Abstract :
[en] 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.
Funding text :
We are forever grateful to Gilles Fontaine, who started thisproject with Betsy Green two decades ago. Thanks to the data and recordsthat he kept, we were able to bring his work to completion and achieve muchmore than would have been possible in 2005. G.F. would have thanked PierreBergeron for his help in the very early phase of this project. We are thankful for those who shared with us their unpublished data to help us track downpulsators and RV variable stars: W. Zong, S. Geier, F. Mattig, H. Dawson, andV. Schaffenroth. M.L. acknowledges funding from the Deutsche Forschungsgemeinschaft (grant LA 4383/4-1). M.D. was supported by the Deutsches Zentrumf\u00FCr Luft- und Raumfahrt (DLR) through grant 50-OR-2304. V.V.G. is a F.R.S.-FNRS Research Associate. S.C. acknowledge support from the Centre Nationald\u2019\u00C9tudes Spatiales (CNES, France), focused on the mission TESS. This workhas made use of data from the European Space Agency (ESA) mission Gaia(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by nationalinstitutions, in particular the institutions participating in the Gaia MultilateralAgreement. This work has made use of IRAF, which was distributed by theNational Optical Astronomy Observatory, USA, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. This research has made useof NASA\u2019s Astrophysics Data System Bibliographic Services, of the SIMBADdatabase (Wenger et al. 2000), operated at CDS, Strasbourg, France, and ofthe VizieR catalogue access tool (Ochsenbein et al. 2000), CDS, StrasbourgAstronomical Observatory, France (DOI : 10.26093/cds/vizier). This researchhas made use of TOPCAT (Taylor 2005) and the PYTHON packages pandas(pandas development team 2020) and MATPLOTLIB (Hunter 2007).We are forever grateful to Gilles Fontaine, who started this project with Betsy Green two decades ago. Thanks to the data and records that he kept, we were able to bring his work to completion and achieve much more than would have been possible in 2005. G.F. would have thanked Pierre Bergeron for his help in the very early phase of this project. We are thankful for those who shared with us their unpublished data to help us track down pulsators and RV variable stars: W. Zong, S. Geier, F. Mattig, H. Dawson, and V. Schaffenroth. M.L. acknowledges funding from the Deutsche Forschungsgemeinschaft (grant LA 4383/4-1). M.D. was supported by the Deutsches Zentrum f\u00FCr Luft- und Raumfahrt (DLR) through grant 50-OR-2304. V.V.G. is a F.R.S.-FNRS Research Associate. S.C. acknowledge support from the Centre National d\u2019\u00C9tudes Spatiales (CNES, France), focused on the mission TESS. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has made use of IRAF, which was distributed by the National Optical Astronomy Observatory, USA, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. This research has made use of NASA\u2019s Astrophysics Data System Bibliographic Services, of the SIMBAD database (), operated at CDS, Strasbourg, France, and of the VizieR catalogue access tool (), CDS, Strasbourg Astronomical Observatory, France (DOI : 10.26093/cds/vizier). This research has made use of TOPCAT () and the PYTHON packages pandas () and MATPLOTLIB ().
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