Pulsating subdwarf B stars in the oldest open cluster NGC6791

TL;DR

This study analyzes Kepler data of the open cluster NGC6791, identifying pulsating subdwarf B stars, extending their frequency data, and deriving stellar properties including rotation periods and potential binarity.

Contribution

It extends the known pulsation data for sdB stars in NGC6791 and uses multiplet analysis to determine mode geometry and rotation periods.

Findings

Identified three pulsating sdB stars with extended frequency spectra.

Detected new multiplets and period spacing sequences for mode identification.

Suggested potential binarity in three sdB stars based on radial velocity hints.

Abstract

We report results of our analysis of the Kepler superaperture LC data of the open cluster NGC6791 to search for pulsating sdB stars. We checked all pixels and we found only three sdB stars to be pulsating, KIC2569576 (B3), KIC2438324 (B4) and KIC2437937 (B5). These stars were known to be pulsators before, though we extended data coverage detecting more frequencies and features in their amplitude spectra, i.e. new multiplets and more complete period spacing sequences that we used for identifying geometry of the pulsation modes. The multiplet splittings were also used to derive rotation periods. The remaining known sdBs do not show any pulsation-related light variation down to our detection thresholds. We analyzed already existing spectroscopic observations taken with the HECTOSPEC at the MMT telescope in Smithsonian Arizona and with the GMOS at the Gemini North telescope, and fitted atmospheric parameters using the Balmer lines. Four stars, B3-B6, show atmospheric parameters that are consistent with g-mode dominated sdBs. We detected hints of radial velocity variability in B3, B5, and B6, indicating these three stars may be in binaries.