Neutron-rich freeze-out in viscously spreading accretion discs formed from compact object mergers

Accretion discs with masses ∼10^-3-0.1 M_⊙ are believed to form during the merger of a neutron star (NS) with another NS and the merger of a NS with a black hole (BH). Soon after their formation, such hyperaccreting discs cool efficiently by neutrino emission and their composition is driven neutron-rich by pair captures under degenerate conditions. However, as the disc viscously spreads and its temperature drops, neutrino cooling is no longer able to offset viscous heating and the disc becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disc's remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion discs formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the disc's late-time transition to an advective state. As a result, hyperaccreting discs generically freeze-out neutron-rich (with electron fraction Y_e ∼ 0.2-0.4), and their late-time outflows robustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our Solar system, we constrain the compact object merger rate in the Milky Way to be ≲10^-5 (M_d,0/0.1 M_⊙) ^-1 yr^-1, where M_d,0 is the average initial mass of the accretion disc. Thus, either the NS-NS merger rate is at the low end of current estimates or the average disc mass produced during a typical merger is ≪0.1 M_⊙. Based on the results of current general relativistic merger simulations, the latter constraint suggests that prompt collapse to a BH is a more common outcome of NS-NS mergers than the formation of a transient hypermassive NS. We also show that if most short-duration gamma-ray bursts (GRBs) are produced by compact object mergers, their beaming fraction must exceed f_b ≈ 0.13(M_d,0/0.1 M_⊙), corresponding to a jet half-opening angle ≳30° (M_d,0/0.1 M_⊙)^1/2. This is consistent with other evidence that short-duration GRB outflows are less collimated than those produced in long-duration GRBs.