From 3D hydrodynamic simulations of common-envelope interaction to gravitational-wave mergers

TL;DR

This study uses 3D hydrodynamic simulations to model common-envelope interactions in massive binary stars, revealing that most systems do not merge within a Hubble time, impacting gravitational-wave event predictions.

Contribution

First 3D hydrodynamic simulations of massive common-envelope interactions showing envelope ejection and implications for gravitational-wave merger rates.

Findings

Nearly complete envelope ejection assuming recombination energy aids unbinding.

Most systems do not merge within a Hubble time after common-envelope phase.

Supernova kicks can enable some neutron-star mergers, but reduce overall merger predictions.

Abstract

Modeling the evolution of progenitors of gravitational-wave merger events in binary stars faces two major uncertainties: the common-envelope phase and supernova kicks. These two processes are critical for the final orbital configuration of double compact-object systems with neutron stars and black holes. Predictive one-dimensional models of common-envelope interaction are lacking and multidimensional simulations are challenged by the vast range of relevant spatial and temporal scales. Here, we present three-dimensional hydrodynamic simulations of the common-envelope interaction of an initially $10\,M_{\odot}$ red supergiant primary star with a black-hole and a neutron-star companion. We show that the high-mass regime is accessible to full ab-initio simulations. Nearly complete envelope ejection is reached assuming that all recombination energy still available at the end of our simulation continues to help unbinding the envelope. In contrast to previous assumptions, we find that the dynamical plunge-in of both companions terminates at orbital separations too wide for gravitational waves to merge the systems in a Hubble time. We discuss the further evolution of the system based on analytical estimates. A subsequent mass-transfer episode from the remaining $3\,M_{\odot}$ core of the supergiant to the compact companion does not shrink the orbit sufficiently either. A neutron-star--neutron-star and neutron-star--black-hole merger is still expected for a fraction of the systems if the supernova kick aligns favorably with the orbital motion. For double neutron star (neutron-star--black-hole) systems we estimate mergers in about $9 \%$ ($1 \%$) of cases while about $77 \%$ ($94 \%$) of binaries are disrupted, i.e., supernova kicks actually enable gravitational-wave mergers in our cases; however, we expect a reduction in predicted gravitational-wave merger events. (abbr.)