Detailed chemical abundances of stars in the outskirts of the Tucana II
  ultra-faint dwarf galaxy
  We present chemical abundances and velocities of five stars between 0.3 kpc
to 1.1 kpc from the center of the Tucana II ultra-faint 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]
$\approx -2.8$, indicating the onset of Type Ia SN enrichment and somewhat
extended chemical evolution. The most metal-rich star has strikingly low
[Sc/Fe] = $-1.29 \pm 0.48$ and [Mn/Fe] = $-1.33 \pm 0.33$, implying significant
enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial
velocity gradient in Tucana II
($\text{d}v_{\text{helio}}/\text{d}\theta_1=-2.6^{+3.0}_{-2.9}$ km s$^{-1}$
kpc$^{-1}$) reflecting a lack of evidence for tidal disruption, and derive a
dynamical mass of $M_{1/2} (r_h) = 1.6^{+1.1}_{-0.7}\times 10^6$ M$_{\odot}$.
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.
