Chemical enrichment mechanisms inωCentauri: clues from neutron-capture elements

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 colourmagnitude 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 weak s-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 main s-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.

A Chemical Signature from Fast-rotating Low-metallicity Massive Stars: ROA 276 in ω Centauri*

“…The conclusions of this work depend on the decision to use ROA46 as the reference star. In Figure 4, we plot various combinations of neutron-capture element abundances and compare ROA 276 and ROA 46 with a larger stellar sample from ω Cen (D'Orazi et al 2011). Within the measurement uncertainties, ROA 46 is representative of the primordial population of ω Cen with low abundance ratios for the s-process elements.…”

A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)

“…Many ironcomplex clusters are also relatively massive and tend to host very blue horizontal branches (HB). The massive globular cluster omega Centauri (ω Cen) is the best known and most extreme object from this group, and has been demonstrated by multiple authors to possess: a very blue HB, a range in [Fe/H] that spans about a factor of 100, at least five distinct main stellar populations (each with its own set of primordial, intermediate, and extreme stars), and a strong correlation between metallicity and elements such as Ba and La that are likely produced by the s-process (e.g., Norris & Da Costa 1995;Suntzeff & Kraft 1996;Lee et al 1999;Smith et al 2000;Bellini et al 2010;Johnson & Pilachowski 2010;D'Orazi et al 2011;Marino et al 2011a;Pancino et al 2011;Villanova et al 2014). Less extreme examples also include M22, M2, M54, NGC 1851, NGC 5286, NGC 5824, and Terzan 5 (M22: e.g., Hesser et al 1977;Pilachowski et al 1982;Lehnert et al 1991;Marino et al 2009Marino et al , 2011bMarino et al , 2013Da Costa et al 2009;Roederer et al 2011;Alves-Brito et al 2012;M2: Piotto et al 2012;Lardo et al 2013;Yong et al 2014;Milone et al 2015;M54: e.g., Sarajedini & Layden 1995;Brown et al 1999;Siegel et al 2007;Bellazzini et al 2008;Carretta et al 2010a;NGC 1851: e.g., Yong & Grundahl 2008Milone et al 2009;Yong et al 2009Yong et al , 2015Zoccali et al 2009;Carretta et al ...…”

RUBIDIUM ABUNDANCES IN THE GLOBULAR CLUSTERS NGC 6752, NGC 1904, AND NGC 104 (47 Tuc)

“…If the lack of internal variations is confirmed, the internal polluters responsible for the light-element spreads must not have produced any s-process elements to a significant extent. Obviously the opposite requirement has to be satisfied by the stellar sources responsible for s-process variations in "non-standard" GCs such as M22 (Marino et al 2009), NGC 1851 (Yong & Grundahl 2008), ω Centauri (Johnson & Pilachowski 2010;Marino et al 2011b;D'Orazi et al 2011), which implies that the type of stars that produced the Na-O anticorrelation in all GCs cannot be the same as those that produced the variations in s-process Carretta et al (2007Carretta et al ( , 2009b, Marino et al (2008), Yong et al (2003), Wallerstein et al (2007).…”