Shell helium-burning hot subdwarf B stars as candidates for blue large-amplitude pulsators

TL;DR

This study explores whether shell helium-burning subdwarf B stars can explain blue large-amplitude pulsators (BLAPs), using MESA models to match observational data and considering binary evolution formation channels.

Contribution

It introduces a new model for BLAPs as shell helium-burning subdwarfs, expanding understanding of their origins and internal structures.

Findings

Models with helium core masses of 0.45-0.5 M_sun match observations.

Positive period change rates can evolve to negative.

About half of binary models produce long-period SHeB sdBs that evade detection.

Abstract

Blue large-amplitude pulsators (BLAPs) are a newly discovered type of variable star. Their typical pulsation periods are on the order of a few tens of minutes, with relatively large amplitudes of 0.2-0.4 mag in optical bands, and their rates of period changes are on the order of $10^{-7} yr^{-1}$ (both positive and negative). They are extremely rare objects and attempts to explain their origins and internal structures have attracted a great deal of attention. Previous studies have proposed that BLAPs may be pre-white dwarfs, with masses around $0.3M_\odot$, or core-helium-burning stars in the range of $\sim 0.7-1.1M_\odot$. In this work, we use a number of MESA models to compute and explore whether BLAPs could be explained as shell helium-burning subdwarfs type B (SHeB sdBs). The models that best match existing observational constraints have helium core masses in the range of $\sim 0.45-0.5M_\odot$. Our model predicts that the positive rate of period change may evolve to negative. The formation channels for SHeB sdBs involve binary evolution and although the vast majority of BLAPs do not appear to be binaries (with the exception of HD 133729), the observational constraints are still very poor. Motivated by these findings, we explored the Roche lobe overflow channel. Of the 304 binary evolution models we computed, about half of them are able to produce SHeB sdBs in long-period binaries that evade detection from the limited observations that are currently available.