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 { [ 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 } M _ { \odot } per single event is deduced by associating this frequency with the observed plateau value of { [ Eu / H ] } \sim - 1.3 for { [ Fe / H ] } \gtrsim - 2 .
The observed plateau implies that early Eu enrichment ceases at { [ 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 .