Letter to the Editor

The extended star formation history of ω Centauri^*

¹ Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Universidad de Concepción, Departamento de Física, Casilla 106-C, Concepción, Chile

Corresponding author: M. Hilker, mhilker@astro.uni-bonn.de

Received: 29 April 2004
Accepted: 28 May 2004

Abstract

For the first time, the abundances of a large sample of subgiant and turn-off region stars in ω Centauri have been measured, the data base being medium resolution spectroscopy from FORS2 at the VLT. Absolute iron abundances were derived for ~400 member stars from newly defined line indices with an accuracy of ±0.15 dex. The abundances range between  dex, resembling the large metallicity spread found for red giant branch stars. The combination of the spectroscopic results with the location of the stars in the colour magnitude diagram has been used to estimate ages for the individual stars. Whereas most of the metal-poor stars are consistent with a single old stellar population, stars with abundances higher than  dex are younger. The total age spread in ω Cent is about 3 Gyr. The monotonically increasing age-metallicity relation seems to level off above  dex. Whether the star formation in ω Cen occured continuously or rather episodically has to be shown by combining more accurate abundances with highest quality photometry.