Chemical enrichment mechanisms in ω Centauri: clues from neutron-capture elements ^⋆

¹ INAF − Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: valentina.dorazi@oapd.inaf.it; raffaele.gratton@oapd.inaf.it; sara.lucatello@oapd.inaf.it
² INAF − Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: elena.pancino@oabo.inaf.it; angela.bragaglia@oabo.inaf.it; eugenio.carretta@oabo.inaf.it;
³ Department of Astronomy and McDonald Observatory, The University of Texas, Austin TX 78712, USA
e-mail: chris@verdi.as.utexas.edu

Received: 5 July 2011
Accepted: 18 August 2011

Abstract

Context. In the complex picture of multiple stellar populations in globular clusters (GCs), a special role is played by NGC 5139 (ω Centauri). At variance with the majority of GCs, ω Cen exhibits significant star-to-star variations in metallicity and in relative neutron-capture element abundance ratios with respect to Fe, along with split evolutionary sequences as revealed from colour–magnitude diagrams. Combining information from photometry and spectroscopy, several studies suggested that an age spread of several Gyr has to be invoked to explain (at least partially) some of the observed features. However, a comprehensive understanding of the formation, evolution and chemical enrichment processes is still not at hand.

Aims. Relatively metal-rich ω Cen stars display neutron-capture abundance distributions dominated by contributions from the s-process, but it is not clear what roles have been played by the so-called main and weaks-process components in generating these abundances. To gain better insight into this question we derived lead (Pb) abundances for several ω Cen cluster members, because this element can only be produced by the mains-process.

Methods. We analysed high-resolution UVES@VLT spectra of a sample of twelve red-giant branch stars, deriving abundances of Pb and also of Y, Zr, La, Ce, Eu, and the C+N+O sum. Spectral synthesis was applied to all features, taking into account isotopic shifts and/or hyperfine structure as needed.

Results. We measured for the first time the Pb content in ω Cen, discovering a clear hint for a Pb production occurring at [Fe/H]  >   −1.7 dex. Our data suggest that the role of the weak component in the production of s-process elements is negligible. Moreover, evidence gathered from the abundances of other elements indicates that the main component occurring in this GC is peculiar and shifted towards higher mass polluters than the standard one.