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Abstract Preliminary astrometric data from the fourth data release of the Gaia mission revealed a 33M⊙dark companion to a metal-poor red giant star, deemed Gaia BH3. This system hosts both the most massive known stellar-origin black hole and the lowest-metallicity star yet discovered in orbit around a black hole. The formation pathway for this peculiar stellar–black hole binary system has yet to be determined, with possible production mechanisms that include isolated binary evolution and dynamical capture. The chemical composition of the stellar companion in Gaia BH3 (hereafter BH3⋆) can help constrain the potential formation mechanisms of this system. Here, we conduct the most comprehensive chemical analysis of BH3⋆ to date using high resolution spectra obtained by the Tull Coudé Spectrograph on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory to constrain potential formation mechanisms. We derived 29 elemental abundances ranging from lithium to thorium and find that BH3⋆ is anα-enriched ([α/Fe] = 0.41), r-I neutron-capture star ([Eu/Fe] = 0.57). We conclude that BH3⋆ shows no chemical peculiarities (defined as deviations from the expected chemical pattern of an r-I halo red giant) in any elements, which is in alignment with both the dynamical capture and isolated binary evolution formation scenarios. With an upper limit detection on Th, we use the Th/Eu chronometer to place limits on the cosmochronometric age of this system. These observations lay the groundwork for heavy-element chemical analysis for subsequent black hole and low-metallicity stellar binaries that will likely be found in Gaia DR4.
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Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry
Context. Gravitational waves from black-hole (BH) merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models - and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Aims: Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. Methods: As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. Results: The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 ± 0.82 M⊙ BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. Conclusions: The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way. Full Table B.1 and Table B.2 with Gaia epoch data are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/686/L2
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- Award ID(s):
- 2206828
- PAR ID:
- 10535438
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- EDP Sciences
- Date Published:
- Journal Name:
- Astronomy and astrophysics
- ISSN:
- 1067-8603
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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