NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars^⋆,⋆⋆

¹ GEPI Observatoire de Paris, CNRS, Université Paris Diderot, 92195 Meudon Cedex, France
e-mail: monique.spite@obspm.fr
² Department of Astronomy and Astronomical Observatory, Odessa National University, T.G. Shevchenko Park, 65014, Odessa ; and Isaac Newton Institute of Chile, Odessa Branch, Ukraine
³ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany

Received: 4 January 2012
Accepted: 19 March 2012

Abstract

Context. Calcium is a key element for constraining the models of chemical enrichment of the Galaxy.

Aims. Extremely metal-poor stars contain the fossil records of the chemical composition of the early Galaxy and it is important to compare Ca abundance with abundances of other light elements, that are supposed to be synthesized in the same stellar evolution phases.

Methods. The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI, which allows a very good description of the radiation field.

Results. With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. This abundance is found to be solar. We find that $\overline{\mathrm{[}\mathrm{Ca}\mathit{/}\mathrm{Fe}\mathrm{]}}$ = 0.50±0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. The scatter of the ratios [X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where X is a “light metal” (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or even smaller than, the scatter around the mean value of [X/Ca]. We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity than the abundance deduced from the subordinate lines. Our computations of the effects of convection (3D effects) are not able to explain this difference. A fully consistent 3D NLTE model atmosphere and line formation scheme would be necessary to fully capture the physics of the stellar atmosphere.