Which site dominates the r-process inventory of the Milky Way?
In plain words
Neutron-star mergers, rare collapsing stars and flares from magnetars can all make r-process elements; their shares are unknown.
Precise statement
Determine the fractional contributions of neutron-star mergers (including neutron star-black hole), collapsars and magnetorotational supernovae, and magnetar giant flares to the Galactic r-process mass. Constraints: binary neutron star merger rate from gravitational waves ($\sim 10\text{ to }1700\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$), ejecta per event ($\sim 0.01\text{ to }0.05\,M_{\mathrm{sun}}$), the $[\mathrm{Eu}/\mathrm{Fe}]$ versus $[\mathrm{Fe}/\mathrm{H}]$ trend and its scatter in metal-poor stars, and Pu-244 in deep-sea crusts. Answer: fractions with uncertainties.
What would settle it
A tighter merger rate and kilonova yield sample from gravitational-wave events, together with direct detection of r-process signatures from collapsars.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
2025: delayed MeV emission after the 2004 SGR 1806-20 giant flare was interpreted as radioactive decay of about $10^{-6}\,M_{\mathrm{sun}}$ of fresh r-process matter, adding magnetar flares as a minor site.
Problems built on this one
Special cases
- Do collapsar accretion disks eject neutron-rich r-process matter? · open
- Can neutron-star mergers explain r-process elements in the oldest stars? · open
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