Pulsating subdwarf B stars in the oldest open cluster NGC 6791

Abstract

We report results of our analysis of the Kepler superaperture LC data of the open cluster NGC 6791 to search for pulsating sdB stars. We checked all pixels and we found only three sdB stars to be pulsating, KIC 2569576 (B3), KIC 2438324 (B4), and KIC 2437937 (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 analysed already existing spectroscopic observations taken with the HECTOSPEC at the MMT telescope in Smithsonian Arizona and with the Gemini Multi-Object Spectrograph 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.