The peculiar nature of Liller1 from high-resolution CRIRES+ spectra

The discovery of the complex stellar populations in Terzan 5 and Liller 1, among the most massive stellar systems of the Bulge and more generally of the Galaxy, posed new questions about their true nature and their possible connection with the Bulge at the epoch of its formation and early evolution. Indeed, they host sub-populations with significantly different ages (several Gyrs) and metallicities (about 1 dex) indicate that these stellar systems cannot be genuine globulars. While Terzan 5 has been already photometrically, kinematically and chemically characterized in some detail, Liller 1 still needs a proper chemical investigation. The original work presented in this Thesis has concerned the first high-resolution spectroscopic study of the stellar populations in Liller 1, by using high resolution near-IR spectra recently acquired with CRIRES+ at the VLT. Accurate measurements of the chemical abundance of key species as Fe, Mg, Al, Si, S, Ca and Ti have been obtained for a first sample of 12 member stars. The results obtained in this Thesis confirm the photometric existence of at least two major sub-populations with significantly different iron abundance and [αlpha/Fe] enhancement. The chemistry of Liller 1 is strikingly similar to the one of Terzan 5 and of the Bulge field. This is the first firm spectroscopic evidence of the complex nature of this stellar system and fully confirms that this is not a genuine globular. The available evolutionary, kinematic and chemical information on Terzan 5 and Liller 1 suggest an in-situ (i.e. bulge) formation and evolution for these systems, thus supporting a scenario where their old stellar sub-populations can be fossil fragments of more massive clumps that may have contributed to form the bulge. These fragments survived total disruption and could evolve and possibly self-enrich as independent systems. The younger sub-populations of Liller 1 and Terzan 5 could be the products of such an evolution.