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We present a detailed abundance analysis of the three brightest member starsat the top of the giant branch of the ultra-faint dwarf galaxy Grus~II. Allstars exhibit a higher than expected $\mathrm{[Mg/Ca]}$ ratio compared tometal-poor stars in other ultra-faint dwarf galaxies and in the Milky Way halo.Nucleosynthesis in high mass ($\geqslant 20$M$_\odot$) core-collapse supernovaehas been shown to create this signature. The abundances of this small sample(3) stars suggest the chemical enrichment of Grus~II could have occurredthrough substantial high-mass stellar evolution and is consistent with theframework of a top-heavy initial mass function. However, with only three starsit can not be ruled out that the abundance pattern is the result of astochastic chemical enrichment at early times in the galaxy. The mostmetal-rich of the three stars also possesses a small enhancement in rapidneutron-capture ($r$-process) elements. The abundance pattern of the$r$-process elements in this star matches the scaled $r$-process pattern of thesolar system and $r$-process enhanced stars in other dwarf galaxies and in theMilky Way halo, hinting at a common origin for these elements across a range ofenvironments. All current proposed astrophysical sites of $r$-process elementproduction are associated with high-mass stars, thus the possible top-heavyinitial mass function of Grus~II would increase the likelihood of any of theseevents occurring. The time delay between the $\alpha$ and $r$-process elementenrichment of the galaxy favors a neutron star merger as the origin of the$r$-process elements in Grus~II.

Chemical Analysis of the Ultra-Faint Dwarf Galaxy Grus~II. Signature of high-mass stellar nucleosynthesis