Mode Identification in a Pulsating Subdwarf B Star EPIC 212707862 Observed with K2

Abstract

We present an analysis of K2 observations of EPIC 212707862, a pulsating subdwarf B star. We detected 13 significant frequencies from an 81 day run during Campaign 6. Our goal was to find features that could help us to identify pulsation modes. We could not identify any multiplets, which would also have enabled us to precisely derive a rotation period. Based on amplitude modulation we estimated that period to be around 80 days. We found two period-spacing sequences, and successfully identified modal degrees for 11 out of 13 detected frequencies. We assigned six of them to l=1, and another five to l=2. These results will facilitate future theoretical modeling. This star brings to 19 the number of pulsating subdwarf B stars observed with K2. Radial velocities obtained to date and the spectral energy distribution are consistent with EPIC 212707862 being a single hot subdwarf. Analysis of the spectrum gives atmospheric parameters: T<SUB>eff</SUB>=28 298±162 K, log g[cm/s<SUP>2</SUP>]=5.479±0.025 and log (n(He)/n(H))=-2.752±0.069.