Globular clusters are extreme stellar populations. They have the highest stellar density, and host both the oldest and most metal-poor stellar populations in the Galaxy. Their densities make them excellent testbeds for stellar dynamics, while the properties of their stars allows us to test our understanding of old and metal-poor stellar evolution. This thesis is comprised of three projects studying the two nearest globular clusters, NGC 6397 and Messier 4. By examining high-quality HST photometry of NGC 6397, we have constrained the binary fraction in both the central regions, and beyond the half-light radius. We find a binary fraction of ~0.05 in the core and ~0.015 in the outskirts. In the context of recent N-body simulations by Hurley et al., we interpret the observed binary fraction in the outer field as the primordial binary fraction. This value is lower than typically assumed, and has implications for cluster dynamics and N-body modeling. We report the discovery that young white dwarfs are dynamically hotter than their progenitors. Using the same photometry as mentioned above, and archival HST photometry of Messier 4, we have found that young white dwarfs have an extended radial distribution, and therefore a higher velocity dispersion, compared with older white dwarfs and their progenitors. This implies the existence of a ``natal kick''. Implications for cluster dynamics and stellar evolution are discussed. Finally, we present the spectra of 23 white dwarfs in Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. We find that all white dwarfs are of type DA. Assuming the same DA/DB ratio as is observed in the field, the chance of finding no DBs in our sample due to statistical fluctuations is 0.006. This suggests DB formation is suppressed in the cluster environment. Furthermore, we constrain the mass of these white dwarfs by fitting models to the spectral lines. Our best estimate of the masses of the white dwarfs currently forming in Messier 4 is 0.51+/-0.02 M_sun.This extends the empirical constraint on the initial-final mass relation over the entire range of initial masses that could have formed white dwarfs in a Hubble time.

This thesis has used medium- and high-resolution spectral data to derive elemental abundances, in particular light and heavy s-process elemental abundances, for groups of giant stars in the globular clusters 47 Tuc, NGC 6388 and NGC 362. These analyses were undertaken using both curve-of-growth and spectrum synthesis techniques. The techniques were calibrated with respect to the metal-poor giant star Arcturus in order to reduce systematic errors in the analysis process. A feasibility study was undertaken that compared synthetic spectra at different resolutions throughout the colour-magnitude diagram (CMD) of a metal-rich ([Fe/H] = -0.5 dex) globular cluster. This study identified where on the CMD light and heavy s-process elemental abundances could be derived at medium resolution (R ~ 10,000). Abundance analyses could be undertaken on the giant branches down to just below the horizontal branch and then again on the main sequence below Teff ~ 4500 K. At all other places on the CMD high-resolution spectra (R ~ 30,000) are required to derive these abundances. Performance verification data at R ~ 5,000$ was obtained using the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) and showed that there were no large scale s-process elemental abundance variations in 47 Tuc giant branch stars. The level of precision in this preliminary data was [X/Fe] ~ 0.5dex. A resolution of R ~ 10,000 should be achievable with SALT RSS in the future which will improve this limit. The AAOmega survey of 47 Tuc stars at R ~ 6,500 was more promising in certain aspects of elemental abundance determination. The observed wavelengths included the key features of CN and CH molecular bands, and light (Z < 30) and heavy (38 < Z < 63) element spectral lines. CN indices were measured and calibrated to previous results. The well-known CN bimodality was observed in the 47 Tuc stars, as well as a radial gradient in CN strength. A preliminary subset of ten of the survey stars have undergone an abundance analysis for which the abundances of Fe, Si, and Ca were found to be homogeneous within this cluster. The Na abundances had a large range in values that were observed to correlate with CN strength. The s-process elemental abundance results were inconclusive. The Zr abundances showed little to no enhancement in the sample and the Ba abundances varied considerably due to strong lines of Ba II being extremely sensitive to microturbulence. Various high-resolution studies were carried out using spectra of giant stars in 47 Tuc, NGC 6388 and NGC 362 observed on the Australian National Observatory (ANO) 2.3 m echelle spectrograph and the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope (VLT). The high-resolution analysis of 47 Tuc giant star Lee 2525 found an enhanced Zr abundance in this star which resolved a discrepancy between two previous 47 Tuc elemental abundance studies (Brown & Wallerstein 1992; Wylie et al. 2006). The stars in the VLT dataset that were analysed here included five giant branch stars in 47 Tuc, two in NGC 6388 and thirteen in NGC 362. The low temperatures and gravities of these stars caused departures from local thermodynamic equilibrium in low excitation potential neutral species, particularly Fe and Zr, that needed to be taken into account before reliable stellar parameters and elemental abundances could be determined for these stars. Veiling effects due to circumstellar dust were postulated to have produced artificially low metallicities for the infra-red excess stars in this sample, particularly for the 47 Tuc stars. The element abundance analyses of 47 Tuc, NGC 6388 and NGC 362 stars found the derived metallicities to be homogeneous for each cluster (<[Fe/H]>(47Tuc) = -0.88 +/- 0.09 dex; <[Fe/H]>(NGC6388) = -0.60 +/- 0.06 dex; <[Fe/H]>(NGC362) = -1.21 +/- 0.08 dex). The 47 Tuc sample included Lee 2525 and the five VLT stars. The derived metallicities were in reasonable agreement with previously reported values. The light (ls) and heavy (hs) s-process element abundances were enhanced and homogeneous in the stars of each cluster. The abundances determined for 47 Tuc and NGC 6388 were in good agreement, reflecting the similarity in metallicity of the stars in these two clusters (<[ls/Fe]>(47Tuc) = +0.53 +/- 0.02 dex; <[hs/Fe]>(47Tuc) = +0.40 +/- 0.06 dex; <[ls/Fe]>(NGC6388) = +0.58 +/- 0.13 dex; <[hs/Fe]>(NGC6388) = +0.39 +/- 0.07 dex). The more metal-poor cluster NGC 362 was less enhanced in ls elemental abundances and slightly more enhanced in hs elemental abundances (<[ls/Fe]>(NGC362) = +0.32 +/- 0.10 dex, <[hs/Fe]>(NGC362) = +0.46 +/- 0.09 dex). The clear enhancement in the s-process elemental abundances and homogeneity in the results for each globular cluster is evidence that these stars have been enhanced extrinsically in s-process elements. Pollution events in the history of each cluster has resulted in the abundance distribution in both the light elements and the heavy elements that has been observed in the stars analysed in this thesis. The enhancements in Na, ls and hs elemental abundances favours intermediate mass AGB stars as the source of the pollution.

