X-shooter spectroscopy of Liller 1 giant stars
Deimer Antonio Alvarez Garay, C. Fanelli, L. Origlia, Cristina Pallanca, A. Mucciarelli, L. Chiappino, Chiara Crociati, B. Lanzoni, F. R. Ferraro, R. Michael Rich, E. Dalessandro
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]⟩ = −0.31 ± 0.02 and 1 σ dispersion of 0.08 dex) and with enhanced [ α /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]⟩ = +0.22 ± 0.03 and 1 σ dispersion of 0.06 dex) and roughly solar-scaled [ α /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 12 C/ 13 C 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.