Cobalt in stars and the galaxy

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Abstract
An analysis of the abundance of cobalt in atmospheres of red giants, indicates they can be divided into two groups: stars with the normal [Co/Fe] abundance and those with a small [Co/Fe] excess. A comparative analysis of the spectrograms taking into account the effect of superfine splitting provides evidence for a [Co/Fe] excess in some stars. We have also detected physical and kinematical differences between these groups. Stars with a [Co/Fe] excess are related to the thick-disk population of the Galaxy. These stars are older and less massive, display lower metallicities, and have Galactic velocities corresponding to those of thick-disk objects. It is suggested that the observed pattern of a [Co/Fe] excess in the halo and thick disk reflects the chemical composition of the Galaxy at a very early stage of its evolution, when Population III objects existed. The lower abundance excess in the thick disk compared to the halo and the absence of an excess in the thin disk are due to the contributiuon from Type I supernovae at later stages of the Galaxy’s evolution. We have found that the thick disk of the Galaxy displays gradients of its cobalt and iron abundances, possibly providing evidence that the thick disk formed as a result of the collapse of a protogalactic cloud.

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