Confirmation of a metallicity spread amongst first population stars in globular clusters

Abstract

Stars in massive star clusters exhibit intrinsic variations in some light elements (MPs) that are difficult to explain in a coherent formation scenario. In recent years, HST photometry has led to the characterisation of the global properties of th MPs in an unparalleled level of detail. In particular, the colour-colour diagrams known as chromosome maps have been proven to be very efficient at separating cluster stars with field-like metal abundance distribution (1P) from object with distinctive light-element abundance anti-correlations (2P). The unexpected wide colour ranges covered by the 1P group in the chromosome maps of the majority of the investigated Galactic GCs have been recently attributed to intrinsic metallicity variations up to ~0.30 dex, from the study of SGB stars in two metal rich Galactic GCs by employing appropriate HST filter combinations. On the other hand, high-resolution spectroscopy of small samples of 1P stars in NGC 3201 and NGC 2808 have so far provided conflicting results, with a spread of metallicity detected in NGC 3201 but not in NGC 2808. We present a new method that employs HST near-UV and optical photometry of RGB stars, to independently confirm these results. Our approach has been firstly validated using observational data for M2, a GC hosting a small group of 1P stars with enhanced (by ~0.5 dex) metallicity with respect to the main component. We have then applied our method to three clusters that cover a much larger metallicity range, and have well populated, extended first population sequences in their chromosome maps, namely M92, NGC2808, and NGC6362. We confirm that metallicity spreads are present among 1P stars in these clusters, thus solidifying the case for the existence of unexpected variations up to a factor of two of metal abundances in most GCs. We also confirm the complex behaviour of the mean metallicity differences between 1P and 2P stars.