Evidence from Disrupted Halo Dwarfs that $r$-process Enrichment via Neutron Star Mergers is Delayed by $\gtrsim500$ Myrs

Abstract

The astrophysical origins of r-process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels' distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r-process enrichment in galaxies of similar mass, but differing star-formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way's stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: both have M ≈ 10 8 M , but differing star-formation durations of ≈2 Gyrs and ≈3.6 Gyrs. Here we present R ≈ 50, 000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r-process element Eu in all stars. Stars from both systems have similar [Mg/H]≈ −1, but Kraken has a median [Eu/Mg]≈ −0.1 while GSE has an elevated [Eu/Mg]≈ 0.2. With simple models we argue NSM enrichment must be delayed by 500 − 1000 Myrs to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu-free. In this picture, rCCSNe account for ≈ 50% of the Eu in Kraken, ≈ 25% in GSE, and ≈ 15% in dwarfs with extended star-formation durations like Sagittarius. The inferred delay time for NSM enrichment is 10 − 100× longer than merger delay times from stellar population synthesis -this is not necessarily surprising because the enrichment delay includes time taken for NSM ejecta to be incorporated into subsequent generations of stars. For example, this may be due to natal kicks that result in r-enriched material deposited far from star-forming gas, which then takes ≈ 10 8 − 10 9 years to cool in these galaxies.

The detailed chemical abundance patterns of accreted halo stars from the optical to infrared

“…Their chemical evolution modeling supports the former scenario, in agreement with our results, which we discuss in detail in the following section comparing element abundances to Ba. Indeed, a more recent study by Naidu et al (2021) proposed that the r-process enrichment from NSM is delayed by 0.5 to 1 Gyr to explain their GES Eu abundance.…”

“…Very recently, there have been works by Matsuno et al (2021), Aguado et al (2021), andNaidu et al (2021) that focus on the r-process in GES stars where all found elevated Eu, highlighting the r-process enhancement in the GES system. There are various sites for r-process production (e.g., electron-capture supernovae or magnetorotationally-driven supernovae, Cescutti & Chiappini (2014)) but the simple chemical evolution model from Matsuno et al (2021) best agree with a neutron star merger scenario (e.g.…”

Collapsars as Sites of r-process Nucleosynthesis: Systematic Near-Infrared Follow-up of Type Ic-BL Supernovae

“…Measurements of excess [Ba/Fe] and [Eu/Fe] abundances in the dwarf galaxy Reticulum II argue for not only a rare and prolific event, but one capable of enriching the galaxy early in its history (Ji et al 2016a;Tarumi et al 2020), pointing towards a potential rare SN subtype whose r -process production would follow star formation (Côté et al 2019;Siegel et al 2019). Further evidence of heavy r -process enrichment in the disrupted dwarf galaxy Gaia Sausage Enceladus (∼3.6 Gyr star formation duration) but not in the disrupted dwarf galaxy Kraken (with ≈2 Gyr star formation duration) points towards multiple r -process enrichment sites operating on different timescales (Naidu et al 2021).…”

Common envelope jets supernova r-process yields can reproduce $\rm [Eu/Fe]$ abundance evolution in the Galaxy

“…These astronomical sites have been tested with Galactic chemical evolution models that lead to different conclusions. Recent studies suggest that multiple r-process sites are required to explain the r-process abundances in the Galaxy (e.g., Haynes & Kobayashi 2019;Yamazaki et al 2021;Farouqi et al 2021;Tsujimoto 2021;Naidu et al 2021). Molero et al (2021) argue that the same applies for ultra faint dwarf galaxies.…”