NGC 1850 BH1 is another stripped-star binary masquerading as a black hole

Abstract

We show that the radial velocity-variable star in the black hole candidate NGC 1850 BH1 cannot be a normal $\approx 5M_{\odot }$ subgiant, as was proposed, but is an overluminous stripped-envelope star with mass $\approx 1M_{\odot }$. 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 $\approx 1.5M_{\odot }$. 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 $\sim 5M_{\odot }$ 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.

5 pages, 3 figures. Accepted to MNRAS Letters

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