Blue large-amplitude pulsators formed from the merger of low-mass white dwarfs

TL;DR

This study explores the formation of Blue Large-Amplitude Pulsators (BLAPs) through the merger of low-mass white dwarfs, modeling their evolution and estimating their population in the galaxy, suggesting a plausible origin for these pulsating stars.

Contribution

It demonstrates that BLAPs can originate from low-mass white dwarf mergers with specific mass ranges, providing detailed evolutionary pathways and population estimates.

Findings

BLAPs can form from low-mass white dwarf mergers with total mass 0.32-0.7 M$_\odot$.

Merger products evolve into hot subdwarfs and then into white dwarfs or hybrid objects.

Up to a few hundred BLAPs may currently exist in the Galaxy from this formation channel.

Abstract

Blue large-amplitude pulsators (BLAPs) are a recently discovered group of hot stars pulsating in radial modes. Their origin needs to be explained, and several scenarios for their formation have already been proposed. We investigate whether BLAPs can originate as the product of a merger of two low-mass white dwarfs (WDs) and estimate how many BLAPs can be formed in this evolutionary channel. We used the MESA code to model the merger of three different double extremely low-mass (DELM) WDs and the subsequent evolution of the merger product. We also performed a population synthesis of Galactic DELM WDs using the COSMIC code. We find that BLAPs can be formed from DELM WDs provided that the total mass of the system ranges between 0.32 and 0.7 M$_\odot$. BLAPs born in this scenario either do not have any thermonuclear fusion at all or show off-centre He burning. The final product evolves to hot subdwarfs and eventually finishes its evolution either as a cooling He WD or a hybrid He/CO WD. The merger products become BLAPs only a few thousand years after coalescence, and it takes them 20 to 70 thousand years to pass the BLAP region. We found the instability of the fundamental radial mode to be in fair agreement with observations, but we also observed instability of the radial first overtone. From the population synthesis, we found that up to a few hundred BLAPs born in this scenario can exist at present in the Galaxy. Given the estimated number of BLAPs formed in the studied DELM WD merger scenario, there is a good chance to observe BLAPs that originated through this scenario. Since strong magnetic fields can be generated during mergers, this scenario could lead to the formation of magnetic BLAPs. This fits well with the discovery of two likely magnetic BLAPs whose pulsations can be explained in terms of the oblique rotator model.