Binary-single interactions with different mass ratios Article Swipe
YOU?
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· 2025
· Open Access
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· DOI: https://doi.org/10.1051/0004-6361/202450890
· OA: W4409324389
Context . Dynamical interactions in star clusters are an efficient mechanism to produce the coalescing binary black holes (BBHs) that have been detected with gravitational waves (GWs). Aims . We want to understand how BBH coalescence can occur during – or after – binary-single interactions with different mass ratios. Methods . We perform gravitational scattering experiments of binary-single interactions using different mass ratios of the binary components ( q 2 ≡ m 2 / m 1 ≤ 1) and the incoming single ( q 3 ≡ m 3 / m 1 ). We extract cross-sections and rates for (i) GW capture during resonant interactions; (ii) GW inspiral in between resonant interactions and apply the results to different globular cluster conditions. Results . We find that GW capture during resonant interactions is most efficient if q 2 ≃ q 3 and that the mass-ratio distribution of BBH coalescence due to inspirals is ∝ m 1 −1 q 2.9﹢α , where α is the exponent of the BH mass function. The total rate of GW captures and inspirals depends mostly on m 1 and is relatively insensitive to q 2 and q 3 . We show that eccentricity increase in direct (that is, non-resonant) encounters approximately doubles the rate of BBH inspirals in between resonant encounters. For a given GC mass and radius, the BBH merger rate in metal-rich GCs is approximately double that of metal-poor GCs, because of their (on average) lower BH masses ( m 1 ) and steeper BH mass function, yielding binaries with lower q . Conclusions . Our results enable the mass-ratio distribution of dynamically formed BBH mergers to be translated to the underlying BH mass function. The additional mechanism that leads to a doubling of the inspirals provides an explanation for the reported high fraction of in-cluster inspirals in N-body models of clusters.
