Detection of additional Be+sdO systems from IUE spectroscopy

TL;DR

This study expands the search for Be stars with hot subdwarf companions using archival IUE spectra, identifying 12 new candidates and analyzing their properties and orbital motions.

Contribution

It introduces a new method of using archival FUV spectra to identify Be+sdO systems, increasing the known sample and analyzing their spectral types and orbital characteristics.

Findings

12 new Be+sdO candidates identified

8 candidates show radial velocity variations indicating orbital motion

Surprising lack of later-type Be+sdO systems in the sample

Abstract

There is growing evidence that some Be stars were spun up through mass transfer in a close binary system, leaving the former mass donor star as a hot, stripped-down object. There are five known cases of Be stars with hot subdwarf (sdO) companions that were discovered through International Ultraviolet Explorer (IUE) spectroscopy. Here we expand the search for Be+sdO candidates using archival FUV spectra from IUE. We collected IUE spectra for 264 stars and formed cross-correlation functions (CCFs) with a model spectrum for a hot subdwarf. Twelve new candidate Be+sdO systems were found, and eight of these display radial velocity variations associated with orbital motion. The new plus known Be+sdO systems have Be stars with spectral subtypes of B0 to B3, and the lack of later-type systems is surprising given the large number of cooler B-stars in our sample. We discuss explanations for the observed number and spectral type distribution of the Be+sdO systems, and we argue that there are probably many Be systems with stripped companions that are too faint for detection through our analysis.