Searching for a predicted metallicity variation in the type II globular cluster NGC 362 = Buscando una variación de metalicidad predicha en el cúmulo globular de tipo II NGC 362.

Abstract

According to Milone et al. (2017), NGC 362 is a rare Type II Galactic globular cluster, showing a complex pseudo two-color diagram or ’Chromosome Map’ based on their HST UV Legacy photometry. The clear separation of its stellar populations in the color-magnitude diagram and the distribution of the giant stars in the chromosome map strongly suggests that NGC 362 could host stars with both cluster-nominal as well as enhanced heavy-element abundances, in particular iron, leading to an intrinsic metallicity spread, as shown by at least some other Type II globulars. However, despite previous spectroscopic observations of NGC 362, no such metallicity variation has been detected, but previous studies did not pre-select potential normal and anomalous stars. Our main goal is to search for any internal variation of [Fe/H] which would give us insight into the formation and evolution of this interesting globular cluster and Type II clusters in general. In this work, we present the abundance analysis for a sample of 11 red giant branch members based on high resolution and high S/N spectra obtained with the MIKE echelle spectrograph mounted at the Magellan-Clay telescope. The stars were carefully selected from the HST photometry to maximize the possibility of including a metallicity variation based on the Milone et al. (2017) scenario. HST and GAIA photometry and astrometry has been used to determine atmospheric parameters and membership. We obtained Teff, log(g) and vt for our target stars and measured the mean iron content of the sample and its dispersion with three different methods, which lead to [Fe/H]1=-1.10±0.02, [Fe/H]2=-1.09±0.01 and [Fe/H]3=-1.10±0.01, while the internal dispersion turned out to be σ[Fe/H]1=0.06±0.01, σ[Fe/H]2=0.03±0.01 and σ[Fe/H]3=0.05±0.01 respectively. The error analysis gives an internal dispersion due to observational error of 0.05 dex. Comparing the observed dispersion with the internal errors, we conclude that NGC 362 does not show any trace of an internal iron spread, in particular between the anomalous red stars with presumably higher metallicities and the bulk of the stars that fall along the nominal first and second generation sequences, based on the chromosome map. Thus, our findings do not support the metallicity spread predicted by Milone et al. (2017). This makes NGC 362 the first Type II cluster which definitely does not show such a spread, thus requiring a modification to their prediction