OGLE-BLAP-001 and ZGP-BLAP-08: Two possible magnetic blue large-amplitude pulsators^⋆

¹ Instytut Astronomiczny, Wydział Fizyki i Astronomii, Uniwersytet Wrocławski, Kopernika 11, 51-622 Wrocław, Poland
² Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19c, Bât. B5c, 4000 Liège, Belgium

^⋆⋆ Corresponding author; Andrzej.Pigulski@uwr.edu.pl

Received: 23 July 2024
Accepted: 24 October 2024

Abstract

Blue large-amplitude pulsators (BLAPs) are a newly discovered group of compact pulsating stars whose origins are yet to be explained. Of the existing evolutionary scenarios that could lead to the formation of BLAPs, there are two positing that 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 modes, in terms of frequency, had been found in one, namely, OGLE-BLAP-001. We show that three modes that are similarly equidistant in frequency do exist in yet another BLAP, namely, ZGP-BLAP-08. This perfect separation in frequency is a strong argument for explaining 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 hypothesise that these two BLAPs are magnetic, as equidistant modes in terms of the frequency pulsation would be observed in the presence of a magnetic field whose axis of symmetry does not coincide with the rotational axis. A logical consequence of this hypothesis is to postulate that these two BLAPs could have originated in a merger scenario, similarly to what is believed to be behind the origin of magnetic white dwarfs. We also find that period changes in both stars cannot be interpreted by a constant rate of period change. We discuss the possible origins of these changes.