A Triple Scenario for the Formation of Wide Black Hole Binaries Such as Gaia BH1

Abstract

Recently, several noninteracting black hole–stellar binaries have been identified in Gaia data—for example, Gaia BH1, where a Sun-like star is in a moderately eccentric (e = 0.44) 185 days orbit around a black hole. This orbit is difficult to explain through binary evolution. The present-day separation suggests the progenitor system would have undergone an episode of common-envelope evolution, but a common envelope should shrink the period below the observed one. Since the majority of massive stars form in higher-multiplicity systems, a triple evolution scenario is more likely for the progenitors of BH binaries. Here we show that such systems can indeed be more easily explained via evolution in hierarchical triple systems. von Zeipel–Lidov–Kozai oscillations or instabilities can delay the onset of the common-envelope phase in the inner binary of the triple, so that the black hole progenitor and low-mass star are more widely separated when it begins, leading to the formation of wider binaries. There are also systems with similar periods but larger eccentricities, where the BH progenitor is a merger product of the inner binary in the triple. Such mergers lead to a more top-heavy black hole mass function.