Detailed Chemical Abundances of Stars in the Outskirts of the Tucana II Ultrafaint Dwarf Galaxy

We present chemical abundances and velocities of five stars between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy (UFD) from high-resolution Magellan/MIKE spectroscopy. We find that every star is deficient in metals (-3.6 < [Fe/H] < -1.9) and in neutron-capture elements as is characteristic of UFD stars, unambiguously confirming their association with Tucana II. Other chemical abundances (e.g., C, iron peak) largely follow UFD trends and suggest that faint core-collapse supernovae (SNe) dominated the early evolution of Tucana II. We see a downturn in [alpha/Fe] at [Fe/H] asymptotic to-2.8, indicating the onset of Type Ia SN enrichment and somewhat extended chemical evolution. The most metal-rich star has strikingly low [Sc/Fe] = -1.29 +/- 0.48 and [Mn/Fe] = -1.33 +/- 0.33, implying significant enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial velocity gradient in Tucana II ( dv(helio)/d theta(1)=-2.6-2.9+3.0 -1 kpc(-1)), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of M-1/2(r(h))=1.6(--0.7)(+)(1.1)x10(6 )M(?). We revisit formation scenarios of the extended component of Tucana II in light of its stellar chemical abundances. We find no evidence that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in situ stellar halo formation, then other UFDs should show similar such features. Although not a unique explanation, the decline in [alpha/Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts.

Automated summary

Using high-resolution Magellan/MIKE spectroscopy, we provide chemical abundances and velocities of five stars located between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy. The stars exhibit low metal and neutron-capture element abundances, confirming their association with Tucana II. The chemical abundances suggest the dominance of faint core-collapse supernovae in the early evolution of Tucana II, with a decline in [alpha/Fe] indicating the onset of Type Ia supernova enrichment. The lack of a radial velocity gradient implies a lack of tidal disruption, providing insights into the formation scenarios of the extended component of Tucana II.