HR 6819 is a binary system with no black hole -- revisiting the source with infrared interferometry and optical integral field spectroscopy

TL;DR

This study uses high-resolution infrared interferometry and optical spectroscopy to determine that HR 6819 is a binary star system without a black hole, challenging previous models suggesting a black hole companion.

Contribution

The paper provides the first direct observational evidence that HR 6819 is a binary system with no black hole, using combined interferometric and spectroscopic data.

Findings

HR 6819 is a binary system with a stellar companion at ~1.2 mas separation.

No evidence of a black hole in HR 6819 from high-angular resolution observations.

The companion's orbit is consistent with the known 40-day period.

Abstract

Two scenarios have been proposed to match the existing observational constraints of the object HR 6819. The system could consist of a close inner B-type giant plus a black hole (BH) binary with an additional Be companion in a wide orbit. Alternatively, it could be a binary composed of a stripped B star and a Be star in a close orbit. Either scenario makes HR 6819 a cornerstone object as the stellar BH closest to Earth, or as an example of an important transitional, non-equilibrium phase for Be stars with solid evidence for its nature. We aimed to distinguish between the two scenarios for HR 6819. Both models predict two luminous stars but with very different angular separations and orbital motions. Therefore, the presence of bright sources in the 1-100 milliarcsec (mas) regime is a key diagnostic for determining the nature of the HR 6819 system. We obtained new high-angular resolution data with VLT/MUSE and VLTI/GRAVITY of HR 6819. The MUSE data are sensitive to bright companions at large scales, whilst the interferometric GRAVITY data are sensitive down to separations on mas scales and large magnitude differences. The MUSE observations reveal no bright companion at large separations and the GRAVITY observations indicate the presence of a stellar companion at an angular separation of ~1.2 mas that moves on the plane of the sky over a timescale compatible with the known spectroscopic 40-day period. We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system for quantitatively characterising the immediate outcome of binary interaction and probing how Be stars form.