# A Sun-like star orbiting a black hole

**Authors:** Kareem El-Badry, Hans-Walter Rix, Eliot Quataert, Andrew W. Howard, Howard Isaacson, Jim Fuller, Keith Hawkins, Katelyn Breivik, Kaze W. K. Wong, Antonio C. Rodriguez, Charlie Conroy, Sahar Shahaf, Tsevi Mazeh, Frédéric Arenou, Kevin B. Burdge, Dolev Bashi, Simchon Faigler, Daniel R. Weisz, Rhys Seeburger, Silvia Almada Monter  
**arXiv:** [2209.06833](https://arxiv.org/abs/2209.06833)  
**Date:** 2022-09-14  
**Summary page:** https://www.summarizepaper.com/en/paper/2209.06833v1/

## Key points

- A Sun-like star has been discovered orbiting the nearest known black hole by a factor of three
- The system was identified as a black hole candidate via its astrometric orbital solution from the Gaia mission
- Radial velocity monitoring validated and refined the Gaia solution
- Spectroscopy ruled out significant light contributions from another star
- Joint modeling of radial velocities and astrometry constrained the companion mass to $M_2 = 9.8\pm0.2\,M_{\odot}$
- The spectroscopic orbit alone set a minimum companion mass of $M_2>5\,M_{\odot}$
- The orbital period of this system is longer than that of any known stellar-mass black hole binary at 185.6 days with an eccentricity of 0.45
- Its Galactic orbit is typical of thin-disk stars suggesting that it formed in the Milky Way disk with at most a weak natal kick
- Explaining its formation with standard binary evolutionary models is challenging
- This discovery suggests that there may be a sizable population of dormant black holes in binaries waiting to be discovered through future Gaia releases or other surveys

## Summary

A team of astronomers has discovered a Sun-like star orbiting a black hole, which is the nearest known black hole by a factor of three. The system was identified as a black hole candidate via its astrometric orbital solution from the Gaia mission, and radial velocity monitoring validated and refined the Gaia solution. Spectroscopy ruled out significant light contributions from another star, and joint modeling of radial velocities and astrometry constrained the companion mass to $M_2 = 9.8\pm0.2\,M_{\odot}$. The spectroscopic orbit alone set a minimum companion mass of $M_2>5\,M_{\odot}$; if the companion were a $5\,M_{\odot}$ star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the assumed mass of the luminous star, which appears as a slowly-rotating G dwarf with near-solar metallicity and an unremarkable abundance pattern. The orbital period of this system is longer than that of any known stellar-mass black hole binary at 185.6 days with an eccentricity of 0.45. Its Galactic orbit is typical of thin-disk stars suggesting that it formed in the Milky Way disk with at most a weak natal kick. However, explaining its formation with standard binary evolutionary models is challenging: it is difficult for the luminous star to survive a common envelope event under standard assumptions and difficult for it to end up in such wide orbit afterward. This discovery suggests that there may be a sizable population of dormant black holes in binaries waiting to be discovered through future Gaia releases or other surveys. The team found no plausible astrophysical scenario that can explain this orbit without involving a black hole; highlighting how much we still have to learn about these enigmatic objects and their interactions with their stellar companions.