1.1 Globular Clusters Globular Clusters (GCs) are dense, old stellar clusters, often containing hundreds of thousands of stars. They lie predominantly in the halo of the Milky Way (MW) but are also present in the bulge and thick disk with often extreme internal kinematics and metal-poor stars (Gratton et al. 2019). A robust definition of a GC is still being debated with many factors now taken into account such as the cluster mass, metallicity and which region in the galaxy the cluster belongs to, as well as investigations into chemical properties (Carretta et al. 2010). GCs were originally thought to be simple stellar populations, that is, they are comprised of stars with similar ages and abundances (Bastian and Lardo 2018). However, more complex chemical trends have been observed with large intra-cluster variations for many elements, indicating GCs can have multiple stellar populations (MSP).

A prominent chemical feature of most GCs are the presence of anti-correlations. In particular, the sodium-oxygen (Na-O) and magnesium-aluminium (Mg-Al) anti-correlations (Gratton et al. 2001; Carretta et al. 2009b; Pancino et al. 2017b; Bastian and Lardo 2018). Na and O undergo more internal mixing in stars compared to Mg and Al, therefore, Mg and Al abundances have minimal dependance on a star’s evolutionary stage (Pancino et al. 2017b). This allows the Mg-Al anti-correlation to act as a tool for tracing the chemical enrichment history of GCs. The observed Mg and Al abundances reflect the contributions of previous generations of stars, unaffected by ongoing nucleosynthesis in the observed stars. However, clusters that do exhibit Na-O anti-correlations do not always show Mg-Al anti-correlations as well, therefore the Na-O anti-correlation can be utilised in more clusters to determine the existence of multiple populations (Bastian and Lardo 2018).

There are two main theories of where GCs form, either they are part of their host galaxy’s initial formation, or they have formed externally to then be accreted into another galaxy (Gratton et al. 2019). External accretion has been supported by studying stellar morphology within clusters, and by counting the number of GCs in nearby dwarf galaxies, the Milky Way is proposed to have had approximately seven merger events with cluster-bearing dwarf galaxies (Mackey and Gilmore 2004).

This would generate the roughly 41 external clusters as determined from Mackey and Gilmore 2004. A slightly wider range of 27-47 possible external clusters from 6-8 dwarf galaxies was found in Forbes and Bridges 2010. Today, the Milky Way is known to contain at least 151 GCs with the origin of some still uncertain (Massari et al. 2019).

Spectroscopy is the study of starlight, often in optical or infrared wavelengths, to determine characteristics of stars such as their astrophysical parameters and chemical abundances. As GCs have a tight distribution of stars in the centre, it can be challenging to observe resolved stars in this region. Integrated light observations allow the study of stars within the cluster centre where multiple stars are observed at once in a single spectrum. However, individual stellar spectra can be obtained for stars that lie outside of this dense region. Spectroscopic observations of GCs can be undertaken by either obtaining integrated light spectra, or single stellar spectra where individual cluster members are observed. The chemical information obtained from examining spectra of GCs contains crucial information about the abundance of specific elements in the atmosphere of the stars. By observing many individual cluster stars, detailed intra-cluster variations can be explored to determine possible multiple populations and their origins.

1.2 Research Motivation The primary aim of this thesis is to investigate chemical trends within globular clusters (GCs) using the Gaia-ESO Survey (GES) (Gilmore et al. 2012; Gilmore et al. 2022; Randich et al. 2022) GC sample. Part of this is to assess if additional abundances can be determined to expand the sample. The GES GIRAFFE medium resolution and UVES high resolution spectra will be reassessed to determine if any additional chemical abundances can be measured using iSpec (Blanco-Cuaresma et al. 2014; Blanco-Cuaresma 2019). iSpec is a python wrapper around spectral synthesis codes and in this thesis, will be utilised with the synthesis code MOOG (Sneden et al. 2012). This method results in a synthetic spectrum created to be compared with the observed using a least squares fit. This fit is then minimised, producing the determined chemical abundances for the corresponding star.

The analysis of these abundances will focus on the intra and inter-cluster chemical trends, investigating both individual clusters and the entire GC population. Specifically, the trends for elements that strongly represent an astrophysical process such as a nucleosynthesis and enrichment processes. These elements are also known as tracer elements and are crucial for understanding the underlying contributions to stellar formation within GCs. Using tracer elements can help to determine the existence of any multiple populations and generations of stars with GCs, and their typical chemical trends. A comparison to the wider Milky Way chemical evolution will also be performed to provide insight into how these clusters relate to the MW as a whole, as well as the detection of possible accreted clusters from other galaxies. The presence of any accreted clusters provides opportunity to examine extra-galactic GC abundance trends and implications. These abundance analyses will contribute to understanding the elusive characteristics of GCs.

The secondary aim of this work is to investigate the capabilities of the University of Canterbury Ōtehīwai Mt John Observatory for GC analysis. This capability of the University of Canterbury Ōtehīwai Mt John Observatory will be tested by obtaining integrated light spectroscopy on the 1-metre McLellan telescope and the High Efficiency and Resolution Canterbury University Large Echelle Spectrograph (HERCULES). As with the GES spectra, iSpec will be used except this time, for the determination of astrophysical parameters such as metallicity and radial velocity. A comparison between the literature values of GCs observed and the derived parameters will be used to assess the quality of GC studies at Ōtehīwai Mt John Observatory. As the current observations on the 1-metre McLellan telescope are exclusively bright (V< 9), resolved targets, pushing the limit to fainter targets would provide new research opportunities and future programs.

1.3 Thesis Structure This thesis has the following structure. Chapter 2 discusses the background and theory of topics used in the thesis such as the chemical properties of stars and their application on the wider chemical evolution of the Milky Way galaxy. Chapter 3 outlines the methods, results and brief discussion of integrated light spectroscopy for two GCs obtained at the University of Canterbury’s Ōtehīwai Mt John Observatory. In Chapter 4, the spectral synthesis method for obtaining additional abundances from the Gaia-ESO Survey spectra is described. The abundance results from this are shown in Chapter 5 for the eleven elements investigated in this project and across the fourteen Globular Clusters. The analysis of the chemical trends and multiple populations from the determined abundances is in Chapter 6. Chapter 7 discusses the findings and wider implications of this project to the chemical understanding of the Milky Way. Finally, Chapter 8 concludes the work done in this thesis and provides an overview of any possible future work.