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

## TL;DR

This paper demonstrates that a two-phase interstellar medium model, with specific cooling times and yield injection ratios, successfully explains observed europium trends from neutron star mergers in the Milky Way.

## Contribution

It introduces a two-phase ISM model with differential yield injection to better match r-process element observations, resolving previous discrepancies in chemical evolution models.

## Key findings

- Two-phase ISM model reproduces observed Eu trends.
- A merger timescale of about 150 Myr fits data.
- Europium likely originates from neutron star mergers or similar sources.

## Abstract

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 timescales. 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/alpha] > 0 or [Eu/Fe] > [alpha/Fe] for alpha-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 \tau_{cool} of order 1 Gyr and a larger fraction of NM yields injected directly into the cold star-forming phase relative to alpha-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 order 2, and a characteristic merger timescale \tau_NM=150 Myr. We show that the observed super-solar [Eu/alpha] 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.