The Arizona-Montr\'eal spectroscopic survey of hot subluminous stars

TL;DR

This study analyzes 336 hot subdwarf stars using spectroscopic data and Gaia parallaxes to determine their atmospheric and stellar properties, revealing insights into their formation, composition, and pulsation behaviors.

Contribution

The paper provides a comprehensive analysis of hot subdwarf stars' atmospheric parameters, mass distributions, and pulsation characteristics using homogeneous spectra and Gaia data, highlighting their formation scenarios.

Findings

Mass distribution centered around 0.47 solar masses for H-rich sdBs and sdOs.

He-sdOs have a wider mass distribution, averaging 0.78 solar masses.

Over 80 pulsating stars identified within specific instability regions.

Abstract

Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch. They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). Using the high-quality, homogeneous spectra of 336 hot subluminous star candidates from the Arizona-Montr\'eal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature (Teff), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the $Gaia$ parallax was used to compute the stellar parameters radius, luminosity, and mass. We detected helium stratification in six sdB stars with Teff around 30 kK, making them good candidates for also showing $^3$He enrichment in their atmospheres. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 $\text{M}_\odot$, consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 $\text{M}_\odot$. This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass ($<$0.45$ \text{M}_\odot$) sdBs located below the EHB that might have originated from more massive progenitors. Finally, we identified more than 80 pulsating stars in our sample and found these to fall into well-defined $p$- and $g$-mode instability regions.