Exploring B4: A pulsating sdB star, in a binary, in the open cluster NGC 6791

Abstract

We report on Kepler photometry of the hot subdwarf B (sdB) star B4 in the open cluster NGC 6791. We confirm that B4 is a reflection effect binary with an sdB component and a low-mass main-sequence companion with a circular 0.3985 day orbit. The sdB star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384s to 7643s. Several of the pulsation modes show symmetric splitting by 0.62 μHz. Attributing this to rotational splitting, we conclude that the sdB component has a rotation period of approximately 9.63 days, indicating that tidal synchronization has not been achieved in this system. Comparison with theoretical synchronization time provides a discriminant between various theoretical models. © 2011. The American Astronomical Society. All rights reserved.