Apparent trend of the iron abundance in NGC 3201: The same outcome with different data

Abstract

We further study the unusual trend we found at statistically significant levels in some globular clusters, including NGC 3201: a decreasing iron abundance in red giants toward the cluster centers. We first show that recently published new estimates of iron abundance in the cluster reproduce this trend, in spite of the authors’ statement about no metallicity spread due to a low scatter achieved in the [Fe II/H] ratio. The mean of [Fe II/H] within R ~ 2¢ from the cluster center is lower, by Δ[Fe II/H] = 0.05 ± 0.02 dex, than in the outer region, in agreement with our original estimate for a much larger sample size within R » 9¢. We found that an older data set traces the trend to a much larger radial distance, comparable with the cluster tidal radius, at Δ[Fe/H] ∼ 0.2 dex, due to higher metallicity of distant stars. We conclude the trend is reproduced by independent data sets and find that it is accompanied by both a notable same-sign trend of oxygen abundance that can vary by up to Δ[O/Fe] ∼ 0.3 dex within R » 9¢ and an opposite-sign trend of sodium abundance