Chemical Diversity in the Ultra-faint Dwarf Galaxy Tucana II

TL;DR

This study presents the first detailed chemical abundance analysis of Tucana II, revealing diverse nucleosynthesis signatures and complex star formation history in this ultra-faint dwarf galaxy.

Contribution

It provides new high-resolution spectroscopic data showing chemical diversity and multiple enrichment sources in Tucana II, challenging the idea of it being a simple first galaxy.

Findings

Presence of carbon-enhanced and carbon-normal stars with distinct chemical signatures.

Evidence for multiple nucleosynthesis sources, including Population III stars.

Diversity of chemical signatures indicates complex star formation history.

Abstract

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.