Letter to the Editor

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

¹ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile
e-mail: triviniu@eso.org
² European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
³ Astronomical Institute, Academy of Sciences of the Czech Republic, Boční II 1401, 141 31 Praha 4, Czech Republic
⁴ The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA

Received: 25 March 2020
Accepted: 22 April 2020

Abstract

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 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 M_⊙ 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.