X-shooter spectroscopy of Liller 1 giant stars

Abstract

We present the first comprehensive chemical study of a representative sample of 27 luminous red giant branch (RGB) stars belonging to Liller 1, a complex stellar system in the Galactic bulge. This study is based on medium-resolution near-infrared spectra acquired with X-shooter at the Very Large Telescope. We found a subpopulation counting 22 stars with subsolar metallicity ($<$ Fe/H and 1sigma dispersion of 0.08 dex) and with enhanced alpha /Fe Al/Fe and K/Fe that likely formed early and quickly from gas that was mainly enriched by type II supernovae, and a metal-rich population counting 5 stars with supersolar metallicity ($<$ Fe/H $>$=+0.22pm 0.03 and 1sigma dispersion of 0.06 dex) and roughly solar-scaled alpha /Fe Al/Fe and K/Fe that formed at later epochs from gas that was also enriched by type Ia supernovae. Moreover, both subpopulations show enhanced Na/Fe as in the bulge field, about solar-scaled V/Fe and depletion of C/Fe and 12C/13C with respect to the solar values. This indicates that mixing and extra-mixing processes during the RGB evolution also occur at very high metallicities. Notably, no evidence of a Na-O anticorrelation, which is considered the fingerprint of genuine globular clusters, has been found. This challenges any formation scenarios that invoke the accretion of a molecular cloud or an additional stellar system onto a genuine globular cluster. The results of this study underline the strong chemical similarity between Liller 1 and Terzan 5 and support the hypothesis that these complex stellar systems might be fossil fragments of the epoch of Galactic bulge formation.