Be星的剥离伴星起源：来自假定黑洞HR 6819和LB-1的线索
A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1
摘要：
HR 6819是一颗明亮的蓝色恒星（$V=5.36$），最近被认为是一个包含分离黑洞（BH）的三重星系。我们表明，该系统是一个二进制，不包含BH。利用光谱分解，我们将观测到的复合光谱分解为两个部分：一个快速旋转的Be星和一个缓慢旋转的低表面重力的B星$(\log g \approx 2.75)$。这两颗星都表现出周期性的视向速度（RV）变化，但B星轨道的RV半振幅为$K_{\rm B}= (62.7 \pm 1)\,\rm km\,s^{-1}$，而Be星的RV半振幅只有$K_{\rm Be} = (4.5\pm 2)\,\rm km\,s^{-1}$。这意味着B星的质量至少小10倍。B星的表面丰度带有CNO燃烧的痕迹。我们认为，B星是一个膨胀，最近剥离氦星的质量$\approx 0.5\,M_{\odot}$，目前正在收缩成为一个热的亚矮星。Be星的轨道运动使得不需要BH来解释B星的运动。条纹星模型再现了该系统的观测亮度，而具有B星温度和重力的正常恒星的亮度将高出10倍以上。HR 6819和LB-1可能是通过类似的通道形成的。我们使用MESA模型来研究它们的进化历史，发现它们可能是由中等质量（$3-7\,M_{\odot}$）的初级恒星被质量稍低的次级恒星剥离而形成的，并且是Be + sdOB双星如$ϕ$ Persei的祖先。它们当前演化阶段的寿命平均为$2\times 10^5$年，约为Be阶段总寿命的0.5%。这意味着许多Be星都有热的亚矮星和白矮星伴星，并且相当大一部分（$20-100\%$）的Be场星是通过吸积来自双星伴星的物质形成的。
英文摘要：
HR 6819 is a bright ($V=5.36$), blue star recently proposed to be a triple containing a detached black hole (BH). We show that the system is a binary and does not contain a BH. Using spectral decomposition, we disentangle the observed composite spectra into two components: a rapidly rotating Be star and a slowly rotating B star with low surface gravity $(\log g \approx 2.75)$. Both stars show periodic radial velocity (RV) variability, but the RV semi-amplitude of the B star's orbit is $K_{\rm B}= (62.7 \pm 1)\,\rm km\,s^{-1}$, while that of the Be star is only $K_{\rm Be} = (4.5\pm 2)\,\rm km\,s^{-1}$. This implies that the B star is less massive by at least a factor of 10. The surface abundances of the B star bear imprints of CNO burning. We argue that the B star is a bloated, recently stripped helium star with mass $\approx 0.5\,M_{\odot}$ that is currently contracting to become a hot subdwarf. The orbital motion of the Be star obviates the need for a BH to explain the B star's motion. A stripped-star model reproduces the observed luminosity of the system, while a normal star with the B star's temperature and gravity would be more than 10 times too luminous. HR 6819 and the binary LB-1 probably formed through similar channels. We use MESA models to investigate their evolutionary history, finding that they likely formed from intermediate-mass ($3-7\,M_{\odot}$) primaries stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB binaries such as $ϕ$ Persei. The lifetime of their current evolutionary phase is on average $2\times 10^5$ years, of order half a percent of the total lifetime of the Be phase. This implies that many Be stars have hot subdwarf and white dwarf companions, and that a substantial fraction ($20-100\%$) of field Be stars form through accretion of material from a binary companion.
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