Observational Parameters of Blue Large-amplitude Pulsators

TL;DR

This paper provides new photometric and spectroscopic insights into Blue Large-amplitude Pulsators (BLAPs), revealing their physical properties, period changes, and potential evolutionary status, based on OGLE survey data.

Contribution

It offers the first comprehensive analysis of BLAPs' observational parameters, including period, temperature, gravity, and chemical composition, and establishes period--luminosity and period--gravity relations.

Findings

BLAPs are predominantly single-mode pulsators in the fundamental radial mode.

They exhibit period change rates around 10^{-7} yr^{-1}, with some decreasing.

A split in helium-to-hydrogen content suggests different atmospheric compositions among BLAPs.

Abstract

Blue large-amplitude pulsators (BLAPs) are a recently discovered class of short-period pulsating variable stars. In this work, we present new information on these stars based on photometric and spectroscopic data obtained for known and new objects detected by the Optical Gravitational Lensing Experiment (OGLE) survey. BLAPs are evolved objects with pulsation periods in the range of 3-75 minutes, stretching between subdwarf B-type stars and upper main-sequence stars in the Hertzsprung-Russell diagram. In general, BLAPs are single-mode stars pulsating in the fundamental radial mode. Their phase-folded light curves are typically sawtooth-shaped, but many longer-period objects exhibit an additional bump. The long-term OGLE observations show that the period change rates of BLAPs are usually of the order of $10^{-7}$ yr$^{-1}$ and in a quarter of the sample are negative. The spectroscopic data indicate that the BLAPs form a homogeneous group in the period, surface gravity, and effective temperature spaces. However, we observe a split into two groups in terms of helium-to-hydrogen content. The atmospheres of the He-enriched BLAPs are more abundant in metals (about 5 times) than the atmosphere of the Sun. We discover that BLAPs obey a period--gravity relationship and we use the distance to OGLE-BLAP-009 to derive a period--luminosity relation. Most of the stars observed in the OGLE Galactic bulge fields seem to reside in the bulge, while the remaining objects likely are in the foreground Galactic disk.