K2 observations of five pulsating subdwarf B stars with white dwarf companions

TL;DR

This study analyzes five pulsating subdwarf B stars with white dwarf companions observed by Kepler's K2 mission, revealing their pulsation modes, rotation periods, and internal structure, and comparing them with other similar stars.

Contribution

It provides detailed seismic analysis of five sdBV stars with white dwarf companions, including mode identification, rotation profiles, and comparison with other pulsators, advancing understanding of their internal physics.

Findings

Three stars are g-mode-dominated hybrid pulsators.

Stars rotate subsynchronously to their binary periods.

Detected frequency multiplets indicate differential rotation in one star.

Abstract

We report seismic analyses of five pulsating subdwarf B (sdBV) stars observed during Kepler's K2 mission, each with a white dwarf companion. We find three of the five to be g-mode-dominated hybrid pulsators. For the other two, we only detect g modes. We determine rotation periods from frequency multiplets for four stars and each rotates subsynchronously to its binary period, including PG 0101+039 and PG 0902+124 both with binary periods near 0.57 days and spin periods near 9 days. We detect frequency multiplets in both p and g modes for PG 0101+039 and LT Cnc and determine that PG 0101+039 rotates like a solid body while LT Cnc rotates differentially radially with the envelope spinning faster than deeper layers. Mostly we find these five stars to be quite similar to one another, spectroscopically and seismically. We find the p modes of the three hybrid pulsators to have gaps between regions of power, which we interpret as overtones and apply a technique to assign modes. We examine their g mode period spacings and deviations thereof and again, find the stars to be similar with period spacings near the average of 250 s and deviations mostly under 25 s. We compare Kepler-observed sdBV stars of different binary types and likely-single pulsators.