The role of accretion efficiency, natal kicks, and angular momentum transport in the formation of the Gaia black holes

TL;DR

This study uses population synthesis to explore how accretion efficiency, natal kicks, and angular momentum transport influence Gaia black hole formation, matching observed properties and predicting future discoveries.

Contribution

It identifies key physical assumptions that maximize the likelihood of forming Gaia-like dormant black hole systems consistent with current observations.

Findings

Models with non-conservative mass transfer and wind-driven angular momentum loss best match observed Gaia BHs.

Models with low common-envelope ejection efficiency predict orbital periods too short compared to observations.

Large natal kicks are favored by observed eccentricities, and low-kick models predict many long-period, low-eccentricity BHs for future Gaia data.

Abstract

Gaia has the potential to deliver several tens of new dormant black holes (BHs) with low-mass stellar companions (hereafter, Gaia BHs) in the upcoming fourth data release. Three Gaia BHs are already known, but their formation pathways remain uncertain. Here, we perform a large parametric study to explore the formation of Gaia BHs from isolated binary systems with the population-synthesis code SEVN and compare our models with the properties of the three already reported Gaia BHs. Specifically, we explore the impact of accretion efficiency, mass transfer stability, natal kicks, angular momentum transport, and core-collapse supernova prescriptions. We find that models in which stable mass transfer is highly non-conservative and angular momentum is lost as a wind from the donor surface (Jeans mode) maximize the probability of forming dormant systems that match the properties of the observed Gaia BHs in terms of both orbital period and eccentricity, because such assumptions prevent the initial orbit from shrinking too much when the BH progenitor fills its Roche lobe. If we allow for common-envelope evolution, we find that models with common-envelope ejection efficiency $\alpha{} < 1$ predict dormant systems with orbital periods that are too short compared to the observed Gaia BHs. The eccentricity of the observed Gaia BHs, when combined with information about orbital period and BH mass, favors relatively large natal kicks, similar to those inferred from Galactic neutron stars. Finally, models in which the natal kicks are low - e.g. because they are modulated by fallback - result in the formation of a large population of dormant BHs with long orbital periods ($P_{\rm orb}>10^4$ days) and low eccentricity, which will be tested soon by the fourth Gaia data release.