%0 Journal Article
%T A naked-eye triple system with a nonaccreting black hole in the inner binary
%A D. Baade
%A M. Heida
%A P. Hadrava
%A R. Klement
%A Th. Rivinius
%J -
%D 2020
%R 10.1051/0004-6361/202038020
%X 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<sup>£¿1<sup/> of the inner star and its minimum (probable) mass of 5.0 <i>M<i/><sub>¡Ñ<sub/> (6.3£¿¡À£¿0.7 <i>M<i/><sub>¡Ñ<sub/>) imply a mass of the unseen object of ¡Ý4.2 <i>M<i/><sub>¡Ñ<sub/> (¡Ý5.0£¿¡À£¿0.4 <i>M<i/><sub>¡Ñ<sub/>), that is, a black hole (BH). The spectroscopic time series is stunningly similar to observations of LB-1. A similar triple-star architecture of LB-1 would reduce the mass of the BH in LB-1 from ¡«70 <i>M<i/><sub>¡Ñ<sub/> to a level more typical of Galactic stellar remnant BHs. The BH in HR 6819 probably is the closest known BH to the Sun, and together with LB-1, suggests a population of quiet BHs. Its embedment in a hierarchical triple structure may be of interest for models of merging double BHs or BH + neutron star binaries. Other triple stars with an outer Be star but without BH are identified; through stripping, such systems may become a source of single Be stars
%U https://www.aanda.org/articles/aa/full_html/2020/05/aa38020-20/aa38020-20.html