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Evidence from Disrupted Halo Dwarfs that r-process Enrichment via Neutron Star Mergers is Delayed by ≳500 Myr
- Source :
- The Astrophysical Journal Letters. 926:L36
- Publication Year :
- 2022
- Publisher :
- American Astronomical Society, 2022.
-
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 \textit{Gaia}-Sausage Enceladus (GSE), afford precisely this opportunity: both have $M_{\star}\approx10^{8}M_{\rm{\odot}}$, but differing star-formation durations of ${\approx}2$ Gyrs and ${\approx}3.6$ Gyrs. Here we present $R\approx50,000$ 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]$\approx-1$, but Kraken has a median [Eu/Mg]$\approx-0.1$ while GSE has an elevated [Eu/Mg]$\approx0.2$. 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 $\approx50\%$ of the Eu in Kraken, $\approx25\%$ in GSE, and $\approx15\%$ in dwarfs with extended star-formation durations like Sagittarius. The inferred delay time for NSM enrichment is $10-100\times$ 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 $\approx10^{8}-10^{9}$ years to cool in these galaxies.<br />Comment: Submitted to ApJL. Figure 3 shows the key empirical result, and Figure 5 summarizes the proposed scenario. Comments very welcome!
- Subjects :
- High Energy Astrophysical Phenomena (astro-ph.HE)
Astrophysics - Solar and Stellar Astrophysics
Space and Planetary Science
Astrophysics of Galaxies (astro-ph.GA)
FOS: Physical sciences
Astronomy and Astrophysics
Astrophysics - High Energy Astrophysical Phenomena
Astrophysics - Astrophysics of Galaxies
Solar and Stellar Astrophysics (astro-ph.SR)
Subjects
Details
- ISSN :
- 20418213 and 20418205
- Volume :
- 926
- Database :
- OpenAIRE
- Journal :
- The Astrophysical Journal Letters
- Accession number :
- edsair.doi.dedup.....cd7ad6b1f2299d1e6bdf3d1a82f6c3a1