# Discovery of a second pulsating intermediate helium-enriched sdOB star

## TL;DR

This paper reports the discovery of long-period pulsations in the helium-enriched sdOB star Feige 46, expanding the understanding of pulsating hot subdwarfs outside the typical instability strip and suggesting a possible halo origin.

## Contribution

It is the first detailed analysis of Feige 46's pulsations and atmospheric parameters, revealing similarities with the only other known pulsating He-enriched sdOB star, LS IV -14116.

## Key findings

- Feige 46 exhibits five independent g-mode pulsations.
- Feige 46's atmospheric parameters are similar to LS IV -14116.
- Feige 46 likely belongs to the Galactic halo population.

## Abstract

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. Up until now only one other He-enriched sdOB star (LS IV -14116) 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 fitted our low-resolution, high signal-to-noise optical spectrum of the star with our grid of non-LTE model atmospheres and derived the following atmospheric parameters: Teff = 36120 $\pm$ 230 K, log g = 5.93 $\pm$ 0.04 and log N(He)/N(H) = -0.32 $\pm$ 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV -14116 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 -14116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear but a possible scenario involves the $\epsilon$-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.