# Constraining Collapsar r-Process Models through Stellar Abundances

## TL;DR

This study uses stellar abundance observations to evaluate collapsar models for r-process element production, finding they are inconsistent with observed metal-poor star compositions and suggesting neutron star mergers as a more plausible source.

## Contribution

It provides new constraints on collapsar models by comparing predicted and observed r-process and Fe abundances in metal-poor stars, highlighting the incompatibility of collapsars with these observations.

## Key findings

- Collapsar models predict too high [r/Fe] ratios compared to observations.
- Fe mixing from supernovae cannot explain the observed r-process abundances.
- Neutron star mergers are more consistent with the observed distribution of r-process elements.

## Abstract

We use observations of heavy elements in very metal-poor stars ([Fe/H] < -2.5) in order to place constraints on the viability of collapsar models as a significant source of the r-process. We combine bipolar explosion nucleosynthesis calculations with recent disk calculations to make predictions of the observational imprints these explosions would leave on very metal-poor stars. We find that a source of low (~ 0.1-0.5 $M_\odot$) Fe mass which also yields a relatively high (> 0.08 $M_\odot$) r-process mass would, after subsequently mixing and forming new stars, result in [r/Fe] abundances up to three orders of magnitude higher than those seen in stars. In order to match inferred abundances, 10-10$^3 M_\odot$ of Fe would need be efficiently incorporated into the r-process ejecta. We show that Fe enhancement and hence [r/Fe] dilution from other nearby supernovae is not able to explain the observations unless significant inflow of pristine gas occurs before the ejecta are able to form new stars. Finally, we show that the inferred [Eu/Fe] abundances require levels of gas mixing which are in conflict with other properties of r-process enhanced metal-poor stars. Our results suggest that early r-process production is likely to be spatially uncorrelated with Fe production, a condition which can be satisfied by neutron star mergers due to their large kick velocities and purely r-process yields.