Abundance correlations in mildly metal-poor stars^*

II. Light elements (C to Ca)

¹ Institute of Astrophysics and Geophysics, University of Liège, 17 Allée du 6 Août, 4000 Liège, Belgium e-mail: decauwer@astro.ulg.ac.be
² European Southern Observatory, Casilla 19001, Alonso de Cordova 3107, Vitacura, Santiago, Chile
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 23 March 2004
Accepted: 16 November 2004

Abstract

Accurate relative abundances have been obtained for carbon, oxygen, sodium, aluminium, silicon, and calcium in a sample of mildly metal-poor stars. This analysis complements a previous study carried out by Jehin et al. ([CITE], A&A, 341, 241), which provided the basis for the EASE scenario. This scenario postulates that field metal-poor stars were born in self-enriched proto-globular cluster clouds. By further investigating the correlations between the different α-element abundances, we propose a modified scenario for the formation of intermediate metallicity stars, in which the stars exhibiting lower than average α/Fe abundance ratios would form in low mass clouds, unable to sustain the formation of very massive stars ().
Moreover, the carbon-to-iron ratio is found to decrease as one climbs the so-called Population IIb branch, i.e. when the s-element abundance increases. In the framework of the EASE scenario, we interpret this anticorrelation between the carbon and the s-element abundances as a signature of a hot bottom burning process in the metal-poor AGB stars which expelled the matter subsequently accreted by our Population IIb stars.