Abundances for a Large Sample of Red Giants in NGC 1851: Hints for a Merger of Two Clusters?

Abstract

We present the abundance analysis of a sample of more than 120 red giants in the globular cluster (GC) NGC 1851, based on FLAMES spectra. We find a small but detectable metallicity spread. This spread is compatible with the presence of two different groups of stars with a metallicity difference of 0.06-0.08 dex, in agreement with earlier photometric studies. If stars are divided into these two groups according to their metallicity, both components show a Na-O anticorrelation (signature of a genuine GC nature) of moderate extension. The metal-poor stars are more concentrated than the metal-rich ones. We tentatively propose the hypothesis that NGC 1851 formed from a merger of two individual GCs with a slightly different Fe and α−element content, and possibly an age difference up to 1 Gyr. This is supported also by number ratios of stars on the split subgiant and on the bimodal horizontal branches. The distribution of n-capture process elements in the two components also supports the idea that the enrichment must have occurred in each of the structures separately, and not as a continuum of events in a single GC. The most probable explanation is that the proto-clusters formed into a (now dissolved) dwarf galaxy and later merged to produce the present GC.

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

“…In Figure 6, there is no cor-relation between metallicity, [Fe/H], and C+N+O. In this figure, we also confirm the anticorrelation between O and Na (Carretta et al 2010b(Carretta et al , 2011. Additionally, we identify a positive correlation between Na and C+N+O.…”

“…As noted in the introduction, there is evidence for a small iron abundance dispersion in NGC 1851 (Carretta et al 2010b(Carretta et al , 2011. For our [Fe/H] measurements, the standard deviation is 0.055 ± 0.011 dex.…”

“…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.…”

“…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).…”