$R$-process enhancements of Gaia-Enceladus in GALAH DR3

TL;DR

This study compares r-process element abundances in stars from Gaia-Enceladus and the Milky Way, revealing higher europium ratios in Gaia-Enceladus likely due to neutron-star merger enrichment, informing the origin of r-process elements.

Contribution

It provides the first detailed comparison of r-process element abundances between Gaia-Enceladus and the Milky Way using GALAH DR3 data, highlighting the role of neutron-star mergers.

Findings

Gaia-Enceladus stars show higher [{Eu}/{Mg}] ratios than in-situ stars.

Gaia-Enceladus lacks significant s-process contribution, unlike some dwarf galaxies.

High [{Eu}/{Mg}] can be explained by neutron-star merger enrichment with specific delay times.

Abstract

The dominant site of production of $r$-process elements remains unclear despite recent observations of a neutron star merger. Observational constraints on the properties of the sites can be obtained by comparing $r$-process abundances in different environments. The recent Gaia data releases and large samples from high-resolution optical spectroscopic surveys are enabling us to compare $r$-process element abundances between stars formed in an accreted dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. We aim to understand the origin of $r$-process elements in Gaia-Enceladus. We first construct a sample of stars to study Eu abundances without being affected by the detection limit. We then kinematically select 71 Gaia-Enceladus stars and 93 in-situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of which 50 and 75 stars can be used to study Eu reliably. Gaia-Enceladus stars clearly show higher ratios of [{Eu}/{Mg}] than in-situ stars. High [{Eu}/{Mg}] along with low [{Mg}/{Fe}] are also seen in relatively massive satellite galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand, unlike these galaxies, Gaia-Enceladus does not show enhanced [{Ba}/{Eu}] or [{La}/{Eu}] ratios suggesting a lack of significant $s$-process contribution. From comparisons with simple chemical evolution models, we show that the high [{Eu}/{Mg}] of Gaia-Enceladus can naturally be explained by considering $r$-process enrichment by neutron-star mergers with delay time distribution that follows a similar power-law as type~Ia supernovae but with a shorter minimum delay time.