The formation of black holes in non-interacting isolated binaries

Context. The black holes discovered using Gaia , especially Gaia BH1 and BH2, have low-mass companions of solar-like metallicity in wide orbits. For standard formation channels of isolated binary evolution that include interactions, this extreme mass ratio is unexpected, especially for orbits of hundreds to thousands of days. Aims. We investigate a non-interacting formation path for isolated binaries to explain the formation of Gaia BH1 and BH2. Methods. We used single star models computed with MESA to constrain the main characteristics of possible progenitors of long-period black hole binaries such as Gaia BH1 and BH2. Then, we incorporated these model grids into the binary population synthesis code POSYDON to explore whether the formation of the observed binaries at solar metallicity is indeed possible. Results. We find that winds of massive stars (≳80 M ⊙ ), especially during the Wolf-Rayet phase, tend to cause a plateau in the relation of the initial stellar mass to final black hole mass (at about 13 M ⊙ in our default wind prescription). However, stellar winds at earlier evolutionary phases are also important at high metallicity, as they prevent the most massive stars from expanding (< 100 R ⊙ ) and filling their Roche lobe. Consequently, the strength of the applied winds affects the range of the final black hole masses in non-interacting binaries, which enables the formation of systems similar to Gaia BH1 and BH2. Conclusions. We deduce that wide binaries with a black hole and a low-mass companion can form at high metallicity without binary interactions. There could be hundreds of such systems in the Milky Way. The mass of the black hole in binaries that evolved through the non-interacting channel might provide insights into the wind strength during the progenitor evolution.