A comparison of abundance analyses of first generation stars in multiple populations in 47 Tuc and NGC 3201

Abstract

The distinction of the stellar content in globular clusters (GCs) in multiple stellar populations characterized by different amounts of proton-capture elements has been well understood for some time. On the other hand, the existence of noticeable variations in metallicity among GC stars is still debated. In particular, recent spectroscopic analyses claimed the presence of a small variation in metallicity, ∼0.1 dex, for the first generation (FG) stars in NGC 3201 and NGC 104 (also called 47 Tuc). However, in both cases the claim is not robust because of the internal error of 0.1 dex associated with the [Fe/H] values. To verify the existence of a metallicity variation, we compared two analyses performed by the same authors using an identical methodology. We found trends of metallicity as a function of the spectroscopically derived effective temperatures. However, the trends are in opposite directions; in NGC 3201, cooler (and brighter) stars have higher [Fe/H] values, whereas in 47 Tuc they show lower metallicities. The trend is not statistically significant in the former case, but it is in the latter. The dependence of metallicity on the luminosity along the red giant branch seems to indicate problems in the abundance analysis for 47 Tuc. Finally, the effective temperatures do not show a significant variation as a function of the colour spread along the HST pseudo-colour map, which we should observe if the trends of metallicity with temperature were a real effect of intrinsic scatter in iron. According to this comparison, we conclude that with these analyses, and the associated spurious trends, the issue of metallicity variations in FG stars is not yet resolved.