The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy

Abstract

The B emission-line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only 1 of the 10 detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

Velocity monitoring of γ Cas stars reveals their binarity status

“…Furthermore, in observations, there may be hints of a slight trend of increasing secondary masses with larger primary masses, a trend which is expected in stripped-star evolutionary models (Shao & Li 2021). Finally, the range of observed masses is compatible with that expected for stripped stars (Shao & Li 2014;Wang et al 2021).…”

“…Sana et al 2012). Also, such low-mass stars should not display the UV signatures detected for some of the systems listed at the bottom of Table 4; rather, this type of signature is expected for hot stripped stars (Wang et al 2018(Wang et al , 2021. Furthermore, in observations, there may be hints of a slight trend of increasing secondary masses with larger primary masses, a trend which is expected in stripped-star evolutionary models (Shao & Li 2021).…”

“…Cracco et al 2018;Kennea et al 2021). In parallel, the UV radiation of hot, stripped He-stars allowed to detect such companions, but only for a few Be stars as the detection is here also very challenging (Wang et al 2018(Wang et al , 2021. Finally, while orbits could be established for some Be binaries (see Sect.…”

“…(2012) andNemravová et al (2012) are similar -only the latter is quoted here. Velocity shifts have also been recorded for the secondary in HD 194335, although a full orbital coverage has not yet been acquired(Wang et al 2021) hence we do not yet add it to this Table.…”

Ultraviolet Spectropolarimetry:Conservative and Nonconservative Mass Transfer in OB Interacting Binaries

“…These systems are relevant to the current Polstar objective S4 as providing context for the final stages of a nonconservative transfer process happening earlier, but they are also relevant to another objective, S3, which tests the hypothesis that classical Be stars are an intermediate stage of binary mass transfer that eventually produces a BeS object (Jones et al 2021). The sdO object (cf., Wang et al 2021, and references therein) are thought to be the stripped down, CNO processed core of the mass loser that transferred significant mass and angular momentum to the Be star. Consider the following scenario.…”