A naked-eye triple system with a nonaccreting black hole in the inner binary

  title={A naked-eye triple system with a nonaccreting black hole in the inner binary},
  author={Th. Rivinius and Dietrich Baade and Petr Hadrava and Marianne Heida and R{\'o}bert Klement},
  journal={Astronomy \& Astrophysics},
Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple. A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit. The radial-velocity semi-amplitude of 61.3 km s−1 of the inner star and its minimum (probable) mass of 5.0 M⊙ (6.3 ± 0.7 M⊙) imply a mass of the unseen object of ≥4.2 M⊙ (≥5.0 ± 0.4 M⊙), that is, a black hole (BH). The spectroscopic time… 
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