TEST

[Torbjörn Svensson Diaz]

HR 6819 is a hierarchical triple containing a classical Be star in a
wide orbit of unknown period around an inner 40.3 day binary, a B3 III
star and a non-emitting (non-accreting) black hole (≥ 5±0.4 M☉),
designated Ab.[7]

HR 6819 was regarded as a single star,[10] until astronomer Monika
Maintz concluded the spectrum contained the signatures of two stars in
2009. However extensive analysis was prevented by limited observations.
This took place after more extensive radial velocity measurements by
Thomas Rivinius and colleagues, suggesting the presence of an unseen
stellar-mass black hole within the system.[7] Though the HR 6819 system
has been described as a member of the Sco OB2 association of co-moving
stars,[11] it has more recently been suggested to be an older system and
not part of the association.[7]

The spectrum of HR 6819 contains both narrow and broadened lines. The
broad lines originate from the rapidly-rotating Be star, while the
narrow lines are from a more slowly-rotating B-class giant. The radial
velocity variations of the lines indicate that the normal B giant is in
a 40-day orbit, but not with the Be star. Therefore, there is a third,
invisible, body in the system, the other component in the 40-day orbit.
Analysis of the orbital parameters suggests that the third body is
sufficiently massive that it can only be a black hole.[7]