Abstract
ABSTRACT Blue large-amplitude pulsators (BLAPs) are a recently discovered class of pulsating star, believed to be proto-white dwarfs, produced by mass stripping of a red giant when it has a small helium core. An outstanding question is why the stars in this class of pulsator seem to form two distinct groups by surface gravity, despite predictions that stars in the gap between them should also pulsate. We use a binary population synthesis model to identify potential evolutionary pathways that a star can take to become a BLAP. We find that BLAPs can be produced either through common envelope evolution or through Roche lobe overflow, with a main-sequence star or an evolved compact object being responsible for the envelope stripping. The mass distribution of the inferred population indicates that fewer stars would be expected in the range of masses intermediate to the two known groups of pulsators, suggesting that the lack of observational discoveries in this region may be a result of the underlying population of pre-white dwarf stars. We also consider metallicity variation and find evidence that BLAPs at Z = 0.010 (half-solar) would be pulsationally unstable and may also be more common. Based on this analysis, we expect the Milky Way to host around 12 000 BLAPs and we predict the number density of sources expected in future observations such as the Legacy Survey of Space and Time at the Vera Rubin Observatory.
Highlights
Blue Large-Amplitude Pulsators (BLAPs) are hot, faint stars, discovered in both OGLE and ZTF survey data (Pietrukowicz et al 2017; Kupfer et al 2019)
Our objectives are four-fold: (i) We consider whether additional evolution pathways exist that might give rise to stars matching the known properties of BLAPs; (ii) we determine the likely number of BLAPs in the Milky Way - a quantity which has ranged over three orders of magnitude in previous estimates (Meng et al 2020); (iii) we investigate whether the observed mass gap in known systems arises naturally in a synthetic population; (iv) we calculate the expected surface density of these sources in deep, wide-field surveys
We distinguish between BLAP models that have formed through Roche Lobe Overlow (RLOF) and BLAP models that have formed through Common Envelope Evolution (CEE) by comparing the initial and final orbital separations
Summary
Blue Large-Amplitude Pulsators (BLAPs) are hot, faint stars, discovered in both OGLE and ZTF survey data (Pietrukowicz et al 2017; Kupfer et al 2019). They show periodic brightness variations on timescales ranging from 3 minutes to 40 minutes, with amplitudes of up to a few per cent of their total brightness, and effective temperatures (Teff) between 20 000 and 35 000 K. The pulsation driving mechanism in BLAPs is the κ-mechanism, due to the opacity bump generated by iron and nickel in the envelope. An enhancement in the abundance of these elements - produced by the equilibrium between radiative levitation and gravitational settling processes - is necessary to produce a sufficient opacity bump to drive the pulsations (Charpinet et al 1997; Byrne & Jeffery 2020). It has been suggested that BLAPs could be formed from the surviving remnant of a single degenerate Type Ia supernova (Meng et al 2020) or that they could be core-helium burning stars approaching the extended horizontal branch (Wu & Li 2018)
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