Testing the presence of a dormant black hole inside HR 6819

Abstract

Context. HR 6819 was recently reported to be a triple system with a non-accreting black hole (BH). The inner binary system was defined as a B3 III type star (a 5 − 7 M⊙ star estimated to be at the end of its main sequence) and a dormant BH (> 4.2 M⊙). The period of the inner binary was estimated to be ∼40 days with an eccentricity in the range 0.02 − 0.04. As the inner binary is not resolved, the third component may actually just be spatially coinciding with the inner binary. Aims. In this study we test whether the system’s 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. Methods. 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. Results. 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. Conclusions. 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.