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

¹ Uniwersytet Pedagogiczny, Obserwatorium na Suhorze, ul. Podchorążych 2, 30-084 Kraków, Poland
e-mail: sfbaran@cyf-kr.edu.pl
² Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA
³ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
⁴ Armagh Observatory, College Hill, Armagh BT61 9DG, UK
⁵ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland

Received: 5 September 2016
Accepted: 11 October 2016

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.