Binary evolution pathways of blue large-amplitude pulsators

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 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 12000 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.

The OmegaWhite Survey for Short-Period Variable Stars VII: High amplitude, short period blue variables

“…Moving to b=15 • results in a very low number of BLAPs being predicted. Our search for BLAPs in the OW survey did not reach as faint as g=20 mag but with an observed number density of ∼0.01 deg −1 down to g ∼ 19, our findings appear reasonably consistent with the predictions of Byrne et al (2021), giving good grounds to suggest that 12,000 BLAPs may exist in our Galaxy.…”

“…We have identified four stars which have short photometric periods and high amplitudes with followup spectroscopic observations indicate they are BLAPs, implying one BLAP per ∼0.01 deg −1 . Byrne et al (2021) made predictions of how many BLAPs would be expected in our Galaxy using the Binary Population Spectral Synthesis code (Eldridge et al 2017). They found the numbers were very dependent on Galactic latitude (concentrated towards the plane) and longitude (highest the Bulge) although regions of high extinction could make this very variable.…”

“…In addition, Byrne & Jeffery (2020) also concluded that a channel where BLAPs were formed from binaries in which Roche-lobe overflow was occuring merited further investigation. Byrne et al (2021) explore possible reasons for why stars with gravities between the high gravity and low gravity BLAPs do not appear to pulsate despite models predicting they should.…”

HD 133729: A blue large-amplitude pulsator in orbit around a main-sequence B-type star

“…Both models proposed for BLAPs require a significant part of the initial stellar mass to be stripped off during pre-BLAP evolution. This is possible in binaries (e.g., Byrne et al 2021), but no BLAP was known to reside in a binary. In this context, the solution proposed by Meng et al (2020) seemed to solve the problem.…”

Discovery of post-mass-transfer helium-burning red giants using asteroseismology

“…A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf-B star [1][2][3] . However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not.…”