The r-process in Magnetorotational Supernovae
  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.
