A Sun-like star orbiting a black hole (2209.06833v3)

Abstract: We report discovery of a bright, nearby ($G = 13.8;\,\,d = 480\,\rm pc$) 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 modeling of radial velocities and astrometry constrains the companion mass to $M_2 = 9.62\pm 0.18\,M_{\odot}$. The spectroscopic orbit alone sets 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 mass of the luminous star, which appears as a slowly-rotating G dwarf ($T_{\rm eff}=5850\,\rm K$, $\log g = 4.5$, $M=0.93\,M_{\odot}$), with near-solar metallicity ($\rm [Fe/H] = -0.2$) 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, $P_{\rm orb}=185.6$ days, is longer than that of any known stellar-mass black hole binary. The system's modest eccentricity ($e=0.45$), high metallicity, and thin-disk Galactic orbit suggest that it was born in the Milky Way disk 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.

• The paper presents the discovery of Gaia BH1, a binary system where a Sun-like star is gravitationally influenced by a non-accreting black hole with a mass of approximately 9.62 solar masses.
• It details the methodology that combines Gaia astrometric data with radial velocity and spectroscopic measurements to determine the system’s orbital period of 185.6 days and eccentricity of 0.45.
• The study highlights that the findings point to a potentially abundant population of dormant black holes in the galaxy, thereby reshaping our understanding of binary evolution and compact object formation.

Gaia BH1: A Sun-like Star Orbiting a Black Hole

The paper presents the discovery and analysis of a binary system, Gaia BH1, in which a Sun-like star orbits a stellar-mass black hole (BH). The system was identified through its astrometric orbital solution derived from data by the Gaia satellite. This paper is a significant contribution to the effort of understanding the demographics of non-accreting BHs in the galaxy, providing empirical data that informs both the processes of binary star evolution and compact object formation.

Methodology and Observations

The discovery of Gaia BH1 utilized the astrometric capabilities of the Gaia mission. The system was flagged as a potential BH candidate due to its particular orbital characteristics. Complementary follow-up observations, including radial velocity (RV) measurements and spectroscopic analysis, were conducted to confirm the nature of the companion and to refine its mass estimate.

By integrating astrometric and spectroscopic data, the authors characterized the system; the observed G-type star exhibits a clear RV variation, instrumental in calculating the system's mass function and further supporting the presence of a dark, massive companion. Through joint modeling of the vast data spectrum, the mass of the unseen object was constrained to about 9.62 ± 0.18 solar masses, which incontrovertibly suggests a BH nature given the lack of detected luminosity from the companion.

Numerical Results and Analysis

The system's orbital parameters include a period of 185.6 days and an eccentricity of 0.45, features uncommon among known BH binaries. The companion's mass, derived directly from dynamical measurements, effectively rules out any ordinary stellar object given the lack of accompanying light. Furthermore, this mass significantly exceeds the neutron star maximum, leaving a BH as the only plausible matter configuration, especially given that strong efforts to rule out massive star remnants were employed. Importantly, this analysis does not rely on uncertain dynamical models or luminous star mass assumptions, providing a robust case for the first unambiguous detection of a BH in a system non-interacting with its companion.

Implications on Astrophysical Theory

The existence of Gaia BH1, being relatively nearby at approximately 480 parsecs, proposes that such non-accreting BH binaries might be more common in the galaxy than previously thought. The system challenges prevalent theories regarding the formation of BH binaries, particularly scenarios involving common envelope evolution, as traditional models struggle to reproduce such wide orbital separations without leading to a merger.

The discovery implies a broader population of dormant BHs, residing in binaries with normal stars, that could potentially be detected by future astrometric data releases or flagged by complementary observation techniques such as X-ray or radio surveys. It emphasizes the importance of the Gaia mission in unmasking a subpopulation of stellar-mass BHs and opens prospects for unraveling the complexities of binary star evolution pathways.

Conclusion and Future Perspectives

Gaia BH1 presents a novel and compelling case of a dormant BH detected via its gravitational influence on a stellar companion, offering a glimpse into an observationally elusive class of binaries. This detection establishes a precedent for identifying such systems in the Galactic field.

Going forward, expanded Gaia data releases will likely unveil additional BH candidates with longer and highly secure periods, enabling refined understanding and efficiency optimization for BH searches in the Milky Way. The synergy of astrometric precision and supplementary observational strategies promises to elevate our knowledge of BH formation channels and their lifecycles within diverse Galactic environments.