A Mass-Dependent Yield Origin of Neutron-Capture Element Abundance
  Distributions in Ultra-Faint Dwarfs
  One way to constrain the nature of the high-redshift progenitors of the Milky
Way is to look at the low-metallicity stellar populations of the different
Galactic components today. For example, high-resolution spectroscopy of very
metal poor (VMP) stars demonstrates remarkable agreement between the
distribution of [Ti/Fe] in the stellar populations of the Milky Way halo (MW)
and ultra-faint dwarf (UFD) galaxies. In contrast, for the neutron capture (nc)
abundance ratio distributions [(Sr,Ba)/Fe], the peak of the small UFD sample (6
stars) exhibits a signicant under-abundance relative to the VMP stars in the
larger MW halo sample (~ 300 stars). We present a simple scenario that can
simultaneously explain these similarities and differences by assuming: (i) that
the MW VMP stars were predominately enriched by a prior generation of stars
which possessed a higher total mass than the prior generation of stars that
enriched the UFD VMP stars; and (ii) a much stronger mass-dependent yield (MDY)
for nc-elements than for the (known) MDY for Ti. Simple statistical tests
demonstrate that conditions (i) and (ii) are consistent with the observed
abundance distributions, albeit without strong constraints on model parameters.
A comparison of the broad constraints for these nc-MDY with those derived in
the literature seems to rule out Ba production from low-mass SNs and affirms
models that primarily generate yields from high-mass SN. Our scenario can be
confirmed by a relatively modest (factor of ~ 3-4) increase in the number of
high-resolution spectra of VMP stars in UFDs.
