Detailed chemical abundances of stars in the outskirts of the Tucana II ultra-faint dwarf galaxy

Abstract

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 [α/Fe] at [Fe/H] ≈ −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 /dθ 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 × 10 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 [α/Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts.

Metal Mixing in the r-process Enhanced Ultrafaint Dwarf Galaxy Reticulum II*

“…For matched stars in each sample, we calculate a weighted mean velocity offset. After removing this offset, stars with velocity variations inconsistent with a chi-squared test with p < 0.01 are identified as likely binaries (e.g., Chiti et al 2022). We iterate this process until convergence, resulting in five binary stars: Stars 2, 13, 18, 19, and 21.…”

Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

“…We remeasure the velocities of Gru1-032 (DES J225658.06-501357.9) and Gru1-038 (DES J225629.92-500433.3) from the MIKE spectra presented in Ji et al (2019), following the steps outlined in Chiti et al (2022). We derive a velocity of v MIKE = −139.6 ± 1.2 km s −1 for Gru1-032, and v MIKE = −143.3 ± 1.2 km s −1 for Gru1-038.…”