A Sun-like star orbiting a black hole

We report discovery of a bright, nearby (G = 13.8; d = 480 pc) Sun-like star orbiting a dark object. We identified the system as a black hole candidate via its astrometric orbital solution from the Gaia mission. Radial velocities validated and refined the Gaia solution, and spectroscopy ruled out significant light contributions from another star. Joint modelling of radial velocities and astrometry constrains the companion mass of M-2 = 9.62 +/- 0.18 M-circle dot. The spectroscopic orbit alone sets a minimum companion mass of M-2 > 5 M-circle dot; if the companion were a 5 M-circle dot star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the mass of the luminous star, which appears as a slowly rotating G dwarf (T-eff = 5850 K, log g = 4.5, M = 0.93 M-circle dot), with near-solar metallicity ([Fe/H] = -0.2) and an unremarkable abundance pattern. We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole. The orbital period, P-orb = 185.6 d, is longer than that of any known stellar-mass black hole binary. The system's modest eccentricity (e = 0.45), high metallicity, and thin-disc Galactic orbit suggest that it was born in the Milky Way disc with at most a weak natal kick. How the system formed is uncertain. Common envelope evolution can only produce the system's wide orbit under extreme and likely unphysical assumptions. Formation models involving triples or dynamical assembly in an open cluster may be more promising. This is the nearest known black hole by a factor of 3, and its discovery suggests the existence of a sizable population of dormant black holes in binaries. Future Gaia releases will likely facilitate the discovery of dozens more.

Automated summary

We have discovered a bright and nearby Sun-like star orbiting a dark object. The system has been identified as a black hole candidate through astrometric orbital analysis from the Gaia mission. The modeling of radial velocities and astrometry provides constraints on the companion mass, and the spectroscopic orbit sets a minimum companion mass. This discovery suggests the existence of a sizable population of dormant black holes in binaries.