The Origin of the Spurious Iron Spread in the Globular Cluster NGC 3201

Abstract

NGC 3201 is a globular cluster suspected to have an intrinsic spread in the iron content. We re-analysed a sample of 21 cluster stars observed with UVES-FLAMES at the Very Large Telescope and for which Simmerer et al. found a 0.4 dex wide [Fe/H] distribution with a metal-poor tail. We confirmed that when spectroscopic gravities are adopted, the derived [Fe/H] distribution spans ∼0.4 dex. On the other hand, when photometric gravities are used, the metallicity distribution from Fe I lines remains large, while that derived from Fe II lines is narrow and compatible with no iron spread. We demonstrate that the metal-poor component claimed by Simmerer et al. is composed by asymptotic giant branch stars that could be affected by non local thermodynamical equilibrium effects driven by iron overionization. This leads to a decrease of the Fe I abundance, while leaving the Fe II abundance unaltered. A similar finding has been already found in asymptotic giant branch stars of the globular clusters M5 and 47 Tucanae. We conclude that NGC 3201 is a normal cluster, with no evidence of intrinsic iron spread.

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

“…These differences are comparable to the internal precision of our measurements. A similar but possibly relevant issue related to differences in [Fe/ H] derived from 1D LTE analyses of RGB, as opposed to AGB, stars has been noted by Ivans et al (2001;NGC 5904), Lapenna et al (2014;NGC 104), and Mucciarelli et al (2015;NGC 3201). These authors found that the derived [Fe I/H] abundances were ∼0.10-0.15 dex lower than the [Fe II/H] abundances for AGB, but not RGB, stars when the model atmosphere parameters were determined via spectroscopic methods.…”

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…and M22 (Pilachowski et al 1982;Da Costa et al 2009;Marino et al 2009Marino et al , 2011b, may host multiple populations with distinct [Fe/H] ratios. However, recent studies by Mucciarelli et al (2015b) and Lardo et al (2016) Lapenna et al (2014), and Mucciarelli et al (2015c) found that [Fe/H] determinations for RGB and AGB stars can differ systematically by >0.1 dex, and that mixing RGB and AGB stars in a sample can produce an artificial metallicity spread. A common thread connecting these issues is the method by which a star's surface gravity is determined (spectroscopic versus photometric).…”

Light and Heavy Element Abundance Variations in the Outer Halo Globular Cluster NGC 6229

“…For example, Figure 6 shows that NGC 6229 has significantly lower [Na/Fe] 8 We note that Simmerer et al (2013) found a potential [Fe/H] spread in NGC 3201, and that the cluster has a fainter absolute magnitude than NGC 6229 with M V = -7.45 (Harris 1996). However, Mucciarelli et al (2015) Carretta et al (2009b,c), and represent the general trends exhibited by ∼15 monometallic clusters. The cyan symbols represent the abundance trends of several clusters (filled symbols) and field stars (open circles) associated with the Sagittarius dwarf spheroidal galaxy (Bonifacio et al 2000;Cohen 2004;Sbordone et al 2007;Carretta et al 2010a;McWilliam et al 2013;Carretta et al 2014bCarretta et al , 2017.…”