The Chemical Evolution of the Globular Cluster ω Centauri (NGC 5139)

Abstract

We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster u Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are deÐned as a function of the standard metallicity indicator iron. The lowest metallicity stars in u Cen have [Fe/H] D [1.8, and the initial abundance distribution in the cluster is established at this metallicity. The stars in the cluster span a range of [Fe/H] D [1.8 to [0.8. At the lowest metallicity, the heavy-element abundance is found to be well characterized by a scaled solar system r-process distribution, as found in other stellar populations at this metallicity. As iron increases, the s-process heavy-element abundances increase dramatically. Comparisons of the s-process increases with recent stellar models Ðnds that s-process nucleosynthesis in 1.5È3 asymptotic giant branch stars (AGB) Ðts well the heavy-element abundance M _ distributions. In these low-mass AGB stars, the dominant neutron source is 13C(a, n)16O. A comparison of the Rb/Zr abundance ratios in u Cen Ðnds that these ratios are consistent with the 13C source. The reason u Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGB ejecta, because of their low velocity winds, are more efficiently retained in the cluster relative to the much faster moving Type II supernova ejecta. SigniÐcant s-process enrichment relative to Fe, from the lower mass AGB stars, would require that the cluster was active in star formation for quite a long interval of time, of the order of 2È3 Gyr. The AGB ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of a-rich elements shows that no signiÐcant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] D [0.6 (derived here for the Ðrst time in this cluster) Ðnds a plausible interpretation.

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

“…To compare with homogeneous literature sets, we considered preferentially large (≥10 stars) samples of high-resolution measurements of stars belonging to ω Cen. Unfortunately, there are no high-resolution studies of subgiants 13 , so we resorted to red giants surveys (Norris & Da Costa 1995;Smith et al 2000;Johnson et al 2009). This will help us to cross-identify sub-populations in the SGB region with those already identified on the RGB.…”

“…From the pioneering studies in the 60 s to the latest high-quality data and models, more and more details of its multiple stellar populations have come to light, but the picture did not become as clear as expected. Excellent photometries and astrometric catalogues have recently been produced by both groundbased (such as Lee et al 1999;Pancino et al 2000;van Leeuwen et al 2000;Hilker & Richtler 2000;Hughes & Wallerstein 2000;Sollima et al 2005a;Calamida et al 2009;Bellini et al 2009) and space telescopes (e.g., Ferraro et al 2004;Bedin et al 2004;Bellini et al 2010), complemented by high-quality spectroscopic abundance studies both at high (e.g., Norris & Da Costa 1995;Smith et al 2000;Cunha et al 2002;Pancino 2003;Johnson et al 2008Johnson et al , 2009Johnson & Pilachowski 2010;Marino et al 2010) and low (Norris et al 1996;Suntzeff & Kraft 1996;Sollima et al 2005b;Stanford et al 2006;Villanova et al 2007) resolution. Nevertheless, several puzzles still await solutions.…”

CNO abundances in the globular clusters NGC 1851 and NGC 6752★

“…These molecular line strength variations are driven by star-to-star abundance variations for the light elements from C to Al (see reviews by Kraft 1994and Gratton et al 2004, 2012a for details.) Secondly, a star-to-star dispersion in iron-peak elements, and other elements, has long been known to exist in the globular cluster ω Centauri (e.g., Freeman & Rodgers 1975;Cohen 1981;Norris & Da Costa 1995;Smith et al 2000;Johnson & Pilachowski 2010). More recently, abundance dispersions have also been identified in a number of globular clusters including M2 , M22 (Marino et al 2009Roederer et al 2011), M54 (Carretta et al 2010a) Carretta et al 2011), NGC 3201 1 (Simmerer et al 2013), NGC 5824 2 ) and Terzan 5 (Ferraro et al 2009;Origlia et al 2013), although the shape of the metallicity distribution function differs between these objects.…”

Globular cluster abundances: the role of asymptotic giant branch stars

“…There are exceptions noted in the literature, including the metal-rich cluster 47 Tuc (Wylie et al 2006) and NGC 1851 (Yong et al 2009). The other important exception is the massive cluster ω Centauri, whose age and metallicity spread, along with a rise in s-process element abundances with increasing [Fe/H], suggests that 162 A. I. Karakas it evolved very differently from other GCs and may have an extragalactic origin (Smith et al 2000).…”