Two p-mode-dominated subdwarf B pulsators in binaries with F-star companions observed with K2

Abstract

This paper presents spectroscopic and seismic analyses of two rare hybrid pulsating subdwarf B (sdB) stars, PG 0048+091 and PG 1315-123, which are dominated by pressure (p)-mode pulsations. These are only the second and third sdB pulsators with detected frequency multiplets in both the p- and g (gravity)-mode regions. This allows an examination of the radial rotation profiles and p- and g-mode overtone spacings. Frequency multiplets indicate that PG 1315-123 rotates uniformly, as a solid body, while PG 0048+091 is rotating faster in the outer envelope. g- and p-mode overtone spacings for PG 1315-123 agree with previous findings and model predictions at 237 and ∼1000 s while those for PG 0048+091 are significantly shorter at 207 and <250 s, respectively. The cause of PG 0048+091's discrepancies is likely related to core structural differences which could be related to progenitor mass. Spectroscopic findings show that both stars lie on the hot end of the instability region and are most likely in long-period binaries with main-sequence companions. Their high temperatures challenge driving theory, which produces g-mode pulsations only at cooler temperatures. Another challenge is the appearance of pulsations with both driven and stochastic properties. Such a variety of pulsation properties makes these stars very interesting and important for understanding hot horizontal-branch stars.