The role of natal kicks in forming asymmetric compact binary mergers

Abstract

In their most recent observing run, the LIGO-Virgo-KAGRA (LVK) Collaboration observed gravitational waves (GWs) from compact binary mergers with highly asymmetric mass ratios, including both binary black holes (BBHs) and neutron star--black holes (NSBHs). It appears that NSBHs with mass ratios $q\simeq0.2$ are more common than equally asymmetric BBHs, but the reason for this remains unclear. We use the binary population synthesis code COSMIC to investigate the evolutionary pathways leading to the formation and merger of asymmetric compact binaries. We find that within the context of isolated binary stellar evolution, most asymmetric mergers start off as asymmetric stellar binaries. Because of the initial asymmetry, these systems tend to first undergo a dynamically unstable mass transfer phase. However, after the first star collapses to a compact object, the mass ratio is close to unity and the second phase of mass transfer is usually stable. According to our simulations, this stable mass transfer fails to shrink the orbit enough on its own for the system to merge. Instead, the natal kick received by the second-born compact object during its collapse is key in determining how many of these systems can merge. For the most asymmetric systems with mass ratios $q\leq0.1$, the merging systems in our models receive an average kick magnitude of 255 km s$^{-1}$ during the second collapse, while the average kick for non-merging systems is 59 km s$^{-1}$. Because lower mass compact objects, like NSs, are expected to receive larger natal kicks than higher mass BHs, this may explain why asymmetric NSBH systems merge more frequently than asymmetric BBH systems.