Merger Sites of Double Neutron Stars and Their Host Galaxies

Abstract

Using the StarTrack population synthesis code, we analyze the formation channels possibly available to double neutron star binaries and find that they can be richer than previously thought. We identify a group of short-lived, tight binaries, which do not live long enough to escape their host galaxies, despite their large center-of-mass velocities. We present our most recent results on all possible evolutionary paths leading to the formation of double neutron stars, calculate their coalescence rates, and also revisit the question of the distribution of merger sites around host galaxies. For a wide variety of binary evolution models and galaxy potentials, we find that most of the neutron star mergers take place within galaxies. Our results stem from allowing for radial and common envelope evolution of helium-rich stars (testable in the future with detailed stellar-structure and hydrodynamic calculations) and indicate that double neutron star binaries may not be excluded as gamma-ray burst progenitors solely on the basis of their spatial distribution around host galaxies. We also find, in contrast to Bethe & Brown, that in a significant fraction of common envelope (CE) phases, neutron stars do not accrete enough material to become black holes, and thus the channels involving CEs are still open for the formation of double neutron stars.