The origin of the first neutron star – neutron star merger

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
e-mail: kbelczyn@astrouw.edu.pl
² Zentrum für Astronomie - Astronomisches Rechen-Institut, Weberstrasse 13, 69120 Heidelberg, Germany
³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Center for Gravitational Wave Astronomy, University of Texas Rio Grande Valley, Brownsville, TX, USA
⁶ National Astronomical Observatories of China, Chinese Academy of Sciences NAOC/CAS, 20A Datun Rd., Chaoyang District, Beijing 100012, PR China
⁷ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁸ School of Astronomy & Space Science, University of the Chinese Academy of Sciences, Beijing 100012, PR China

Received: 6 December 2017
Accepted: 28 March 2018

Abstract

The first neutron star-neutron star (NS-NS) merger was discovered on August 17, 2017 through gravitational waves (GW170817) and followed with electromagnetic observations. This merger was detected in an old elliptical galaxy with no recent star formation. We perform a suite of numerical calculations to understand the formation mechanism of this merger. We probe three leading formation mechanisms of double compact objects: classical isolated binary star evolution, dynamical evolution in globular clusters, and nuclear cluster formation to test whether they are likely to produce NS-NS mergers in old host galaxies. Our simulations with optimistic assumptions show current NS-NS merger rates at the level of 10⁻² yr⁻¹ from binary stars, 5 × 10⁻⁵ yr⁻¹ from globular clusters, and 10⁻⁵ yr⁻¹ from nuclear clusters for all local elliptical galaxies (within 100 Mpc³). These models are thus in tension with the detection of GW170817 with an observed rate of 1.5_−1.2^+3.2 yr⁻¹ (per 100 Mpc³; LIGO/Virgo 90% credible limits). Our results imply that either the detection of GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is a statistical coincidence; that physics in at least one of our three models is incomplete in the context of the evolution of stars that can form NS-NS mergers; or that another very efficient (unknown) formation channel with a long delay time between star formation and merger is at play.