SIMBAD references

The analysis of stellar abundances for odd-Z Fe-peak elements requires accurate non-local thermodynamic equilibrium (NLTE) modelling of spectral lines fully taking into account the hyperfine structure (HFS) splitting of lines. Here, we investigate the statistical equilibrium of Co in the atmospheres of cool stars and the influence of NLTE and HFS on the formation of Co lines and abundances. Significant departures from LTE level populations are found for Coi; number densities of excited states in Coii also differ from LTE at low metallicity. The NLTE level populations are used to determine the abundance of Co in solar photosphere, logε = 4.95±0.04dex, which is in agreement with that in Ci meteorites within the combined uncertainties. The spectral lines of Coi were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For Coii, the first laboratory measurements of HFS A and B factors were performed. These highly accurate A factor measurements (errors of the order of 3-7 per cent) allow, for the first time, reliable modelling of Coii lines in the solar and stellar spectra and, thus, a test of the Coi/Coii ionization equilibrium in stellar atmospheres. A differential abundance analysis of Co is carried out for 18 stars in the metallicity range -3.12 < [Fe/H] < 0. The abundances are derived by the method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for Coi lines are as large as +0.6,…, + 0.8dex. Thus, LTE abundances of Co in metal-poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of Coii are lower by ∼0.5-0.6dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar Coii line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor stars, as calculated from the Coi lines under NLTE, can be explained if Co is overproduced relative to Fe in massive stars. The models of Galactic chemical evolution are wholly inadequate to describe this trend suggesting that the problem is in supernova yields.