Radial velocity variability and evolution of hot subdwarf stars

Abstract

Hot subdwarf stars represent a late and peculiar stage in the evolution of low-mass stars, because they are likely formed by close binary interactions. Here we performed a radial velocity (RV) variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectra. Atmospheric parameters and RVs were taken from the literature. For stars with archival spectra but without literature values, we determined the parameters by fitting model atmospheres. In addition, we redetermined the atmospheric parameters and RVs for all the He-enriched sdO/Bs. This large sample allowed us to study RV-variability as a function of the location in the T eff − log gand T eff − log n(He)/n(H) diagrams in a statistically significant way. As diagnostics we used the fraction of RV-variable stars and the distribution of the maximum RV variations ∆RV max . Both indicators turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely different behaviour of He-poor and He-rich hot subdwarfs. While the former have a high fraction of close binaries, almost no significant RV variations could be detected for the latter. This led us to the conclusion that there likely is no evolutionary connection between these subtypes. Intermediate He-rich-and extreme He-rich sdOB/Os on the other hand are likely related. We conclude further that the vast majority of this population is formed via one or several binary merger channels. Hot subdwarfs with temperatures cooler than ∼ 24 000 K tend to show less and smaller RV-variations. These objects might constitute a new subpopulation of binaries with longer periods and late-type or compact companions. The RV-variability properties of the extreme horizontal branch (EHB) and corresponding post-EHB populations of the He-poor hot subdwarfs match and confirm the predicted evolutionary connection between them. Stars found below the canonical EHB at somewhat higher surface gravities show large RV-variations and a high RV-variability fraction, which is consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries.

Hot Subdwarf Stars Identified in Gaia DR2 with Spectra of LAMOST DR6 and DR7. I. Single-lined Spectra

“…Hence, we could not obtain a deeper conclusion on this field before the binary nature of the sample are confirmed. Geier et al (2022) studied radial velocity (RV) variability for a sample of 646 hot subdwarfs with multi-epoch radial velocities from SDSS and LAMOST. They found that only a small fraction (e.g., ≈ 3%, see Section 3 and Fig 2 in their study) of He-rich hot subdwarf stars (i.e., log(nHe/nH) -1) show RV variability, while a much higher fraction of He-poor hot subdwarf stars (i.e., log(nHe/nH) <-1) presents RV variability (e.g., ≈ 30%).…”

“…Note that, all the stars analyzed by Geier et al (2022) were single-lined hot subdwarf stars, while composite hot subdwarfs were not included. We cross-matched our 131 composite hot subdwarfs identified with LAMOST DR8 LRS Multiple Epoch Catalog, in which 7,912,959 sources with 2 times or more multiple observations were collected, and found 61 common records.…”