Arizona-Montreal survey 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. 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.

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 Teff>~14kK. We detected the presence of helium stratification in six sdB stars with T eff around 30kK, making them good candidates for also showing 3 He enrichment in their atmospheres.

Our sdB distribution along the EHB shows a gap near 33kK, visible in both the Kiel (logg-Teff ) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47M_{sun}_, 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 0.78M_{sun}_. In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1M_{sun}_. This strongly favors a merger origin for these He-rich objects.

We identified a small number of candidate low-mass (<0.45M_{sun}_) 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 find these to fall into well-defined p- and g-mode instability regions.