Low-period spacing core-helium burning giants: `hot subdwarf analogues'?

TL;DR

This paper proposes that low-period spacing core-helium burning giants with certain oscillation signatures are likely hot subdwarf analogues formed through specific mass-loss events, offering insights into stellar evolution pathways.

Contribution

It introduces a hypothesis linking low-period spacing giants to hot subdwarf formation via mass-loss scenarios at the end of the red-giant branch.

Findings

Stars with low period spacings can result from mass-loss events.

These stars are potential hot subdwarf analogues.

Gradual mass loss during core helium burning may produce hot subdwarf progenitors.

Abstract

Global stellar oscillations probe the internal structure of stars. In low- to intermediate-mass red giants, these oscillations provide signatures from both the outer regions of the star as well as from the core. These signatures are imprinted in e.g. the frequency of maximum oscillation power, and in the differences in periods of non-radial oscillations (period spacings), respectively. In core helium burning giants with masses below about 1.7 solar masses, i.e. stars that have gone through a helium flash, the asymptotic period spacings take values of about 220 -350 s at frequency of maximum oscillation power of $\sim$30-50 $\mu$Hz. A set of stars with asymptotic period spacings lower than about 200 s at similar frequencies separations has recently been discovered by Elsworth and collaborators. In this work, we present a hypothesis for the formation scenario of these stars. We find that these stars can be the result of a mass-loss event at the end of the red-giant branch phase of stars massive enough to not have a degenerate core, i.e. one of the scenarios to form hot subdwarf stars. Therefore, these stars can be classified as `hot subdwarf analogues'. Interestingly, if mass loss continues gradually during the core helium burning phase, these stars turn hotter and denser, and could, therefore, be hot subdwarf progenitors as they shed more of their envelope.