Iron and s-elements abundance variations in NGC 5286: comparison with ‘anomalous' globular clusters and Milky Way satellites

Abstract

We present a high resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster NGC 5286. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s-process element abundance ratios, with two main groups, the s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the s-rich stars having higher Fe, e.g. <[Fe/H]> s−rich − <[Fe/H]> s−poor ∼0.2 dex; and (iii) the presence of O-Na-Al (anti-)correlations in both stellar groups. We have defined a new photometric index, c BVI =(B−V)−(V−I), to maximise the separation in the colour-magnitude diagram between the two stellar groups with different Fe and s-element content, and this index is not significantly affected by variations in light elements (such as the O-Na anticorrelation). The variations in the overall metallicity present in NGC 5286 add this object to the class of anomalous GCs. Furthermore, the chemical abundance pattern of NGC 5286 resembles that observed in some of the anomalous GCs, e.g. M 22, NGC 1851, M 2, and the more extreme ω Centauri, that also show internal variations in s-elements, and in light elements within stars with different Fe and s-elements content. In view of the common variations in s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.

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

“…In contrast, Smith et al (2000) find in ω Cen that the s-process production may be best fit by pollution from ∼1.5-3 M e AGB stars. A similar conclusion is reached by Marino et al (2015) between M22, M2, and NGC 5286. These authors suggest that the larger [Ba/Fe] range found in M2 and NGC 5286 stars compared to M22 stars may be due to different classes of polluters enriching the cluster interstellar mediums.…”

“…As can be seen in Figures 14-15, the shape and slope of the [La/Fe] and [La/Eu] distributions are nearly identical for all of the iron-complex clusters (see also Marino et al 2015). Combining the information from Figures 10 and 15 indicates that in all of the iron-complex clusters included here the increase in [La/Eu] is due to almost pure s-process enrichment.…”

“…Although NGC 6273 is only at a distance of ∼9 kpc from the Sun (e.g., Piotto et al 1999), the large and highly variable reddening complicate both cluster RGB target selection and the interpretation of its color- 8 The term "iron-complex" refers to any globular cluster exhibiting a significant (0.10 dex) [Fe/H] dispersion when measured from high resolution spectra. We have adopted this term in order to avoid confusing the word "anomalous," which can refer to either a cluster with a metallicity dispersion (e.g., Marino et al 2015) or a sub-population with peculiar chemistry residing in a cluster (e.g., Pancino et al 2000;Yong et al 2014). We note that clusters with both [Fe/H] and s-process abundance spreads have also been referred to as "s-Fe-anomalous" .…”

“…Although the measured [Fe/H] dispersion is 12% (0.05 dex) for many globular clusters (Carretta et al 2009a), a population of about 8 known clusters exists for which the derived [Fe/H] spread exceeds the measurement errors (e.g., see Marino et al 2015;their Table 10). These "anomalous" clusters, which we refer to as "iron-complex" clusters, 8 are often identified by the following features: (1) a dispersion in [Fe/H] exceeding ∼0.10 dex when measured using moderately high dispersion and signal-to-noise ratio (S/N) spectra, (2) multiple photometric sequences, especially on the SGB, and (3) a significant abundance spread for light elements and also heavy elements that, in the Solar System, are produced by the slow neutron-capture process (s-process).…”

Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

“…1999;Gratton et al 2001;Bedin et al 2004;Norris 2004;Piotto et al 2005;Mackey & Broby Nielsen 2007;Lee et al 2009;Da Costa et al 2014;See Gratton et al 2012 for a recent review). Most of the investigated GCs in the Galaxy are observed to show anti-correlations between light elements (e.g., C, N, and O) of cluster members stars (e.g., Carretta et al 2009; C09) whereas only 8 GCs have been so far confirmed to have star-to-star abundance spreads in heavy elements (e.g., Yong et al 2014;Marino et al 2015). Extended main-sequence turn-offs (eMSTOs) and splits in main-sequence observed in the color magnitude diagrams (CMDs) of some LMC GCs (e.g., Mackey & Broby ⋆ E-mail: kenji.bekki@uwa.edu.au Nielsen 2007; Goudfrooij et al 2014;Milone et al 2016) can be possible evidence for the multiple stellar populations with different ages, though recent observations suggest that internal stellar rotation rather than age spreads could explain the physical properties of LMC clusters with eMSTOs (e.g., Bastian & De Mink 2009;Milone et al 2016;Li et al 2016).…”