Metallicity of the globular cluster NGC 6388 based on high-resolution spectra of more than 160 giant stars

Abstract

NGC 6388 is one of the most massive Galactic globular clusters (GC) and it is an old, metal-rich Galactic bulge cluster. By exploiting previous spectroscopic observations, we were able to bypass the uncertainties in membership related to the contamination from strong field stars. We present the abundance analysis of 12 new giant stars with UVES spectra and 150 giants with GIRAFFE spectra acquired at the ESO-VLT. We derived radial velocities, atmospheric parameters, and iron abundances for all the stars. When combined with the previous data, we obtained a grand total of 185 stars homogeneously analysed in NGC 6388 from high-resolution spectroscopy. The average radial velocity of the 185 stars is 81.2 ± 0.7, rms 9.4 km s−1. We obtained an average metallicity [Fe/H] = −0.480 dex, rms = 0.045 dex (35 stars), and [Fe/H] = −0.488 dex, rms = 0.040 dex (150 stars) from the UVES and GIRAFFE samples, respectively. Comparing these values to the internal errors in abundance, we excluded the presence of a significant intrinsic metallicity spread within the cluster. Since about a third of giants in NGC 6388 is claimed to belong to the ‘anomalous red giants’ in the HST pseudo-colour map defining the so-called type-II GCs, we conclude that either enhanced metallicity is not a necessary requisite to explain this classification (as also suggested by the null iron spread for NGC 362) or NGC 6388 is not a type-II globular cluster.