0 Assigned ID: 6434 ----- Attempting Submission 1 (Thu Apr 10 13:17:23 GMT 2025) ----- 0 ----- Attempting Submission 1 (Wed Jul 23 15:50:14 GMT 2025) ----- HST Phase II Proposal 18058 successfully submitted. Receipt: # 18058-1 ----- Attempting Submission 2 (Thu Aug 14 16:09:12 GMT 2025) ----- The chemical composition of pristine gas through first-population stars in NGC 121 We propose to uncover, for the first time, the original chemical composition of the gas that formed an ancient globular cluster (GC) in the Small Magellanic Cloud (SMC). Our target is NGC 121, the only SMC cluster old enough to trace the properties of primordial star-forming gas. By combining archival and new HST/UVIS WFC3 F275W images, we will analyze stars in this cluster to reconstruct the conditions present in the early Universe. Our focus is on first-population (1P) stars, which preserve the chemical signature of the proto-cluster cloud. In Milky Way GCs, the 1P sequence often shows unexpected features, such as internal metallicity variations up to ~0.3 dex. These findings challenge the long-standing view that 1P stars are chemically homogeneous, suggesting instead that the star-forming gas was either inhomogeneous or later altered by stellar feedback. However, such studies remain limited to Galactic environments. NGC 121 provides a unique opportunity to investigate whether these internal chemical spreads are a universal property of globular clusters or depend on the host galaxy environment. By studying this SMC cluster, we will gain insight into early chemical enrichment processes in low-mass galaxies and explore the broader connection between star cluster formation and galactic evolution. This proposal aims to determine the chemical composition of the primordial gas from which the ancient globular cluster NGC 121, in the Small Magellanic Cloud, formed. To achieve this goal, we will obtain new observations of NGC121 with the WFC3/UVIS camera using the F275W filter. These data will complement existing exposures in the F336W, F438W, and F814W filters obtained under programs GO-13435 and GO-10396, which target overlapping fields centered on NGC 121. By combining the new F275W observations with the archival datasets, we will construct the multi-band photometry necessary to accurately probe the chemical composition of the early gas clouds from which globular cluster stars formed in the early Universe. To allow flexibility in the scheduling of HST orbits, our observation plan consists of 15 separate visits, each using 1 orbit. Each visit will include two F275W exposures of 1297 s, enabling us to achieve a signal-to-noise ratio of ~100 at the base of the red giant branch, sufficient to robustly characterize the chemical properties of the cluster's stellar populations. All exposures will be dithered to optimize PSF sampling, cosmic ray rejection, and astrometric precision. To mitigate CTE degradation, exposures will be flashed according to current STScI recommendations. Over the past two decades, our team has made significant contributions to the study of multiple stellar populations in globular clusters, including key topics related to this proposal. We bring together expertise in photometry, astrometry, spectroscopy, and theoretical modeling to address one of the most compelling questions in stellar astrophysics. Our group includes internationally recognized early-career and senior researchers with deep experience in Hubble Space Telescope data analysis, spectroscopic studies, and the development of stellar evolution and dynamical models. We are fully equipped with the skills, tools, and personnel required to carry out a rigorous data reduction and deliver a robust scientific analysis. (1) A. P. Milone, E. P. Lagioia, G. Cordoni, E. Dondoglio, and M. V. Legnardi have a long experience in photometric and astrometric data reduction and analysis. Moreover, this dataset will be extremely useful for the PhD thesis works of E. Bortolan, F. Muratore, and T. Ziliotto, which are all focused on topics strictly connected with multiple stellar populations. (2) A. F. Marino brings outstanding expertise in spectral analysis, particularly in deriving chemical abundances through the combined use of multi-band photometry and synthetic spectra. (3) M. Tailo is an expert in theoretical stellar evolution, stellar modeling, and the formation scenarios of multiple populations. His expertise will be crucial for interpreting the results of the proposed dataset in a broader theoretical context. (4) A. Mastrobuono-Battisti specializes in stellar dynamics and will provide essential insights into the dynamical implications of multiple stellar populations, complementing our findings with a dynamical perspective. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false true false false false false SUPPORTED EXT-CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F275W POS-TARG DITHER 10 LINE false false 2 270 LINE 1 false /tmp/OPVisit15834710426698467965.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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