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

TL;DR

The star in NGC 1850 BH1 is a stripped-envelope star with lower mass than previously thought, which challenges its classification as a black hole candidate and suggests a different binary evolution scenario.

Contribution

This paper demonstrates that NGC 1850 BH1 is a stripped star, not a black hole, and models its evolution using MESA to explain its properties within known binary evolution pathways.

Findings

The star's density is too low for its mass, indicating it is a stripped-envelope star.

The companion's mass is consistent with a normal main-sequence star, not a black hole.

Evolutionary models match the observed properties with a primary losing its envelope before contracting.

Abstract

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