Radial velocity variability fractions of different types of hot subdwarf stars

Context. It is generally thought that hot subdwarfs are helium-core- or helium-shell-burning objects with extremely thin hydrogen envelopes and that binary interactions are always needed in their formation. Different types of hot subdwarfs may have different origins, which will cause them to present different radial velocity (RV) variability properties. Aims. We plan to study the RV-variability fractions of different types of hot subdwarfs, as well as their distributions in the T eff − log g diagram. This provides insights into the formation of hot subdwarfs. Methods. The cross-correlation function method was adopted to measure RV variations in 434 hot subdwarfs based on spectra obtained with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Light curves from the Transiting Exoplanet Survey were also used to search for short-period binary hot subdwarfs. Results. Only 6 ± 4% of our single-lined He-rich hot subdwarfs that only show spectroscopic features of hot subdwarfs are found to be RV variable, which is lower than the fraction of single-lined He-poor sdB stars (31 ± 3%). Single-lined sdB stars with effective temperatures ( T eff ) ∼ 25 000–33 000 K show an RV-variability fraction of 34 ± 5%, while lower RV-variability fractions are observed for single-lined sdB stars cooler than about 25 000 K (11 ± 4%), single-lined sdB/OB stars with T eff ∼ 33 000–40 000 K and surface gravities ∼ 5.7–6.0 (13 ± 3%), as well as single-lined sdO/B stars with T eff ∼ 45 000–70 000 K (10 ± 7%). Single-lined hot subdwarfs with T eff ∼ 35 000–45 000 K located above the extreme horizontal branch (EHB) show a similar RV-variability fraction of 34 ± 9% as single-lined sdB stars at about 25 000–33 000 K. The largest RV-variability fraction of 51 ± 8% is found in single-lined hot subdwarfs below the canonical EHB. The detected RV-variability fraction of our composite hot subdwarfs with an infrared excess in their spectral energy distributions is 9 ± 3%, which is lower than that fraction of single-lined hot subdwarfs. Since the average RV uncertainty we measured in the LAMOST spectra is about 7.0 km/s, the lower detected RV-variability fraction for composite hot subdwarfs is expected because the RV amplitudes associated with long-period systems are lower. Conclusions. The results here are generally consistent with the canonical binary evolution channels for forming hot subdwarfs. Most single-lined He-rich hot subdwarfs may form through merger channels, while the stable Roche-lobe overflow channel could play an important role in the formation of composite hot subdwarfs. Single-lined hot subdwarfs with T eff ∼ 35 000–45 000 K located above the EHB may have an evolutionary connection to the sdB stars at about 25 000–33 000 K. The different detected RV-variability fractions for the different subclasses of single-lined hot subdwarfs indicate that their formation channels may differ.