Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2019)
This thesis details three distinct projects that explore stellar populations in Milky Way globular clusters. In the first, a method of modelling mass segregation in clusters is presented. The model is fit to 54 clusters and the best fit parameters are presented in tabular form. The newly derived parameter that indicates the amount of mass segregation correlates strongly with other dynamical cluster parameters. In the second study, white dwarf data in the cluster 47 Tucanae are used to construct an empirical relation between temperature and time for these stars. The modified data are compared to theoretical cooling models from four different research groups. We find disagreement between all of the models and the data. The models are also inconsistent with each other. In thethird investigation, new UV white dwarf data in 47 Tuc is used to constrain the hydrogen mass fraction and neutrino production rates in cooling white dwarfs. A much different approach from the second project is used. The data are left untouched and the model is transformed to the space in which the data exist. Using the unbinned maximum likelihood statistic, the model’s parameter space is explored with MCMC sampling. A constraint on the rate of neutrino production in white dwarfs comes from this analysis.
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.
Master's Student Supervision (2010 - 2018)
In this thesis we analyze stellar proper motions in the core of the globular cluster 47 Tucanae to explore the possibility of an intermediate-mass black hole (IMBH) influence on the stellar dynamics. Our use of short-wavelength photometry affords us an unprecedentedly clear view of stellar motions into the very centre of the crowded core, yielding proper motions for >50,000 stars in the central 2 arcminutes. We model the velocity dispersion profile of the cluster using an isotropic Jeans model. The density distribution is taken as a central IMBH point mass added to a combination of King profiles. We individually model the general low-mass cluster objects (main sequence/giant stars), as well as the concentrated populations of heavy binary systems and dark stellar remnants. Using un-binned likelihood model-fitting, we find that the inclusion of the concentrated populations in our model plays a crucial role in fitting for an IMBH mass. Taking into account all of these cluster sub-populations our model predicts an IMBH to cluster mass ratio of 0.06% +/- 0.13%. The concentrated binaries and stellar-mass black holes produce a sufficient enhancement to the velocity dispersion signal in the core as to make an IMBH unnecessary to fit the observations. We additionally determine that a stellar-mass black hole retention fraction of >18% becomes incompatible with our kinematic observations for 47 Tuc.
Blue stragglers (BSS) are stars whose position in the Color-Magnitude Diagram (CMD) places them above the main sequence turn-off point in a given cluster. Three possible origins have been proposed: stellar collisions, evolution of binary systems, and evolution of hierarchical triples. Using data from the core of 47 Tuc in the ultraviolet (UV), we have identified various stellar populations in the CMD, and used their radial distributions to study the evolution and origin of BSS. When we separate the BSS in two samples divided by their magnitude, we find that the bright BSS show a much more centrally concentrated radial distribution and higher mass estimates, suggesting an origin involving triple or multiple stellar systems. In contrast, the faint BSS are less concentrated, with a radial distribution similar to the main sequence (MS) binaries pointing to this populations as their progenitors. A sample of evolved BSS was found on the UV CMD, this put together with available photometric data and MESA evolutionary models resulted in time scales and number of observed and expected stars agreeing nicely with the BSS having a post-MS evolution comparable to that of a normal star of the same mass and a MS BSS lifetime of about 200-300 Myr. We also find that the extra population of the asymptotic giant branch (AGB) stars in 47 Tuc is due to evolved BSS, with the bulk of the contamination being in the red giant branch bump of the BSS that, according to our models, falls in the same magnitude and color range as the observed AGB bump.
Using the Gemini North Telescope at Mauna Kea, Hawaii, we have obtained astrometric and spectroscopic data for stars in the core of the galactic globular cluster Messier 71 (NGC 6838). This data has allowed us to for the first time ever to obtain three dimensional velocity profiles for stars in the vicinity of centre of a globular cluster. Using the Near Infrared Imager withAdaptive Optics and a 3.8 year baseline for our astrometric study we haveresolved the internal proper motion dispersion. The proper motion dispersion is found to be 179±17 µ year⁻¹, we have put a strict limit to the size of any central Intermediate Mass Black Hole at ~150 solar masses at 90% confidence, additionally we find no evidence of core anisotropy. Using our GMOS Integrated Field Unit spectroscopic data we have obtained a radial velocity dispersion of 3.54±0.64 km s⁻¹. Combining our proper motion and radial velocity dispersions we find the geometric distance to the cluster to be 4.1±1.2 kpc. We then compare our geometric distance to a distance found from fitting stellar evolution models. We have developed a new technique for fitting models, using this technique we find the stellar evolution model distance to be 3.9±0.2 kpc. We then discuss how this technique can easily be applied to other clusters in any future work.
This thesis is composed of three chapters, as well as an introduction, which describe three distinct projects. In Chapter 2 we present new measurements of the centers for 65 Milky Way globular clusters. Centers were determined by fitting ellipses to the density distribution as well as the symmetry of the clusters. All of the determinations were done with stellar positions derived from a combination of two single-orbit Advanced Camera for Surveys images of the core of the cluster. We find that the ellipse-fitting method provides remarkable accuracy over a wide range of core sizes and density distributions, while the symmetry method is difficult to use on clusters with very large cores, or low density, requiring a larger field, or a very sharply peaked density distribution.Chapter 3 deals with a re-analysis of previous work on white dwarf natal kicks, and expands on this to analyze the radial distributions of stellar populations in globular clusters at earlier stages of stellar evolution (earlier referring to pre-white dwarf). The effects of stellar incompleteness, and a method to account for this are discussed. Finally, the results of a statistical analysis of completeness corrected radial distributions in 56 globular clusters are presented. No significant evidence of kicks is found, however multiple clusters show evidence that stars along the horizontal branch have not relaxed since undergoing mass loss after leaving the main sequence.In Chapter 4, we present a novel method for determining the distance to a star cluster by fitting spectral energy models to the spectral energy distributions of cluster white dwarfs in multiple filters. The statistics of our fitting method are discussed in detail. This approach results in a true distance modulus of (m-M)₀ = 13.35 ± 0.02 ± 0.06, which corresponds to a physical distance of 4.67 ± 0.04 ± 0.13 kpc. The first error given is random, and the second is systematic.