Multiple populations among very low-mass stars in NGC288

Globular Clusters (GCs) are among the oldest objects in the Universe. Nearly all GCs host two main stellar groups of stars, a first population (1P) with a chemical composition similar to halo field stars with similar metallicity, and the second population (2P) of stars enriched in He, N, Na, and Al and depleted in O and C. The astronomers have proposed two main scenarios to explain the formation of multiple populations in GCs, namely the single- and the multiple-generation scenarios. Each of them has major implications in Cosmology, Galactic Archae- ology, and stellar evolutionary models. For these reasons, the knowledge of GCs’ formation and evolution is crucial to shed light on the formation and evolution of the Galaxy. Nowadays, most of the studies on multiple populations are focused on stars more massive than ∼ 0.5−0.6 M⊙, whereas the Very Low-Mass (VLM) regime is almost unexplored. Nonetheless, VLM stars would provide crucial information to constrain the formation scenarios. Indeed, the different scenarios predict different chemical abundances for 2P stars with different masses. In the single-generation scenario, 2P stars with different masses also have different light-element abundance, while the multiple-generation scenarios predict that all 2P stars share the same chem- ical composition. In this research, I use Hubble Space Telescope images to investigate the GC NGC 288 and characterize, for the first time, its multiple stellar populations among VLM stars. To this, I derive the mF 160W vs mF 110W − mF 160W Color- Magnitude Diagram (CMD), which is a very efficient tool to disentangle multiple stellar populations below the main sequence (MS) knee. As expected, I detected two distinct MSs of M-dwarfs and I showed that the MS split is due to stellar populations with different oxygen abundances. I associated the blue and red stellar sequences with 1P and 2P stars, respectively. Based on isochrones and synthetic spectra with appropriate chemical com- positions that I computed for this work, I derived the colors of 1P and 2P stars corresponding to different abundances of He, C, N, and O. The comparison be- tween these isochrones and the observed CMD allowed me to infer the oxygen abundance of 1P and 2P stars. I found that 2P M-dwarfs are enhanced in oxygen by ∆ [O/Fe] = 0.33 ± 0.03 dex with respect to 1P VLM stars. Such oxygen variation is comparable with that derived from high-resolution spectroscopy of red-giant branch stars. The finding that the chemical composition of 2P stars does not depend on stellar mass is consistent with the predictions of the multiple-generation scenarios.