OGLE-BLAP-001 and ZGP-BLAP-08: two possible magnetic blue large-amplitude pulsators

TL;DR

This paper investigates two blue large-amplitude pulsators, proposing they are magnetic due to equidistant pulsation modes, and suggests their origin may be linked to stellar mergers, supported by amplitude and phase variations.

Contribution

It presents evidence that two BLAPs are magnetic stars with equidistant pulsation modes, supporting an oblique pulsator model and a merger origin hypothesis.

Findings

Presence of equidistant in frequency modes in both BLAPs.

Support for oblique pulsator model based on amplitude and phase changes.

Hypothesis that these BLAPs are magnetic and formed via stellar mergers.

Abstract

Blue large-amplitude pulsators (BLAPs) are a newly discovered group of compact pulsating stars whose origin needs to be explained. Of the existing evolutionary scenarios that could lead to the formation of BLAPs, there are two in which BLAPs are the products of the merger of two stars, either a main sequence star and a helium white dwarf or two low-mass helium white dwarfs. Among over a hundred known BLAPs, three equidistant in frequency modes had been found in one, OGLE-BLAP-001. We show that similar three equidistant in frequency modes exist in yet another BLAP, ZGP-BLAP-08. This perfect separation in frequency is a strong argument to explain the modes in terms of an oblique pulsator model. This model is supported by the character of the changes of the pulsation amplitude and phase with the rotational phase. Consequently, we hypothesize that these two BLAPs are magnetic, as equidistant in frequency pulsation modes should be observed in the presence of a magnetic field whose axis of symmetry does not coincide with the rotation axis. A logical consequence of this hypothesis is to postulate that these two BLAPs could have originated in a merger scenario, just how the origin of magnetic white dwarfs is explained. We also find that period changes in both stars cannot be interpreted by a constant rate of period change and discuss the possible origin of these changes.