Successful Common Envelope Ejection and Binary Neutron Star Formation in\n 3D Hydrodynamics

Abstract

A binary neutron star merger has been observed in a multi-messenger detection\nof gravitational wave (GW) and electromagnetic (EM) radiation. Binary neutron\nstars that merge within a Hubble time, as well as many other compact binaries,\nare expected to form via common envelope evolution. Yet five decades of\nresearch on common envelope evolution have not yet resulted in a satisfactory\nunderstanding of the multi-spatial multi-timescale evolution for the systems\nthat lead to compact binaries. In this paper, we report on the first successful\nsimulations of common envelope ejection leading to binary neutron star\nformation in 3D hydrodynamics. We simulate the dynamical inspiral phase of the\ninteraction between a 12$M_\\odot$ red supergiant and a 1.4$M_\\odot$ neutron\nstar for different initial separations and initial conditions. For all of our\nsimulations, we find complete envelope ejection and final orbital separations\nof $a_{\\rm f} \\approx 1.3$-$5.1 R_\\odot$ depending on the simulation and\ncriterion, leading to binary neutron stars that can merge within a Hubble time.\nWe find $\\alpha_{\\rm CE}$-equivalent efficiencies of $\\approx 0.1$-$2.7$\ndepending on the simulation and criterion, but this may be specific for these\nextended progenitors. We fully resolve the core of the star to $\\lesssim 0.005\nR_\\odot$ and our 3D hydrodynamics simulations are informed by an adjusted 1D\nanalytic energy formalism and a 2D kinematics study in order to overcome the\nprohibitive computational cost of simulating these systems. The framework we\ndevelop in this paper can be used to simulate a wide variety of interactions\nbetween stars, from stellar mergers to common envelope episodes leading to GW\nsources.\n