The boring history of Gaia BH3 from isolated binary evolution

¹ Physics and Astronomy Department Galileo Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122, Padova, Italy
² INFN – Padova, Via Marzolo 8, 35131 Padova, Italy
³ INAF – Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Institut für Theoretische Astrophysik, ZAH, Universität Heidelberg, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
⁵ Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
⁶ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
⁷ SISSA, via Bonomea 365, 34136 Trieste, Italy
⁸ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁹ INAF, Osservatorio Astronomico di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Gran Sasso Science Institute, Via F. Crispi 7, 67100 L’Aquila, Italy
¹¹ INFN – Laboratori Nazionali del Gran Sasso, 67100 L’Aquila, Italy
¹² INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, s.n.c., 64100 Teramo, Italy

^★ Corresponding authors; giuliano.iorio.astro@gmail.com; stefano.torniamenti@uni-heidelberg.de; mapelli@uni-heidelberg.de

Received: 26 April 2024
Accepted: 31 July 2024

Abstract

Gaia BH3 is the first observed dormant black hole (BH) with a mass of ≈30 M_⊙, and it represents the first confirmation that such massive BHs are associated with metal-poor stars. Here, we explore the isolated binary formation channel for Gaia BH3, focusing on the old and metal-poor stellar population of the Milky Way halo. We used the MIST stellar models and our open-source population synthesis code SEVN to evolve 5.6 × 10⁸ binaries, exploring 20 sets of parameters that encompass different natal kicks, metallicities, common envelope efficiencies and binding energies, and models for the Roche-lobe overflow. We find that systems such as Gaia BH3 form preferentially from binaries initially composed of a massive star (40–60 M_⊙) and a low-mass companion (<1 M_⊙) in a wide (P > 10³ days) and eccentric orbit (e > 0.6). Such progenitor binary stars do not undergo any Roche-lobe overflow episode during their entire evolution, so the final orbital properties of the BH-star system are essentially determined at the core collapse of the primary star. Low natal kicks (≲ 10 km/s) significantly favour the formation of Gaia BH3-like systems, but high velocity kicks up to ≈220 km/s are also allowed. We estimated the formation efficiency for Gaia BH3-like systems in old (t >10 Gyr) and metal-poor (Z < 0.01) populations to be ∼4 × 10⁻⁸ M_⊙⁻¹ (for our fiducial model), representing ~3% of the whole simulated BH-star population. We expect up to ≈4000 BH-star systems in the Galactic halo formed through isolated evolution, of which ≈100 are compatible with Gaia BH3. Gaia BH3-like systems represent a common product of isolated binary evolution at low metallicity (Z < 0.01), but given the steep density profile of the Galactic halo, we do not expect more than one at the observed distance of Gaia BH3. Our models show that even if it was born inside a stellar cluster, Gaia BH3 is compatible with a primordial binary star that escaped from its parent cluster without experiencing significant dynamical interactions.