuous time-series photometry using the Kepler spacecraft. We find that every sdB star cooler than $\approx 27\,500\,$K that Kepler has observed (seven so far) is a long-period pulsator of the V1093~Her (PG~1716) class or a hybrid star with both short and long periods. The apparently non-binary long-period and hybrid pulsators are described here. The V1093~Her periods range from one to 4.5~h and are associated with $g-$mode pulsations. Three stars also exhibit short periods indicative of $p-$modes with periods of 2 to 5~m and in addition, these stars exhibit periodicities between both classes from 15 to 45~m. We detect the coolest and longest-period V1093~Her-type pulsator to date, KIC010670103 ($T_eff\approx 20\,900\,$K, $P_max\approx 4.5$~h) as well as a suspected hybrid pulsator, KIC002697388 which is extremely cool ($T_{\rm eff}\approx 23\,900\,$K) and for the first time hybrid pulsators which have larger $g-$mode amplitudes than $p-$mode ones. All of these pulsators are quite rich with many frequencies and we are able to apply asymptotic relationships to associate periodicities with modes for KIC010670103. Kepler data are particularly well-suited for these studies as they are long-duration, extremely high duty cycle observations with well-behaved noise properties.