http://arxiv.org/abs/2111.07925
We show that the radial velocity-variable star in the black hole candidate NGC 1850 BH1 cannot be a normal $\sim 5\,M_{\odot}$ subgiant, as was proposed, but is an overluminous stripped-envelope star with mass $\sim 1 M_{\odot}$. The result follows directly from the star’s observed radius and the orbital period — density relation for Roche lobe-filling stars: the star’s density, as constrained by the observed ellipsoidal variability, is too low for its mass to exceed $\sim 1.5\,M_{\odot}$. This lower mass significantly reduces the implied mass of the unseen companion and qualitative interpretation of the system, such that a normal main-sequence companion with mass $(2.5-5)\,M_{\odot}$ is fully consistent with the data. We explore evolutionary scenarios that could produce the binary using MESA and find that its properties can be matched by models in which a $\sim5\,M_{\odot}$ primary loses most of its envelope to a companion and is observed in a bloated state before contracting to become a core helium burning sdOB star. This is similar to the scenario proposed to explain the binaries LB-1 and HR 6819. Though it likely does not contain a black hole, NGC 1850 BH1 provides an interesting test case for binary evolution models, particularly given its membership in a cluster of known age.
K. El-Badry and K. Burdge
Tue, 16 Nov 21
75/97
Comments: 5 pages, 3 figures, comments welcome. Submitted to MNRAS letters