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We present high-resolution of spectroscopy of four stars in two candidateultra-faint dwarf galaxies (UFDs) Grus I (Gru I) and Triangulum II (Tri II).Neither object currently has a clearly determined velocity dispersion, placingthem in an ambiguous region of parameter space between dwarf galaxies andglobular clusters. No significant metallicity difference is found for the twoGru I stars, but both stars are deficient in neutron-capture elements. Weverify previous results that Tri II displays significant spreads in metallicityand [$\alpha$/Fe]. Neutron-capture elements are not detected in our Tri IIdata, but we place upper limits at the lower envelope of Galactic halo stars,consistent with previous very low detections. Stars with similarly lowneutron-capture element abundances are common in UFDs, but rare in otherenvironments. This signature of low neutron-capture element abundances traceschemical enrichment in the least massive star-forming dark matter halos, andfurther shows that the dominant sources of neutron-capture elements inmetal-poor stars are rare. In contrast, all known globular clusters havesimilar ratios of neutron-capture elements to those of halo stars, suggestingthat globular clusters form as part of relatively massive galaxies rather thanin their own dark matter halos. The low neutron-capture element abundances maybe the strongest evidence that Gru I and Tri II are (or once were) galaxiesrather than globular clusters, and we expect future observations of thesesystems to robustly find non-zero velocity dispersions or signs of tidaldisruption. However, the nucleosynthetic origin of this low neutron-captureelement floor remains unknown.

astrophysical journal/the astrophysical journal

Chemical Abundances in the Ultra-Faint Dwarf Galaxies Grus I and  Triangulum II: Neutron-Capture Elements as a Defining Feature of the Faintest  Dwarfs