Chemically homogeneous evolution: a rapid population synthesis approach

Abstract

We explore chemically homogeneous evolution (CHE) as a formation channel for massive merging binary black holes (BBHs). We develop methods to include CHE in a rapid binary population syn- thesis code, Compact Object Mergers: Population Astrophysics and Statistics (COMPAS), which combines realistic models of binary evolution with cosmological models of the star-formation his- tory of the Universe. For the rst time, we simultaneously explore conventional isolated binary star evolution under the same set of assumptions. This approach allows us to constrain popula- tion properties and make simultaneous predictions about the gravitational-wave detection rates of BBH mergers for the CHE and conventional formation channels. The overall mass distribution of detectable BBHs is consistent with existing gravitational-wave observations. We nd that the CHE channel may yield up to $\sim 70\%$ of all gravitational-wave detections of BBH mergers coming from isolated binary evolution.

Rates of compact object coalescences

“…Chemically homogeneous evolution Belczynski et al (2017) Population III stars Fragione and Loeb (2019a,b) Triples/Multiples We do not quote predictions for the number of detections per year for a given instrument because such predictions must take into account the variation in both the merger rate and the mass distribution of merging binaries with redshift for detectors sensitive to cosmological distances, and this information is frequently not readily available in the literature. Derivations of how to do such calculations are provided by, e.g., Belczynski et al (2014) and de Mink and Mandel (2016); both analytical fits (e.g., Fishbach and Holz 2017) and numerical codes (e.g., Gerosa 2017; Team COMPAS: Riley et al 2021) are available.…”

“…The observational evidence ranges from tentative to contradictory (Almeida et al 2015;Mandel and de Mink 2016;Abdul-Masih et al 2021) and theoretical models of mixing differ in the predicted thresholds on its efficiency. Moreover, much of the parameter space in this channel likely requires over-contact binaries (Marchant et al 2016;du Buisson et al 2020;Riley et al 2021), which bring extra modelling challenges and uncertainties. On the other hand, a star could be spun up to rapid rotation and chemically homogeneous evolution by mass accretion from the companion (Cantiello et al 2007).…”

The redshift evolution of the binary black hole merger rate: a weighty matter

“…We sample birth metallicities with a probability distribution that is flat-in-log in the range 10 −4 ≤ Z ≤ 0.03. Sampling metallicities from a smooth probability distribution is an improvement over discrete sets of metallicity, which is the most common technique in binary population synthesis studies (but see, for example, Riley et al 2021 for an exception). Smoothly sampling birth metallicity avoids artificial peaks in the BH mass distribution (Kummer 2020).…”

“…If a zero age main sequence (ZAMS) star is rotating faster than the metallicity-dependent rotational frequency threshold described in Riley et al (2021), the binary is assumed to evolve chemically homogeneously. In this work, we focus on the 'classical' pathway of isolated binaries towards merging BBHs and thus we exclude chemically homogeneously evolving stars from our sample.…”

“…Two formation channels which exhibit a preference for the formation of more massive BBHs are chemicallyhomogeneous evolution (CHE; e.g. de Mink et al 2009;Song et al 2013Song et al , 2015Mandel & de Mink 2016;Marchant et al 2016;Riley et al 2021) and Population III binaries (e.g. Marigo et al 2001;Belczynski et al 2004;Kinugawa et al 2014;Inayoshi et al 2017).…”