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 ≈ 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 ${\approx}0.5\, \mathrm{ M}_{\odot }$ 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 (Modules for Experiments in Stellar Astrophysics) models to investigate their evolutionary history, finding that they likely formed from intermediate-mass ($3\!-\!7\, \mathrm{ M}_{\odot }$) 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 × 105 yr, of the order of half a per cent 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 ($20\!-\!100{{\ \rm per\ cent}}$) of field Be stars form through accretion of material from a binary companion.

中文翻译：

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Be 星的剥离伴星起源：来自假定黑洞 HR 6819 和 LB-1 的线索

HR 6819 是一颗明亮的 (V = 5.36) 蓝星，最近提出它是一个包含分离黑洞 (BH) 的三重星。我们证明该系统是二进制的并且不包含 BH。使用光谱分解，我们将观察到的复合光谱分解为两个部分：快速旋转的 Be 星和缓慢旋转的低表面重力 (log g ≈ 2.75) 的 B 星。两颗恒星都显示出周期性的径向速度（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 星只有 $K_{\rm Be} = (4.5\pm 2)\, \rm km\, s^{-1}$。这意味着 B 星的质量至少要小 10 倍。B 星的表面丰度带有 CNO 燃烧的印记。我们认为 B 星是一颗膨胀的、最近被剥离的氦星，质量为 ${\approx}0.5\，\mathrm{ M}_{\odot }$ 目前正在收缩成为一颗炽热的亚矮星。Be 星的轨道运动不需要 BH 来解释 B 星的运动。剥离星模型再现了观测到的系统光度，而具有 B 星温度和引力的普通恒星的亮度将超过 10 倍。HR 6819 和二元 LB-1 可能是通过类似的渠道形成的。我们使用 MESA（恒星天体物理学实验模块）模型来研究它们的进化历史，发现它们很可能是由中等质量 ($3\!-\!7\, \mathrm{ M}_{\odot }$) 原色形成的被质量稍低的次级星剥离，是 Be + sdOB 双星的祖先，例如 φ Persei。它们当前进化阶段的寿命平均为 2 × 105 年，占 Be 相总寿命的 0.5%。这意味着许多 Be 星都有炽热的亚矮星和白矮星伴星，并且相当大一部分 ($20\!-\!100{{\ \rm per\cent}}$) 的 Be 星是由来自二进制伴侣。