Multiple stellar populations in the globular cluster NGC 1851

Abstract

We present a detailed chemical tagging of individual stellar populations in the Galactic globular cluster (GC) NGC 1851. Abundances are derived from FLAMES spectra for the largest sample of giants (124) and the most extensive number of elements ever analysed in this peculiar GC. The chemistry is characterised using homogeneous abundances of proton-capture (O, Na, Mg, Al, Si), α-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). We confirm the presence of an [Fe/H] spread larger than the observational errors in this cluster, but too small to clearly separate different sub-populations. We instead propose a classification scheme using a combination of Fe and Ba (which is much more abundant in the more metal-rich group) by means of a cluster analysis. With this approach, we separated stars into two components of a metal-rich (MR) and a metal-poor (MP) population. Each component displays a Na-O anticorrelation, which is a signature of a genuine GC, but has different ratios of primordial (FG) to polluted (SG) stars. Moreover, clear (anti)correlations of Mg and Si with Na and O are found for each component. The level of [α/H] tracks iron and is higher in the MR population, which might therefore have received an additional contribution from core-collapse supernovae. When considering all s-process elements, the MR population shows a larger enrichment than the MP one. This is probably due to the contribution of intermediate-low mass stars, because we find that the level of heavy s-process elements is higher than that of light s-process nuclei in the MR stars; however, a large contribution from low mass stars is unlikely, because it would likely cancel the O-Na anticorrelation. Finally, we confirm the presence of correlations between the amount of proton-capture elements and the level of s-process elements previously found by other investigations, at least for the MR population. This finding apparently requires a quite long delay for the second generation of the MR component. Scenarios for the formation of NGC 1851 appear complex, and are not yet well understood. A merger of two distinct GCs in a parent dwarf galaxy, each cluster with a different Ba level and an age difference of ∼1 Gyr, might explain (i) the double subgiant branch; (ii) a possible difference in C content between the two original GCs; and (iii) the Strömgren photometry of this peculiar cluster. However, the correlation existing between p-capture and n-capture elements within the MR population requires the additional assumption of a long delay for its second generation. More observations are required to fully understand the formation of this GC.

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

“…These values were determined by comparing the observed and rest wavelengths for about 80 atomic lines in each program star. We find an average value of +320.2 ± 1.3 km s −1 (σ = 4.8 km s −1 ) which agrees with values from the literature: +320.5 ± 0.6 km s −1 (Harris 1996(Harris , updated in 2010 catalogue; +320.26 km s −1 (rms = 3.74 km s −1 ) (Carretta et al 2011); +320.0 ± 0.4 km s −1 (Scarpa et al 2011); +318.2 ± 0.5 km s −1 (Gratton et al 2012c); +319.5 ± 0.5 km s −1 . That is, all program stars are likely cluster members.…”

“…We defer our comparison with the latter until Section 4.2. For the seven stars in common with Carretta et al (2010b) and Carretta et al (2011), we find the following differences in the sense "this study − Carretta et al": ∆RV = +0.8 ± 0.3 km s −1 ; ∆T eff = −10 ± 14 K; ∆log g= −0.05 ± 0.01 cgs; ∆ξt= +0.20 ± 0.14 km s …”

“…For NGC 1851, a subset of our program stars were studied in Carretta et al (2010b), Carretta et al (2011) and Villanova et al (2010). We defer our comparison with the latter until Section 4.2.…”

“…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.…”

“…In the case of NGC 1851, the double subgiant branch ) could be composed of two coeval populations with different mixtures of C+N+O abundances (Cassisi et al 2008). Since the discovery of the double subgiant branch in NGC 1851, understanding its nature and formation history has been an active area of research Han et al 2009;Lee et al 2009;Milone et al 2009;Olszewski et al 2009;Ventura et al 2009;Zoccali et al 2009;Carretta et al 2011Carretta et al , 2012Bekki & Yong 2012;Gratton et al 2012c,b;Lardo et al 2012;Joo & Lee 2013;Marino et al 2014). Indeed, it has been suggested that NGC 1851 may be the product of the merger of two clusters (Carretta et al 2010b).…”