[Paper Review] NGC 1850 BH1 is another stripped-star binary masquerading as a black hole

Kareem El-Badry, Kevin B. Burdge|arXiv (Cornell University)|Nov 15, 2021

Stellar, planetary, and galactic studies28 references59 citations

TL;DR

The paper argues that NGC 1850 BH1, previously proposed as a 11 M⊙ black hole candidate, is instead a massive binary system hosting a stripped, low-mass (≈1 M⊙) star in a 5.04-day orbit with a normal main-sequence companion (2.5–5 M⊙). This reinterpretation arises from the star’s low density—implied by its ellipsoidal variability and orbital period—ruling out a 5 M⊙ subgiant and showing the system can be explained without invoking a black hole.

ABSTRACT

We show that the radial velocity-variable star in the black hole candidate NGC 1850 BH1 cannot be a normal $\approx 5\,M_{\odot}$ 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.

Motivation & Objective

• Re-evaluate the nature of the unseen companion in NGC 1850 BH1, previously inferred to be a 11 M⊙ black hole.
• Challenge the assumption that the radial velocity-variable star is a normal 5 M⊙ subgiant based on its observed radius and orbital period.
• Assess whether the system's photometric and spectroscopic data are consistent with a normal main-sequence companion instead of a black hole.
• Investigate evolutionary pathways that could produce a bloated, low-mass stripped star in a close binary system.
• Use the cluster’s known age to constrain the initial mass of the donor star and test binary evolution models.

Proposed method

• Apply the orbital period–density relation for Roche lobe-filling stars to infer the donor’s density from ellipsoidal variability amplitudes.
• Use HST/WFC3 photometry and MIST isochrones to constrain the donor’s effective temperature, radius, and mass, accounting for extinction and metallicity.
• Model the system’s evolution using MESA, simulating mass transfer in an Algol-type binary with initial masses of 4.8 M⊙ and 3 M⊙ and an initial period of 5 days.
• Test the consistency of the observed orbital period, donor radius, and mass function with a normal main-sequence companion.
• Assess the impact of unresolved luminous companions on photometric and spectroscopic measurements.
• Compare the system’s properties to those of LB-1 and HR 6819, which were similarly misclassified as black hole candidates.

Experimental results

Research questions

• RQ1 Can the observed radial velocity variability and ellipsoidal light curve of NGC 1850 BH1 be explained by a low-mass, stripped star rather than a 5 M⊙ subgiant?
• RQ2 What is the maximum possible mass of the donor star given its observed radius and orbital period, assuming Roche lobe overflow?
• RQ3 Is a normal main-sequence companion (2.5–5 M⊙) consistent with the observed mass function, radial velocity amplitude, and lack of X-ray emission?
• RQ4 What evolutionary pathway can produce a bloated, low-mass, helium-core star in a 5-day binary system?
• RQ5 Why does NGC 1850 BH1 lack emission lines seen in similar systems like LB-1 and HR 6819, and what does this imply about mass transfer and spin-up?

Key findings

• The donor star in NGC 1850 BH1 has a radius of 5.8 ± 0.3 R⊙, implying a density too low to be consistent with a 5 M⊙ subgiant.
• The star’s mass is constrained to ≈1 M⊙, not 5 M⊙, based on its radius and orbital period, ruling out a normal subgiant evolution.
• The implied companion mass is reduced to (2.5–5) M⊙, which is fully consistent with a normal main-sequence star and rules out a 11 M⊙ black hole.
• MESA simulations show that a 4.8 M⊙ primary can lose most of its envelope via conservative mass transfer, becoming a bloated, low-mass, core helium-burning sdOB star.
• The system’s properties are best explained by a post-mass-transfer binary in a state similar to LB-1 and HR 6819, but without a detectable disk or emission lines.
• The lack of X-ray emission is consistent with a non-accreting companion, and the absence of emission lines may reflect insufficient spin-up or a transient disk.