Formation of rotating supergiants via stellar mergers in dense clusters: Implications for black hole natal spins

Abstract

We investigate how massive stellar mergers in young star clusters imprint on black hole spin distributions and the broader implications for gravitational wave sources. The central hypothesis is that angular momentum transferred during stellar mergers substantially affects the spins of the merger products and resulting black holes, with some merger products evolving into collapsar-like objects that retain thick accretion disks that enable efficient spin up. This is in contrast to the more general expectation that black holes form with very small spins, having shed most of their envelope angular momentum via winds and expansion before core collapse. Using roughly 150 N-body models generated with the $\texttt{Cluster Monte Carlo}$ code, $\texttt{CMC}$, we analyze stellar mergers that lead to black hole formation, prioritizing ``significant'' events with mass ratio $q>0.1$. After identifying optimal candidates from our $\texttt{CMC}$ models, we explore detailed stellar structure and post-merger evolution implications with MESA stellar evolution models to capture angular momentum injection and pre-collapse profiles most relevant for the BH natal spin. In our current dataset representative of Milky Way-like globular clusters, up to roughly half of black holes are formed from such mergers, including up to roughly $10\%$ from significant mergers with $q>0.1$. Preliminary angular momentum estimates indicate substantial spin-up during the merger, and trends with mass ratio and stellar properties suggest strong correlations with the final black hole spin. In some cases, dimensionless spin parameters of $a\simeq 0.5$ or more are expected. This process has important implications for the dynamical formation and retention of gravitational wave sources in clusters.