NLTE analysis of Mn and Co in metal-poor stars

Abstract

We analyse the statistical equilibrium of Mn and Co in the atmospheres of subdwarfs and subgiants of different metallicities. Significant departures from local thermodynamic equilibrium (LTE) level populations are found for the neutral ions. They are related to overionization in Co I; deviations from LTE in Mn I are due to the strong radiative processes in discrete transitions as long as the solar metallicities and temperatures are considered. In environments where the radiation field is amplified, such as metal-poor stars with solar or supersolar effective temperatures, overionization in Mn I is the dominant process. The ground states of the singly ionized species are almost unaffected by non-local thermodynamic equilibrium (NLTE). Differential analyses of Mn and Co is carried out for 18 stars in the metallicity range - 2.5<[Fe/H]<0. The abundances are derived by the method of spectrum synthesis. The NLTE abundances of Mn and Co in metal-poor stars are higher than the LTE abundances. The difference [El/Fe]NLTE- [El/Fe]LTE increases with decreasing [Fe/H] and reaches 0.4 dex for Mn and 0.6 dex for Co in the most metal-poor stars. [Mn/Fe] is slightly subsolar all over the metallicity range studied here; [Co/Fe] ratios steadily increase with decreasing Fe abundances in halo and thick-disc stars.