Radial velocity variability fractions of different types of hot subdwarf stars

TL;DR

This study investigates the radial velocity variability among different types of hot subdwarf stars, revealing correlations with temperature, surface gravity, and spectral features, and highlighting differences between single-lined and composite systems.

Contribution

It provides the first comprehensive analysis of RV variability fractions across various hot subdwarf subclasses, linking variability to stellar properties and binary status.

Findings

Single-lined He-rich hot subdwarfs show lower RV variability than He-poor ones.

RV variability fraction peaks at 51% in hot subdwarfs below the EHB.

Composite hot subdwarfs exhibit significantly lower RV variability, likely due to observational limitations.

Abstract

Different types of hot subdwarfs may have different origins, which will cause them to present different radial velocity (RV) variability properties. Only 6$\pm$4% of our single-lined He-rich hot subdwarfs that only show spectroscopic features of hot subdwarfs are found to be RV variable, which is lower than the fraction of single-lined He-poor sdB stars (31$\pm$3%). Single-lined sdB stars with effective temperatures ($T_{\rm eff}$) $\sim$ 25,000 $-$ 33,000 K show an RV-variability fraction of 34$\pm$5%, while lower RV-variability fractions are observed for single-lined sdB stars cooler than about 25,000 K (11$\pm$4%), single-lined sdB/OB stars with $T_{\rm eff}$ $\sim$ 33,000 $-$ 40,000 K and surface gravities about 5.7 $-$ 6.0 (13$\pm$3%), as well as single-lined sdO/B stars with $T_{\rm eff}$ $\sim$ 45,000 $-$ 70,000 K (10$\pm$7%). Single-lined hot subdwarfs with $T_{\rm eff}$ $\sim$ 35,000 $-$ 45,000 K located above the extreme horizontal branch (EHB) show a similar RV-variability fraction of 34$\pm$9% as single-lined sdB stars at about 25,000 $-$ 33,000 K. The largest RV-variability fraction of 51$\pm$8% is found in single-lined hot subdwarfs below the canonical EHB. The detected RV-variability fraction of our composite hot subdwarfs with an infrared excess in their spectral energy distributions is 9$\pm$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.