The Detection and Characterization of Be+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.

B型发射线星（Be星）是一类快速旋转天体，其快速旋转大概率由密近双星系统中通过物质传递产生的质量与角动量吸积作用加速形成。物质传递过程会剥离供体星的包层，最终形成小型炽热的亚矮星残余天体。本研究报道了基于哈勃空间望远镜（Hubble Space Telescope）搭载的STIS（空间望远镜成像摄谱仪，Space Telescope Imaging Spectrograph）对一组Be星样本开展的远紫外光谱观测结果：通过计算观测光谱与理论模型光谱的互相关函数，我们证实了样本中存在炽热的sdO型亚矮星（subdwarf O-type star）伴星。在本次研究的13颗样本恒星中，我们于10颗中清晰探测到了sdO型亚矮星的光谱特征，且这些光谱信号表明，sdO型亚矮星正如理论模型所预测的那样，处于高温、相对暗弱且慢速旋转的状态。将其温度与半径同恒星演化轨迹进行对比后可知，这些sdO型亚矮星正处于寿命相对较长的氦核燃烧阶段。在本次研究开展前，10颗探测到伴星的样本中仅1颗曾被认定为双星系统，这凸显了利用光学光谱法搜寻此类Be+sdO双星系统的难度。然而，本研究结果与其他同类研究均表明，多数Be星大概率都携带有炽热亚矮星伴星。