The Arizona-Montréal spectroscopic survey of hot subluminous stars

TL;DR

Using 336 high-quality hot subluminous spectra from the Arizona-Montréal survey, this study derives $T_{ ext{eff}}$, $\\log g$, and log $N$(He)/$N$(H) through NLTE/LTE atmospheric models and then determines radii, luminosities, and masses by fitting SEDs with Gaia parallaxes. The results show a canonical mass peak near $0.47\,M_\\odot$ for H-rich sdBs/sdOs, while He-rich sdOs have a broader, higher-mass distribution around $0.6$–$1.0\,M_\\odot$, supporting a merger origin for He-rich objects; helium stratification is detected in several sdBs around $T_{ ext{eff}}\sim 28$–31 kK, and more than 80 pulsators fall into defined $p$- and $g$-mode instability regions. A subset of MMT data reveals helium stratification and systematic differences due to limited wavelength coverage, underscoring the importance of Balmer lines for reliable mass estimates. The work also identifies underluminous, low-mass sdBs below the EHB and a substantial IR-excess binary population, providing a cohesive view of hot subdwarf formation channels, binary evolution, and stellar pulsations.

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-Montréal 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.

Figures (27)

• Figure 1: Distance and sky distribution of the stars in the Bok sample. Six objects are located beyond 4000 pc, but they are all most likely MS B-type stars (see Sect. \ref{['sec:res:bok:stellar']}). The red line indicates the position of the galactic plane on the sky projection.
• Figure 2: Surface gravity (log $g$) as function of $T_{\text{eff}}$ (Kiel diagram) for the stars in the Bok sample. The six spectral groups are indicated with symbols of different colors. Stars with IR excess are marked with an additional orange circle (see Sect. \ref{['sec:res:bok:stellar']}). The ZAEHB and TAEHB computed with STELUM for a core mass of 0.47 $\text{M}_\odot$ are shown with red dashed lines. The ZAEHB extension below 20 kK (solid red line) is from BaSTI models. Two BaSTI evolutionary tracks are shown in purple, the solid part represents the core-He burning phase while the dotted part is the post-EHB phase. The dashed purple track is from a late-flasher model. The dashed-dotted red line is the ZAHeMS with the stellar masses indicated along the line. References for theoretical models are listed in Sect. \ref{['sec:res:bok:atmo']}.
• Figure 3: Helium abundance as a function of $T_{\text{eff}}$ for the stars in the Bok sample. The spectral groups are indicated following the same color scheme as in Fig. \ref{['fig:Kiel']}. The solar helium abundance is indicated with the dotted line.
• Figure 4: Bok spectrum (black) and best fit solution (red) for four stars in our sample. PB7352 is a typical hydrogen-rich sdB and PG1708+602 is a hot sdO. PG1247+554 is an iHe-sdB located below the EHB in the Kiel diagram. LSIV+10$^{\circ}$4 is a He-sdO where the Heii Pickering series blends with the Balmer lines.
• Figure 5: Luminosity of our stars versus their $T_{\text{eff}}$ (i.e., HRD). The different spectral types are color-coded as in the previous figures. We indicate the composite objects with an additional orange circle around the symbols. The evolutionary tracks are the same as in the Kiel diagram (Fig. \ref{['fig:Kiel']}). The four luminous post-AGB stars mentioned in Sect. \ref{['sec:res:bok:atmo']} are outside the luminosity range shown here.