First Kepler results on compact pulsators V: Slowly pulsating subdwarf B stars in short-period binaries

TL;DR

This paper analyzes two hot subdwarf B stars observed by Kepler, revealing their pulsation characteristics and exploring the relationship between their pulsations and binary orbital dynamics, with implications for stellar rotation understanding.

Contribution

First detailed Kepler analysis of g-mode pulsations in sdB stars with close binary companions, examining rotational effects and pulsation properties.

Findings

Detected g-mode pulsations in two sdB stars

Observed no clear rotational splitting despite expectations

Identified period spacings suggestive of high-overtone g-modes

Abstract

The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for nonradial pulsations. We present our analysis of two sdB stars that are found to be g-mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M-dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer-duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal-to-noise, perhaps revealing lower amplitude periodicities that could confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high-overtone g-mode pulsations.