The r-process in Magnetorotational Supernovae

TL;DR

This paper investigates the early universe's rapid enrichment of r-process elements, especially europium, in dwarf galaxies, supporting magnetorotational supernovae as the primary source based on chemical evolution and nucleosynthesis models.

Contribution

It constrains the properties of Eu production in early dwarf galaxies and links observed abundance patterns to magnetorotational supernovae as the source.

Findings

Eu production events are about one per 100-200 core-collapse supernovae.

Eu enrichment ceases at [Fe/H] ≈ -2 in early stars.

Eu yields from models match observed chemical evolution data.

Abstract

One of the hottest open issues involving the evolution of r-process elements is fast enrichment in the early Universe. Clear evidence for the chemical enrichment of r-process elements is seen in the stellar abundances of extremely metal poor stars in the Galactic halo. However, small-mass galaxies are the ideal testbed for studying the evolutionary features of r-process enrichment given the potential rarity of production events yielding heavy r-process elements. Their occurrences become countable and thus an enrichment path due to each event can be found in the stellar abundances. We examine the chemical feature of Eu abundance at an early stage of ${\rm[Fe/H]} \lesssim -2$ in the Draco and Sculptor dwarf spheroidal (dSph) galaxies. Accordingly, we constrain the properties of the Eu production in the early dSphs. We find that the Draco dSph experienced a few Eu production events, whereas Eu enrichment took place more continuously in the Sculptor dSph due to its larger stellar mass. The event rate of Eu production is estimated to be about one per $100$-$200$ core-collapse supernovae, and a Eu mass of $\sim (1-2) \times 10^{-5}$\ms per single event is deduced by associating this frequency with the observed plateau value of ${\rm [Eu/H]} \sim -1.3$ for ${\rm [Fe/H]} \gtrsim -2$. The observed plateau implies that early Eu enrichment ceases at ${\rm [Fe/H]} \approx -2$. Such a selective operation only in low-metallicity stars supports magnetorotational supernovae, which require very fast rotation, as the site of early Eu production. We show that the Eu yields deduced from chemical evolution agree well with the nucleosynthesis results from corresponding supernovae models.