Mass Ratios of Merging Double Neutron Stars as Implied by the Milky Way Population

TL;DR

This paper analyzes the mass ratios of merging double neutron stars in the Milky Way, showing most will have nearly equal masses and estimating the amount of material ejected during mergers, with implications for electromagnetic signals and element formation.

Contribution

It demonstrates that the majority of Galactic DNS mergers have near-unity mass ratios when accounting for observational biases and merger times, and estimates the ejecta mass using hydrodynamic simulation-based formulas.

Findings

98% of merging DNSs have mass ratio q > 0.9.

Ejected material at merger is approximately 0.004-0.010 solar masses.

Electromagnetic counterparts may reach luminosities of ~10^41 erg/s.

Abstract

Of the seven known double neutron stars (DNS) with precisely measure masses in the Milky Way that will merge within a Hubble time, all but one has a mass ratio, $q$, close to unity. Recently, precise measurements of three post-Keplerian parameters in the DNS J1913$+$1102 constrain this system to have a significantly non-unity mass ratio of 0.78$\pm$0.03. One may be tempted to conclude that approximately one out of seven (14\%) DNS mergers detected by gravitational wave observatories will have mass ratios significantly different from unity. However J1913$+$1102 has a relatively long merger time of 470 Myr. We show that when merger times and observational biases are taken into account, the population of Galactic DNSs imply that $\simeq98\%$ of all merging DNSs will have $q>$0.9. We then apply two separate fitting formulas informed by 3D hydrodynamic simulations of DNS mergers to our results on Galactic DNS masses, finding that either $\simeq$0.004 ${M_{\odot}}$ or $\simeq$0.010 ${M_{\odot}}$ of material will be ejected at merger, depending on which formula is used. These ejecta masses have implications for both the peak bolometric luminosities of electromagnetic counterparts (which we find to be $\sim$10$^{41}$ erg s$^{-1}$) as well as the $r$-process enrichment of the Milky Way.