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

Of the seven known double neutron stars (DNSs) 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 postKeplerian parameters in the DNS J1913+1102 constrain this system to have a significantly non-unity mass ratio of 0.78.+/-.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 lifetime (pulsar characteristic age plus the system's merger time due to gravitational-wave radiation) of approximate to 3 Gyr. We show that when system lifetimes and observational biases are taken into account, the population of Galactic DNSs implies that similar or equal to 98% 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 similar or equal to 0.004 M-circle dot or similar or equal to 0.009 M-circle dot 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 similar to 10(41) erg s(-1)) as well as the r-process enrichment of the Milky Way.