Exploring B4: A Pulsating sdB star, in a Binary, in the Open Cluster NGC 6791

TL;DR

This study analyzes Kepler data of the sdB star B4 in NGC 6791, revealing its pulsation modes, binary nature, and non-synchronized rotation, providing insights into stellar pulsation and tidal interactions.

Contribution

It is the first detailed analysis of B4's pulsations and binary characteristics, offering new observational constraints on tidal synchronization models in sdB binaries.

Findings

B4 is a reflection effect binary with a 0.3985-day orbit.

The sdB star exhibits g-mode pulsations with periods from 2384 to 7643 seconds.

The star's rotation period is about 9.63 days, indicating non-synchronization.

Abstract

We report on Kepler photometry of the hot 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 d orbit. The sdB star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s to 7643 s. Several of the pulsation modes show symmetric splitting by 0.62 microHz. Attributing this to rotational splitting, we conclude that the sdB component has a rotation period of approximately 9.63 d, indicating that tidal synchronization has not been achieved in this system. Comparison with theoretical synchronization time provides a discriminant between various theoretical models.