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

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.

Automated summary

The B emission-line stars are rapid rotators likely spun up by mass and angular momentum accretion through mass transfer in an interacting binary, leading to the formation of hot sdO companions. Observations of a sample of Be stars using Hubble Space Telescope/STIS show the presence of hot sdO stars, which are hot, relatively faint, and slowly rotating, consistent with theoretical predictions. Comparison with evolutionary tracks suggests that the sdO stars are in the long-lived, He-core burning stage.