On the challenging variability of LS IV-14{\deg}116: pulsational instabilities excited by the {\epsilon}-mechanism

TL;DR

This paper demonstrates that the long-period pulsations in the He-enriched subdwarf star LS IV-14{ extdegree}116 are driven by the { extepsilon}-mechanism in He-burning shells, potentially making it the first known pulsator with this excitation mechanism.

Contribution

It provides the first evidence that the { extepsilon}-mechanism can excite pulsations in a star, specifically in LS IV-14{ extdegree}116, through detailed nonadiabatic pulsation modeling.

Findings

Pulsations are excited by the { extepsilon}-mechanism in He-burning shells.

LS IV-14{ extdegree}116 may be the first pulsating star with { extepsilon}-mechanism excitation.

The period range of unstable modes depends on the shell's location, aiding evolutionary scenario discrimination.

Abstract

We investigate the pulsation driving mechanism responsible for the long-period photometric variations observed in LS IV-14{\deg}116, a subdwarf B star showing a He-enriched atmospheric composition. To this end, we perform detailed nonadiabatic pulsation computations over fully evolutionary post He-core-flash stellar structure models, appropriate for hot subdwarf stars at evolutionary phases previous to the He-core burning stage. We found that the variability of LS IV-14{\deg}116 can be attributed to nonradial g-mode pulsations excited by the {\epsilon}-mechanism acting in the He-burning shells that appear before the star settles on the He-core burning stage. Even more interestingly, our results show that LS IV-14{\deg}116 could be the first known pulsating star in which the {\epsilon}-mechanism of mode excitation is operating. Last but not least, we find that the period range of destabilized modes is sensitive to the exact location of the burning shell, something that might help to distinguish between the different evolutionary scenarios proposed for the formation of this star.