Chemical Analysis of the Ultra-Faint Dwarf Galaxy Grus~II. Signature of
  high-mass stellar nucleosynthesis
  We present a detailed abundance analysis of the three brightest member stars
at the top of the giant branch of the ultra-faint dwarf galaxy Grus~II. All
stars exhibit a higher than expected $\mathrm{[Mg/Ca]}$ ratio compared to
metal-poor stars in other ultra-faint dwarf galaxies and in the Milky Way halo.
Nucleosynthesis in high mass ($\geqslant 20$M$_\odot$) core-collapse supernovae
has been shown to create this signature. The abundances of this small sample
(3) stars suggest the chemical enrichment of Grus~II could have occurred
through substantial high-mass stellar evolution and is consistent with the
framework of a top-heavy initial mass function. However, with only three stars
it can not be ruled out that the abundance pattern is the result of a
stochastic chemical enrichment at early times in the galaxy. The most
metal-rich of the three stars also possesses a small enhancement in rapid
neutron-capture ($r$-process) elements. The abundance pattern of the
$r$-process elements in this star matches the scaled $r$-process pattern of the
solar system and $r$-process enhanced stars in other dwarf galaxies and in the
Milky Way halo, hinting at a common origin for these elements across a range of
environments. All current proposed astrophysical sites of $r$-process element
production are associated with high-mass stars, thus the possible top-heavy
initial mass function of Grus~II would increase the likelihood of any of these
events occurring. The time delay between the $\alpha$ and $r$-process element
enrichment of the galaxy favors a neutron star merger as the origin of the
$r$-process elements in Grus~II.
