EPIC 211779126: a rare hybrid pulsating subdwarf B star richly pulsating in both pressure and gravity modes

Abstract

We present our analysis of EPIC 211779126, a pulsating subdwarf B star discovered with the Kepler spacecraft during K2 Campaign 5. We found 154 frequencies in the g -mode region as well as 29 frequencies in the p -mode region. This makes EPIC 211779126 a rare hybrid pulsator with a rich pulsation spectrum in both regions. We successfully identified modal degrees and relative radial orders of most of the g -modes using asymptotic period spacing, and modal degrees of some of the p -modes using rotational splitting. We detected trapped modes, which are a very important feature for constraining theoretical models. Our ground-based spectroscopic observations revealed no companion, therefore EPIC 211779126 is likely a single sdB star. Using p -mode multiplets, we derived a rotation period of approximately 16 days, making EPIC 211779126 the fastest rotating non-binary subdwarf B pulsator observed with Kepler . However, we do not find any resolved multiplets among the high-amplitude g -mode pulsations that correspond to the rotation rate inferred from the p -mode splittings. This may indicate that the star’s core is rotating more slowly than its envelope.