Detection and characterization of hot subdwarf companions of rapidly rotating Be stars

Rapidly rotating Be stars are suggested to be product of mass transfer through close binary interaction. The initial more massive star inflated and evolved away from the main sequence, transferring its mass and angular momentum to the gainer star under Roche-lobe overflow. The formal donor stars are now observed as the faint, hot, and envelope stripped subdwarf (sdO) stars. We report the detection and characterization of 13 Be+sdO candidate binary systems using the Space Telescope Imaging Spectrograph from Hubble Space Telescope. These candidate systems were previously identified from International Ultraviolet Explorer through FUV spectroscopy. Applying the cross-correlation algorithm, we confirm the detection of sdOs in ten Be+sdO binary systems. The cross-correlation functions (CCFs) of the detections display composite features of a broad component contributed from the Be star and a sharp peak associated with the sdO companion. We measured the radial velocity (RV) of the Be component for all candidate systems. CCFs of the sdO stars were obtained from a direct subtraction of Be contribution from the composite features. We also measured the RVs, effective temperature (Teff), and Vsini of the sdO stars through spectral model fitting. Combined the FUV observations with archived optical photometric measurements, we have estimated the radii of each stellar component for all 13 candidate Be+sdO binary systems. Compared the measured Teff and radius of the sdO stars with evolutionary tracks, the model suggests that most of the sdOs in these binary systems are at a He-core burning stage. Future optical observations will be needed to constrain the orbits and masses of these binary systems.