The chemical evolution of r-process elements from neutron star mergers: the role of a 2-phase interstellar medium

Neutron star mergers (NM) are a plausible source of heavy r-process elements such as Europium, but previous chemical evolution models have either failed to reproduce the observed Europium trends for Milky Way thick disc stars (with [Fe/H] ≈ -1) or have done so only by adopting unrealistically short merger time-scales. Using analytic arguments and numerical simulations, we demonstrate that models with a single-phase interstellar medium (ISM) and metallicity-independent yields cannot reproduce observations showing [Eu/α] > 0 or [Eu/Fe] > [α/Fe] for α-elements such as Mg and Si. However, this problem is easily resolved if we allow for a 2-phase ISM, with hot-phase cooling times τ_cool of the order of 1 Gyr and a larger fraction of NM yields injected directly into the cold star-forming phase relative to α-element yields from core-collapse supernovae (ccSNe). We find good agreement with observations in models with a cold phase injection ratio f_c,NM/f_c,ccSN of the order of 2, and a characteristic merger time-scale τ_{NM}=150 Myr. We show that the observed supersolar [Eu/α] at intermediate metallicities implies that a significant fraction of Eu originates from NM or another source besides ccSNe, and that these non-ccSN yields are preferentially deposited in the star-forming phase of the ISM at early times.