Investigating the chemistry of stars in the Large Magellanic Cloud

Abstract

The Large Magellanic Cloud (LMC) is one of the Milky Way's most prominent dwarf galaxy satellites. Positioned within the Milky Way's gravitational influence, the LMC offers a unique opportunity to explore near-field cosmology in the context of stellar evolution, star formation, interstellar medium behaviour, and galaxy evolution. This thesis presents detailed abundance measurements for various elements in stars within the LMC, shedding light on their chemical composition and evolution.

The first study presents a search for multiple populations in a 1.95 Gyr old LMC star cluster NGC 1846, which is a direct test of the 2 Gyr minimum boundary for the age of massive star clusters exhibiting chemical abundance spread. High-resolution VLT/FLAMES spectra, complemented by HST photometric data and non-local thermodynamic equilibrium corrections, reveal a lack of intrinsic star-to-star spread in Na and O abundances, implying no evidence of multiple populations in NGC 1846. However, a significant spread in carbon abundances indicates varying evolutionary mixing on the red giant branch. This work enhances our understanding of the existence of multiple populations in intermediate-age massive star clusters in galaxies.

The second study presents a search for extremely metal-poor (EMP) stars in the LMC, crucial for unraveling information about the earliest stars and conditions during the initial stages of star formation in dwarf galaxies. Utilising photometric data from SkyMapper DR3 and kinematic data from Gaia DR2, a selection of EMP candidates was made using various photometric and kinematic cuts. A low-resolution spectroscopic follow-up observation was done using ANU 2.3/WiFeS. The study identifies seven stars with [Fe/H] < -2.75, including two with [Fe/H] < -3. The radial velocities of these stars are consistent with the LMC's outer rotation curve, firmly establishing them as members of the LMC. These stars represent the most metal-poor stars discovered in the LMC to date.

The third study delves into detailed abundance results for the seven LMC stars mentioned in the previous study. Based on UVES high dispersion spectra, the stars are confirmed as the most metal-poor stars ever found in the Magellanic Clouds. While their element abundance ratios are generally consistent with Milky Way halo stars of similar [Fe/H], only two of the more metal-rich stars in the sample exhibit enhancements in r-process elements. The absence of r-process enrichment in stars with lower [Fe/H] values imply a minimum timescale of ~100 Myr for the neutron star binary merger process to generate substantial r-process enhancements in the LMC. This finding challenges previous results and provides insights into the early stages of star formation in the LMC and galaxies of similar mass.

Together, these studies contribute valuable insights into the chemical composition, evolutionary history, and early star formation processes within the LMC, advancing our understanding of stellar and galactic evolution.