Chemical Diversity in the Ultra-faint Dwarf Galaxy Tucana II
  We present the first detailed chemical abundance study of the ultra-faint
dwarf galaxy Tucana II based on high-resolution Magellan/MIKE spectra of four
red giant stars. The metallicity of these stars ranges from [Fe/H] = -3.2 to
-2.6, and all stars are low in neutron-capture abundances ([Sr/Fe] and [Ba/Fe]
< -1). However, a number of anomalous chemical signatures are present. Three
stars are carbon-enhanced, including the most metal-rich star. This star
([Fe/H]=-2.6) shows [Na,$\alpha$,Sc/Fe] < 0, suggesting an extended star
formation history with contributions from AGB stars and Type Ia supernovae. The
other carbon-enhanced stars have [Fe/H] < -3 and may be consistent with
enrichment by faint supernovae, if such supernovae can produce neutron-capture
elements. A fourth star with [Fe/H] = -3 is carbon-normal, and exhibits
distinct light element abundance ratios from the carbon-enhanced stars. The
carbon-normal star implies that at least two distinct nucleosynthesis sources,
both possibly associated with Population III stars, contributed to the early
chemical enrichment of this galaxy. Despite its very low luminosity, Tucana II
shows a diversity of chemical signatures that preclude it from being a simple
"one-shot" first galaxy, but still provide a window to star and galaxy
formation in the early universe.
