Testing the presence of a dormant black hole inside HR 6819

TL;DR

This study investigates whether the reported inner binary of HR 6819, potentially hosting a dormant black hole, can be explained by known binary evolution models, concluding it is unlikely under current formation scenarios.

Contribution

The paper provides the first detailed modeling attempt to reproduce HR 6819's inner binary using isolated and dynamical evolution, challenging the black hole interpretation.

Findings

Cannot reproduce HR 6819-like binaries in globular clusters

Unlikely to form HR 6819-like binaries in the Galactic field

Models suggest the dormant black hole hypothesis is inconsistent with standard evolution

Abstract

HR 6819 was recently reported to be a triple system with a dormant black hole (BH). The inner binary system was defined as a star estimated to be at the end of its main sequence and a dormant BH. As the inner binary is not resolved, the third component may actually just be spatially coinciding with the inner binary. In this study we test whether the system inner binary can be reconstructed using the isolated binary evolution in the Galactic field or through the dynamical evolution within globular star clusters. Our goal is to understand the formation of the HR 6819 inner binary. To simulate the inner binary evolution we assumed that the influence of the third body on the inner binary is negligible. We created synthetic populations of BH main-sequence binaries for the Galactic disc and the Galactic globular clusters to compare to the reported parameters of the HR 6819 inner binary. We have adopted very optimistic input physics, in terms of common envelope evolution and BH formation, for the formation of binaries similar to the reported inner HR 6819 binary. Despite our optimistic assumptions we cannot form systems like the inner HR 6819 binary in globular clusters. Even with our extreme assumptions, the formation of an HR 6819-like binary in the Galactic field population is not expected. We argue that if a dormant BH actually exists in the reported configuration inside HR 6819, its presence cannot easily be explained by our models based on isolated and dynamical binary evolution.