title: Probing Nucleosynthesis and Chemical Properties: Infrared Spectroscopy in CEMP Stars and Obscured Star Clusters
Creators: name: Mura Guzman, Aldo
Publisher: name: The Australian National University
Year: 2023

description: Several mechanisms have contributed to the chemical enrichment of our Galaxy throughout its history, from the light elements produced in the Big Bang to the heavier elements formed inside stars and ejected back into the interstellar medium at the time of their death. Understanding the details of nucleosynthesis processes along with the structural properties of the stars are crucial to determine the relative contribution of each source to the Galactic chemical budget. In this context, observations are fundamental to verify and constrain theoretical models to improve our current knowledge and better address the question, "How are the elements produced and recycled through galaxies?" In this Thesis, we discuss chemical abundances from two sources: (i) 12 stars from the intermediate-age globular cluster (GC) GLIMPSE-C01, and (ii) 8 Carbon Enhanced Metal-Poor (CEMP) stars. Their abundances provide valuable insights, at two different metallicity regimes, on nucleosynthesis and chemical evolution. We use high-resolution, high signal-to-noise ratio infrared observations to extract chemical abundance information of several species in carefully selected stellar targets. The spectra were obtained using the Immersion GRating INfrared Spectrometer (IGRINS) during its time at the 4.3-meter Lowell Discovery Telescope (formerly known as the Discovery Channel Telescope) and at the 8-meter class telescope, Gemini South. Analysis of the chemical abundances was conducted via synthetic spectra fitting using the thermodynamic equilibrium code MOOG, coupled with 1D LTE atmosphere models, following a classical spectroscopic method. (i) The nature of the candidate globular cluster GLIMPSE-C01 has been long debated. Previous efforts trying to pin down its age and metallicity demonstrated the difficulties of working with this extremely obscured (AV=14-22) and differential reddened object. We discuss the advantages and analysis of infrared observations to derive stellar parameters and chemical abundances in GLIMPSE-C01. Our result from this new infrared data set reveals super-solar iron abundances in members of the cluster, with no evidence of any intrinsic spread, and indications of multiple populations from Na star-to-star abundance variations. Abundances of the alpha elements Ca and Si follow the Bulge trend. These results show that GLIMPSE-C01 is the youngest and most metal-rich globular cluster in our Galaxy and offers fundamental new constraints on the origin and formation of the Galactic globular cluster system. (ii) The distinct chemical patterns observed in the abundance ratios of different types of CEMP stars, indicate different formation mechanisms, providing crucial insights into stellar nucleosynthesis in the early Galaxy. Fluorine abundances obtained from the HF molecular absorption at ~2.3um in the near-infrared, offer a valuable test for nucleosynthesis models. However, there are limited observational data at low metallicities due to the strict set of conditions required to enable HF detections. We present, in two parts, the results of carefully selected and carefully designed observations of 8 CEMP stars to derive their F abundances. The metallicity range of the sample spans from [Fe/H] = -3.87 to [Fe/H] = -2.20, with two 2-sigma detections, including the F detection at the lowest metallicity so far and upper limits. The results are compared (when available) to state-of-the-art theoretical models of Asymptotic Giant Branch nucleosynthesis and binary evolution, and Faint Supernova nucleosynthesis from Population III stars. We discuss the inconsistencies found between the models and the observed F abundances, and demonstrate the power of fluorine measurements to constrain the physical properties of the first chemical enrichment mechanisms in the Galaxy.