2026A&A...705A.248L

Context. 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 (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45-0.5 M_⊙ range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population.
Aims. Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios.
Methods. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature (T_eff), 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.
Results. Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with T_eff > 14 kK. We detected the presence of helium stratification in six sdB stars with T_eff around 30 kK, making them good candidates for also showing ³He enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (logg-T_eff) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 M_⊙, 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 M_⊙. In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 M_⊙. This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass (<0.45 M_⊙) sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p- and g-mode instability regions.