OGLE-BLAP-009 - A Case Study for the Properties and Evolutionary Status of Blue Large-Amplitude Pulsators

Blue large-amplitude pulsators (BLAPs) make up a rare class of hot pulsating stars with effective temperatures of ~30,000 K and surface gravities of 4.0 - 5.0 dex (cgs). The evolutionary origin and current status of BLAPs is not well understood, largely based on a lack of spectroscopic observations and no available mass constraints. However, several theoretical models have been proposed that reproduce their observed properties, including studies which identify them as pulsating helium-core pre-white dwarfs (He-core pre-WDs). Here, we present follow up high-speed photometry and phase-resolved spectroscopy of one of the original 14 BLAPs, named OGLE-BLAP-009, which was discovered and classified during the Optical Gravitational Lensing Experiment (OGLE) survey of the Galactic Bulge and Disk. We aim to explore this star’s pulsation characteristics and determine its stellar properties such as mass and radius in order test the consistency of these results with theoretical He-core pre-WD models. Multi-color photometry shows a period of 31.936 ± 0.002 minutes with amplitudes of 0.249 ± 0.003 mag and 0.219 ± 0.004 mag in the g and r-bands, respectively. Phase resolved spectroscopy shows large radial velocity (RV), effective temperature (Teff) and surface gravity (log(g)) fluctuations about means of 53.36 ± 01.44 km/s, 29,315 ± 331 K and 4.38 ± 0.03 with peak-to-peak amplitudes of 135 ± 5 km/s, 10,868 ± 784 K and 0.64 ± 0.01, respectively. Fitting a spectral energy distribution which utilized spectroscopic measurements resulted in a mass of 0.28 ± 0.15 M_sol and radius of 0.55 ± 0.06 R_sol. Evolutionary models of He-core pre-WDs which include linear adiabatic pulsation period calculations for both the fundamental and first-overtone radial-modes were constructed using the stellar evolution codes MESA and GYRE. Comparing the observed Teff, log(g) and pulsation period shows close agreement with a He-core pre-WD pulsating in the first-overtone radial-mode. Using the stellar dynamical frequency (ω_dyn) and the observed frequency (ω), a dimensionless frequency factor (f = ω / ω_dyn) was calculated for each model. Combining this value with the surface gravity measurement resulted in a mass of 0.35 ± 0.14 M_sol and a radius of 0.62 ± 0.11 R_sol for the first-overtone mode. These values are consistent with the spectral energy distribution fitting, constraining the evolutionary channel of this object and potentially others in this class to those that produce low-mass He-core pre-WDs of ≈0.30 M_sol.