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

TL;DR

This study uses HST/STIS far-ultraviolet spectroscopy to detect hot subdwarf companions in Be star binaries, revealing that many Be stars likely have such companions which are hard to detect with optical methods.

Contribution

The paper demonstrates a new method for detecting hot sdO companions in Be stars using FUV spectroscopy, significantly increasing known binary detections.

Findings

sdO companions detected in 10 of 13 Be stars

sdO stars are hot, faint, and slowly rotating

Most Be stars may host undetected hot subdwarf companions

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 one of the ten 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.