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Neutron star mergers (NM) are a plausible source of heavy r-process elementssuch as Europium, but previous chemical evolution models have either failed toreproduce the observed Europium trends for Milky Way thick disc stars (with[Fe/H] ~ -1) or have done so only by adopting unrealistically short mergertimescales. Using analytic arguments and numerical simulations, we demonstratethat models with a single-phase interstellar medium (ISM) andmetallicity-independent yields cannot reproduce observations showing [Eu/alpha]> 0 or [Eu/Fe] > [alpha/Fe] for alpha-elements such as Mg and Si. However, thisproblem is easily resolved if we allow for a 2-phase ISM, with hot-phasecooling times \tau_{cool} of order 1 Gyr and a larger fraction of NM yieldsinjected directly into the cold star-forming phase relative to alpha-elementyields from core collapse supernovae (ccSNe). We find good agreement withobservations in models with a cold phase injection ratio f_{c,NM}/f_{c,ccSN} oforder 2, and a characteristic merger timescale \tau_NM=150 Myr. We show thatthe observed super-solar [Eu/alpha] at intermediate metallicities implies thata significant fraction of Eu originates from NM or another source besidesccSNe, and that these non-ccSN yields are preferentially deposited in thestar-forming phase of the ISM at early times.

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