In order to understand the origin of globular clusters, large samples of their stars need to be observed and analyzed for their chemical composition. This is especially true for the complex, multimetallic cluster ω Centauri, with its large range of iron, carbon, nitrogen, oxygen, sodium and barium abundances. In order to accomplish this, an automated spectral matching pipeline was developed to determine these abundances. This thesis made use of photometry and low resolution spectroscopy to analyze the chemical composition of evolved stars in three clusters: ω Cen, 47 Tuc and NGC 6752. The latter two clusters are monometallic and selected due to their similar metallicities to the metal-rich and metal-poor stars in ω Cen. This allowed them to be used as test-cases for the spectral matching pipeline. For ω Cen, two analyses were performed. In the first, 221 giant branch stars were selected that had known [O/Fe]. These stars showed the expected anticorrelation in [C/Fe]to [N/Fe]. In the second, spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen-rich or oxygen-poor. From this a catalogue of abundances of iron, carbon and barium of 848 giant branch stars were determined, of which 557 also had well-defined nitrogen abundances. k-means clustering analysis was used to group the stars in ω Cen into four homogeneous groups based upon these abundances. These groups suggest that there were at least four main periods of star formation in the cluster. The exact order of these star formation events is not yet understood, with some models predicting the groups formed from iron-poorest to iron-richest, while others suggest the potential for iron-poorer groups to form after iron-rich groups. These results compare well with those found from higher resolution studies and show the value of more extensive lower resolution spectral surveys. They also highlight the need for large samples of stars when working with a complex object like ω Cen.

