Discovery of a second pulsating intermediate helium-enriched sdOB star

Abstract

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. So far, only one other He-enriched sdOB star (LS IV−14 ° 116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fit our optical spectrum of the star with our grid of non-local thermodynamic equilibrium (NLTE) model atmospheres and derived the following atmospheric parameters: Teff = 36120 ± 230 K, log g = 5.93 ± 0.04, and log N(He)/N(H) = −0.32 ± 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV−14 ° 116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV−14 ° 116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear, but a possible scenario involves the ϵ-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.