The formation of the Milky Way halo and its dwarf satellites: a NLTE-1D
  abundance analysis. IV. Segue 1, Triangulum II, and Coma Berenices UFDs
  We present atmospheric parameters and abundances for chemical elements from
carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices
(three stars), and Triangulum II (one star) ultra-faint dwarf galaxies (UFDs).
The effective temperatures rely on new photometric observations in the visible
and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian
observatory. Abundances of up to fourteen chemical elements were derived under
the non-local thermodynamic equilibrium (NLTE) line formation, and LTE
abundances were obtained for up to five more elements. For the first time we
present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri
II S40, potassium in Seg 1 S1-S6 and ComaBer S1-S3, and barium in Seg 1 S7.
Three stars in Segue 1, two stars in Coma Berenices, and Triangulum II star
have very low [Na/Mg] of -1.08 to -1.67 dex, which is usually attributed in the
literature to an odd-even effect produced by nucleosynthesis in massive
metal-free stars. We interpret this chemical property as a footprint of first
stars, which is not blurred due to a small number of nucleosynthesis events
that contributed to chemical abundance patterns of the sample stars. Our NLTE
abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulum II report on
lower [Sr/Ba] abundance ratio in the UFDs compared to that in classical dwarf
spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in
massive galaxies, [Sr/Ba] is not constant and it can be higher than the pure
r-process ratio. We suggest a hypothesis of Sr production in metal-poor
binaries at the earliest epoch of galactic evolution.