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This thesis has used medium- and high-resolution spectral data to derive elemental abundances, in particular light and heavy s-process elemental abundances, for groups of giant stars in the globular clusters 47 Tuc, NGC 6388 and NGC 362. These analyses were undertaken using both curve-of-growth and spectrum synthesis techniques. The techniques were calibrated with respect to the metal-poor giant star Arcturus in order to reduce systematic errors in the analysis process. A feasibility study was undertaken that compared synthetic spectra at different resolutions throughout the colour-magnitude diagram (CMD) of a metal-rich ([Fe/H] = -0.5 dex) globular cluster. This study identified where on the CMD light and heavy s-process elemental abundances could be derived at medium resolution (R ~ 10,000). Abundance analyses could be undertaken on the giant branches down to just below the horizontal branch and then again on the main sequence below Teff ~ 4500 K. At all other places on the CMD high-resolution spectra (R ~ 30,000) are required to derive these abundances. Performance verification data at R ~ 5,000$ was obtained using the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) and showed that there were no large scale s-process elemental abundance variations in 47 Tuc giant branch stars. The level of precision in this preliminary data was [X/Fe] ~ 0.5dex. A resolution of R ~ 10,000 should be achievable with SALT RSS in the future which will improve this limit. The AAOmega survey of 47 Tuc stars at R ~ 6,500 was more promising in certain aspects of elemental abundance determination. The observed wavelengths included the key features of CN and CH molecular bands, and light (Z < 30) and heavy (38 < Z < 63) element spectral lines. CN indices were measured and calibrated to previous results. The well-known CN bimodality was observed in the 47 Tuc stars, as well as a radial gradient in CN strength. A preliminary subset of ten of the survey stars have undergone an abundance analysis for which the abundances of Fe, Si, and Ca were found to be homogeneous within this cluster. The Na abundances had a large range in values that were observed to correlate with CN strength. The s-process elemental abundance results were inconclusive. The Zr abundances showed little to no enhancement in the sample and the Ba abundances varied considerably due to strong lines of Ba II being extremely sensitive to microturbulence. Various high-resolution studies were carried out using spectra of giant stars in 47 Tuc, NGC 6388 and NGC 362 observed on the Australian National Observatory (ANO) 2.3 m echelle spectrograph and the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope (VLT). The high-resolution analysis of 47 Tuc giant star Lee 2525 found an enhanced Zr abundance in this star which resolved a discrepancy between two previous 47 Tuc elemental abundance studies (Brown & Wallerstein 1992; Wylie et al. 2006). The stars in the VLT dataset that were analysed here included five giant branch stars in 47 Tuc, two in NGC 6388 and thirteen in NGC 362. The low temperatures and gravities of these stars caused departures from local thermodynamic equilibrium in low excitation potential neutral species, particularly Fe and Zr, that needed to be taken into account before reliable stellar parameters and elemental abundances could be determined for these stars. Veiling effects due to circumstellar dust were postulated to have produced artificially low metallicities for the infra-red excess stars in this sample, particularly for the 47 Tuc stars. The element abundance analyses of 47 Tuc, NGC 6388 and NGC 362 stars found the derived metallicities to be homogeneous for each cluster (<[Fe/H]>(47Tuc) = -0.88 +/- 0.09 dex; <[Fe/H]>(NGC6388) = -0.60 +/- 0.06 dex; <[Fe/H]>(NGC362) = -1.21 +/- 0.08 dex). The 47 Tuc sample included Lee 2525 and the five VLT stars. The derived metallicities were in reasonable agreement with previously reported values. The light (ls) and heavy (hs) s-process element abundances were enhanced and homogeneous in the stars of each cluster. The abundances determined for 47 Tuc and NGC 6388 were in good agreement, reflecting the similarity in metallicity of the stars in these two clusters (<[ls/Fe]>(47Tuc) = +0.53 +/- 0.02 dex; <[hs/Fe]>(47Tuc) = +0.40 +/- 0.06 dex; <[ls/Fe]>(NGC6388) = +0.58 +/- 0.13 dex; <[hs/Fe]>(NGC6388) = +0.39 +/- 0.07 dex). The more metal-poor cluster NGC 362 was less enhanced in ls elemental abundances and slightly more enhanced in hs elemental abundances (<[ls/Fe]>(NGC362) = +0.32 +/- 0.10 dex, <[hs/Fe]>(NGC362) = +0.46 +/- 0.09 dex). The clear enhancement in the s-process elemental abundances and homogeneity in the results for each globular cluster is evidence that these stars have been enhanced extrinsically in s-process elements. Pollution events in the history of each cluster has resulted in the abundance distribution in both the light elements and the heavy elements that has been observed in the stars analysed in this thesis. The enhancements in Na, ls and hs elemental abundances favours intermediate mass AGB stars as the source of the pollution.

We carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and their relevant physical properties available in the catalogues, among them were considered the density, variable stars known, colour-magnitude diagrams, and luminosity. The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time images. The camera was configured to take 10 frames per second. Due to this, the brighter stars observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly, it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that also enabled us to produce images with better angular resolution than previously achieved with conventional CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that obtained with space telescopes. The detection of the stars in each of the globular clusters and the photometry was performed via difference image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated to produce high-precision time-series photometry of very crowded stellar regions. We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the variable star population in the globular clusters. In NGC 6715, we found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type. In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2 SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables (16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study. Ephemerides and photometric measurements for the variable stars are available in electronic form through the Strasbourg Astronomical Data Centre.

In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster omega Centauri in one of the parallel Hubble Space Telescope (HST) fields, located at about 3.5 hal-flight radii from the center of the cluster. Thanks to the over 15 yr long baseline and the exquisite astrometric precision of the HST cameras, well-measured stars in our proper-motion catalog have errors as low as similar to 10 mu as yr(-1), and the catalog itself extends to near the hydrogen-burning limit of the cluster. We show that second-generation (2G) stars are significantly more radially anisotropic than first-generation (1G) stars. The latter are instead consistent with an isotropic velocity distribution. In addition, 1G stars have excess systemic rotation in the plane of the sky with respect to 2G stars. We show that the six populations below the main-sequence (MS) knee identified in our first paper are associated with the five main population groups recently isolated on the upper MS in the core of cluster. Furthermore, we find both 1G and 2G stars in the field to be far from being in energy equipartition, with eta(1G) = -0.007 +/- 0.026 for the former and eta(2G) = 0.074 +/- 0.029 for the latter, where eta is defined so that the velocity dispersion sigma(mu) scales with stellar mass as sigma(mu) proportional to m(-eta). The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in omega Centauri and other globular clusters. We make our astro-photometric catalog publicly available.

We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances ([Mg/Fe]. < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] greater than or similar to-1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution.

Globular clusters are extreme stellar populations. They have the highest stellar density, and host both the oldest and most metal-poor stellar populations in the Galaxy. Their densities make them excellent testbeds for stellar dynamics, while the properties of their stars allows us to test our understanding of old and metal-poor stellar evolution. This thesis is comprised of three projects studying the two nearest globular clusters, NGC 6397 and Messier 4. By examining high-quality HST photometry of NGC 6397, we have constrained the binary fraction in both the central regions, and beyond the half-light radius. We find a binary fraction of ~0.05 in the core and ~0.015 in the outskirts. In the context of recent N-body simulations by Hurley et al., we interpret the observed binary fraction in the outer field as the primordial binary fraction. This value is lower than typically assumed, and has implications for cluster dynamics and N-body modeling. We report the discovery that young white dwarfs are dynamically hotter than their progenitors. Using the same photometry as mentioned above, and archival HST photometry of Messier 4, we have found that young white dwarfs have an extended radial distribution, and therefore a higher velocity dispersion, compared with older white dwarfs and their progenitors. This implies the existence of a ``natal kick''. Implications for cluster dynamics and stellar evolution are discussed. Finally, we present the spectra of 23 white dwarfs in Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. We find that all white dwarfs are of type DA. Assuming the same DA/DB ratio as is observed in the field, the chance of finding no DBs in our sample due to statistical fluctuations is 0.006. This suggests DB formation is suppressed in the cluster environment. Furthermore, we constrain the mass of these white dwarfs by fitting models to the spectral lines. Our best estimate of the masses of the white dwarfs currently forming in Messier 4 is 0.51+/-0.02 M_sun.This extends the empirical constraint on the initial-final mass relation over the entire range of initial masses that could have formed white dwarfs in a Hubble time. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the MAgellanic Stellar History (SMASH), SMASH 1 is a compact (r(h) 9.1(-3.4)(+5.9)pc) and very low luminosity (M-V = -1.0 +/- 0.9, L-V = 10(2.3 +/- 0.4) L-circle dot) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone located at a distance modulus of similar to 18.8, i.e., a distance of similar to 57 kpc. Situated at 11 degrees.3 from the LMC in projection, its three-dimensional distance from the Cloud is similar to 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least 16 kpc. Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only less than or similar to 19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (epsilon = 0.62(-0.21)(+0.17)) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.