A multi-wavelength, Hubble space telescope study of two globular clusters

Abstract

Globular clusters (GCs) are among the densest and oldest stellar aggregates in the Galaxy, and are thought to date from around the time that the Galaxy first formed. The high central densities that characterise GCs lead to frequent stellar interactions and the formation of exotic stellar populations, making GCs excellent laboratories for studying the stellar dynamics of dense environments. The ability to observe many stars which are equidistant and (approximately) the same age makes GCs invaluable in understanding stellar structure and evolution.

This thesis describes surveys of two Galactic GCs: far- and near-ultraviolet data from the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope (HST) were used to study the core region of M80, and far-ultraviolet to Iband data from the Space Telescope Imaging Spectrograph (STIS), ACS and the Wide Field Camera 3 (WFC3) on-board HST were used to carry out an in-depth, multi-wavelength survey of NGC6752. In both studies, the properties of key stellar populations resident in globular clusters are investigated.

In M80, it was discovered that the fainter (redder) blue straggler stars are more centrally concentrated than the brighter (bluer) ones. This is contrary to expectations, and suggests that blue stragglers might get a ‘kick’ at formation, before settling back towards the core. In a search for counterparts to known X-ray sources in M80, one X-ray source was shown to be the remnant of the classical nova T Scorpii. This source was undergoing a dwarf nova outburst during the observations. A variability study of the GC also revealed three variable sources, including an RR Lyrae that was observed around maximum brightness, an SXPhoenicis star with a ? 55minute period, and a longer period variable which might be another RR Lyrae or a Cepheid variable.

In NGC6752, two known cataclysmic variables were revealed to be dwarf novae (DNe), which underwent outbursts during the observations. This takes the number of known DNe in NGC6752 to three, more than any other cluster. Some of the global parameters describing NGC6752 were also investigated. A new estimate of the cluster’s centre position was determined and used to show that the stellar density profile cannot be fit using a single ‘King’ profile, indicating that the cluster is in a core-collapsed state. Finally, a search for broadening in the main-sequence of the colour-magnitude diagram found evidence of small-scale broadening, suggesting the presence of multiple populations. A radial trend in the level of observed broadening was also suggested, with more broadening found in the core than the outer parts of the cluster.

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