New theoretical instability regions, period-luminosity relations and masses for blue large-amplitude pulsators

TL;DR

This paper uses linear non-adiabatic pulsation analysis via MESA-RSP to map BLAP instability regions across a wide grid of stellar models ($M=0.3$–$1.0M_{\odot}$, $Z=0.01$–$0.07$, $T_{ m eff}=20{,}000$–$35{,}000$ K) and to derive new period relations. It assesses how metallicity $Z$, helium $Y$, and convection impact instability domains, and provides a general $MLTZP$ relation that shows period scales with $L$, $M$, $T_{ m eff}$, and $Z$, with low-mass models ($0.3$–$0.5M_{\odot}$ aligning best with observed slopes. The Petersen diagram indicates potential multi-mode pulsation possibilities and shows how $P_{1O}/P_{F}$ varies with $M$, $Z$, and $L$, while the asteroseismic mass of the multiperiodic OGLE-BLAP-030 is tightly constrained to $M=0.62$–$0.64M_{\odot}$ for $Z=0.07$. Overall, the results favor a predominantly low-mass BLAP population but acknowledge a possible HG-BLAP subgroup, highlighting the need for non-linear modeling and spectroscopic metallicities to refine mass estimates and mode identifications.

Abstract

Blue large-amplitude pulsators (BLAPs) are a recently discovered group of hot pulsating stars whose evolutionary status remains uncertain. Their supposed progenitors are either $\simeq 0.3M_{\odot}$ shell H-burning stars or $\simeq 1.0M_{\odot}$ core He-burning stars, both relying on mass loss or a merger event in a (rarely observed) close interacting binary system. With the goal to understand the stellar masses of BLAPs, we therefore carried out a linear non-adiabatic analysis of a grid of models computed using mesa-rsp, with appropriate input stellar parameters $ZXMLT_{\rm eff}$ and convection parameter sets. We discuss the impact of stellar mass, metallicity, helium abundance and convection parameters on the theoretical instability regions of BLAPs. We also derive new theoretical period relations; our theoretical period relations using low stellar masses seem to be in better agreement with the observed period relations. Although only two BLAPS have been observed to be multi-periodic oscillator so far, we analyse theoretical $P_{1O}/P_F$ ratios and compare these values with other classical pulsators. Furthermore, we provide the first asteroseismic mass estimate for the triple-mode pulsator, OGLE-BLAP-030 which seems to be well-constrained in the range of $0.62-0.64 M_{\odot}$ with a high metallicity of $Z=0.07$, albeit with a few sources of uncertainty involved. This would place the BLAP star intermediate to the two proposed mass scenarios so far.

Figures (18)

• Figure 1: Sensitivity of the pulsation period (top panels) and its growth rate (bottom panels) in the fundamental mode of BLAP models for different ($N, N_{\rm outer}$) combinations as a function of different stellar masses, chemical combinations and convection parameter sets. The header of each sub-plot has the format ($Z,X,M/M_{\odot},L/L_{L_{\odot}},T_{\rm eff}$, Convection set). The grey shaded region indicates the combination finally chosen for the analysis; $(P_F)_0$ and $(\Gamma_F)_0$ correspond to the pulsation period and its growth rate obtained corresponding to the chosen ($N, N_{\rm outer}$) combination.
• Figure 2: Same as Fig. \ref{['convergence1']} but for different ($T_{\rm anchor}, T_{\rm inner}$) combinations.
• Figure 3: Instability regions for different stellar masses with $Z=0.05$ computed using convective parameter set A. The black square represents the estimated location of BLAPs on the HR diagram as defined by pietrukowicz17 and pietrukowicz24 while the black stars indicate observationally determined BLAPs from pigulski22 and bradshaw2024.
• Figure 4: A comparison of the observed BLAPs with the theoretical period relations obtained from the linear BLAP models computed using convection parameter set A. Dashed vertical gray lines represent the observed pulsation period range of BLAPs ($3 \leq P[\rm min] \leq 77$) in all the sub-plots. In addition, observed BLAPs with known stellar parameters are highlighted with star-shaped symbols. a)$P$-$T_{\rm eff}$ relation, for $L \sim 200 \: \rm L_{\odot}$. b)$P$-$L$ relation, for $T_{\rm eff} = 30000 \: \rm K$, which is roughly an average of all BLAPs with known $T_{\rm eff}$. Observed BLAPs with determined $L$ from pigulski22 and bradshaw2024 are included. c)$P$-$\log(g)$ relation, for $L \sim 200 \: \rm L_{\odot}$. d)$P$-$R$ relation, for $L \sim 200 \: \rm L_{\odot}$. The observed relations were calculated using $R = \sqrt{\frac{G M}{g}}$ based on our $P$-$\log(g)$ relation, for two values of mass. The plot also includes both values of $R$ determined by bradshaw2024 for OGLE--BLAP--009.
• Figure 5: Petersen diagram from our grid of linear models at a fixed luminosity of $L \simeq 200 L_{\odot}$