Chemical composition of metal-poor stars in Coma Berenices ultra-faint dwarf galaxy as a proxy to individual chemical enrichment events.

We present NLTE abundances and atmospheric parameters for three metal-poor stars ([Fe/H]<-2) in Coma Berenices ultra-faint dwarf galaxy (UFD). The derived results are based on new photometric observations obtained with the 2.5-m telescope of the SAI MSU Caucasian observatory and spectra from the archive of the 10-m Keck telescope. Effective temperatures were determined from V-I, V-K, V-J colours. For each star, Teffs derived from different colours agree within 20K. Surface gravities (log g) were calculated using a relation between log g, MV, BC, distance, Teff, and mass, adopted as 0.8MSun. The NLTE abundances for Na, Mg, Ca, Ti, Fe, Ni, Sr, and Ba were determined. A revision of atmospheric parameters and abundances, based on new photometric observations and accurate modelling of spectral line formation resulted in reinterpretation of the star formation history in Coma Berenices UFD. The derived chemical abundance patterns for three stars differ from each other and from that, which is typical for the MW halo stars of similar [Fe/H]. The S1 star shows solar [alpha/Fe] and unprecedentedly low [Na/Mg] of -1.46, which is the lowest value among metal-poor stars known to date. Abundance pattern of the S1 star is well reproduced by nucleosynthesis model in metal-free massive stars explosion. The stars S2 and S3, in contrast to S1, show high [alpha/Fe] ratios, for example, [Mg/Fe]=0.8 in S2, while stars with -3.5<[Fe/H]<-2 in the other dwarf galaxies and the MW show a typical ratio [Ca,Mg/Fe] of 0.3 dex. All the three stars show low Sr and Ba abundances. These peculiarities in chemical composition argue for a small number of nucleosynthesis events contributed to chemical abundances of these stars. A wide range of metallicity, 0.65 dex, observed in Coma Berenices is likely produced by inhomogeneous mixing of the interstellar medium, but not an increase in [Fe/H] during a prolonged star formation.