Merging black hole binaries with the SEVN code

Abstract

Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 × 10 8 binary systems, with metallicity in the range 10 −4 ; 4 × 10 −2 . From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is ∼ 30, 45 and 55 M at metallicity Z = 2×10 −2 , 6×10 −3 , and 10 −4 , respectively. A few massive single BHs may also form ( < ∼ 0.1% of the total number of BHs), with mass up to ∼ 65, 90 and 145 M at Z = 2 × 10 −2 , 6 × 10 −3 , and 10 −4 , respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814 and GW170104) originate from metal poor (Z < ∼ 6 × 10 −3 ) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10 −4 . The BHB merger rate in the local Universe obtained from our simulations is ∼ 90Gpc −3 yr −1 , consistent with the rate inferred from LIGO-Virgo data.

Impact of the Rotation and Compactness of Progenitors on the Mass of Black Holes

“…We have investigated the impact of rotation and compactness on the mass of black holes (BHs), by implementing rotating stellar evolution models (Limongi & Chieffi 2018) into our population synthesis code sevn (Spera et al 2015;Spera et al 2019).…”

“…Here we present a new version of the populationsynthesis code sevn (Spera et al 2015;Spera et al 2019) in which we include stellar rotation by means of the franec stellar evolution tracks (Limongi et al 2000;Chieffi & Limongi 2004;Limongi & Chieffi 2006;Chieffi & Limongi 2013;Limongi & Chieffi 2018). We discuss the impact of stellar rotation on compact-object mass.…”

Modeling Dense Star Clusters in the Milky Way and Beyond with the CMC Cluster Catalog

“…As discussed in Spera et al (2019), massive stellar mergers occurring through binary evolution may have important consequences for the formation of massive BHs, specifically BHs lying within the so-called upper mass-gap expected from (pulsational) pair instability SNe (e.g., Belczynski et al 2016b;Woosley 2016;Spera & Mapelli 2017). In principle, dynamically-mediated stellar collisions of massive stars in GCs may have similar implications for BH formation.…”

Revising Natal Kick Prescriptions in Population Synthesis Simulations

“…In particular, Bray & Eldridge (2016) and Bray & Eldridge (2018) propose a new linear relation between the mass of the ejecta (to account for the effect of asymmetries), divided by the mass of the compact object (to conserve linear momentum), and the natal kick. Moreover, natal kicks from electron-capture SNe (ECSNe), which are less energetic than CCSNe, are expected to be significantly low (Dessart et al 2006;Schwab et al 2015;Gessner & Janka 2018;Giacobbo & Mapelli 2019). Fur-thermore, stars in close binary systems might undergo ultra-stripped SNe, i.e.…”

“…Electron-capture SNe (ECSNe) are modeled as described in Giacobbo & Mapelli (2019). Core-collapse SNe (CCSNe) are described as in Fryer et al (2012), including both the rapid and the delayed model: the mass of the compact object formed via a CCSN is determined by the final mass of the carbon-oxygen core and by the amount of fallback.…”