In this study we present GALEX UV, Sloan Digital Sky Survey, and Spitzer infrared imaging, along with an optical Hα map of the interacting galaxy pair Arp 107. IRAF photometry was used to analyze 29 star forming regions identified in previous work done with Spitzer colors. In this paper further analysis of the clumps of newly formed stars was done by creating Spectral Energy Distribution plots to sort the clumps as quiescent or starbursts. Color - color plots were used to estimate the ages and extinctions of the clumps by comparing magnitudes to stellar population synthesis models. Results seem to agree with previous models, and indicate a sequence of increasing age around the galaxy arm.

The James Webb Space Telescope (JWST) is a next-generation space telescope that will be capable of making transformative observations of planetary transits. As its launch date grows ever closer, it becomes imperative that astronomers have access to accurate simulations of JWST observations in order to best plan observations and devise data analysis pipelines. Unfortunately, available simulation tools do not provide the most accurate or realistic simulations, including noise and systematic errors. In this thesis, I present an open-source time-domain simulator of planetary transits that is capable of accurately modeling these effects in observations made by JWST.

Theory and methods of processing speckle interferometry data from close visual binary stars are presented and implemented. The effects of the optical systems used for observing close visual binary stars are explained and simulated from both the geometrical and physical optical viewpoints. The atmospheric phase distortion and shot noise responsible for the observed speckle patterns are simulated. The deconvolution technique originally presented by Labeyrie is implemented to extract astrometric data from close visual binary stars. This method is applied to both simulated and experimental data from Kitt Peak National Observatory as validation. Parts of the deconvolution process are optimized to allow for near real time calculations in an automated observatory.

<p>In this thesis, we present theoretical and statistical techniques broadly related to systems of dynamically-interacting particles. We apply these techniques to observations of dense star clusters in order to study gravitational interactions between stars. These include both long- and short-range interactions, as well as encounters leading to direct collisions and mergers. The latter have long been suspected to be an important formation channel for several curious types of stars whose origins are unknown. The former drive the structural evolution of star clusters and, by leading to their eventual dissolution and the subsequent dispersal of their stars throughout the Milky Way Galaxy, have played an important role in shaping its history. Within the last few decades, theoretical work has painted a comprehensive picture for the evolution of star clusters. And yet, we are still lacking direct observational confirmation that many of the processes thought to be driving this evolution are actually occuring. The results presented in this thesis have connected several of these processes to real observations of star clusters, in many cases for the first time. This has allowed us to directly link the observed properties of several stellar populations to the physical processes responsible for their origins.</p> <p>We present a new method of quantifying the frequency of encounters involving single, binary and triple stars using an adaptation of the classical mean free path approximation. With this technique, we have shown that dynamical encounters involving triple stars occur commonly in star clusters, and that they are likely to be an important dynamical channel for stellar mergers to occur. This is a new result that has important implications for the origins of several peculiar types of stars (and binary stars), in particular blue stragglers. We further present several new statistical techniques that are broadly applicable to systems of dynamically-interacting particles composed of several different types of populations. These are applied to observations of star clusters in order to obtain quantitative constraints for the degree to which dynamical interactions affect the relative sizes and spatial distributions of their different stellar populations. To this end, we perform an extensive analysis of a large sample of colour-magnitude diagrams taken from the ACS Survey for Globular Clusters. The results of this analysis can be summarized as follows: (1) We have compiled a homogeneous catalogue of stellar populations, including main-sequence, main-sequence turn-off, red giant branch, horizontal branch and blue straggler stars. (2) With this catalogue, we have quantified the effects of the cluster dynamics in determining the relative sizes and spatial distributions of these stellar populations. (3) These results are particularly interesting for blue stragglers since they provide compelling evidence that they are descended from binary stars. (4) Our analysis of the main-sequence populations is consistent with a remarkably universal initial stellar mass function in old massive star clusters in the Milky Way. This is a new result with important implications for our understanding of star formation in the early Universe and, more generally, the history of our Galaxy. Finally, we describe how the techniques presented in this thesis are ideally suited for application to a number of other outstanding puzzles of modern astrophysics, including chemical reactions in the interstellar medium and mergers between galaxies in galaxy clusters and groups.</p> / Doctor of Philosophy (PhD)

<p>The effect of stellar feedback on the interstellar medium is investigated using numerical simulation. In particular, the roles of supernova feedback and ionization feedback on the star formation rate and structure of the interstellar medium are compared. We use Enzo, an adaptive mesh code, and employ the MUSCL-Hancock hydrodynamics scheme to run simulations of a section of a stratified galactic disk. A turbulent velocity field is imposed in the central region of the disk and self-gravity is applied. Star clusters are formed when density and temperature conditions are met, which, in turn, provide ionization and supernova feedback into the interstellar medium. Simulations were run with and without supernova and ionization feedback and the runs are compared. Ionization feedback is found to dominate over supernova feedback in regulating star formation rates. With no feedback, all the gas is converted to stars by 200 Myr. When supernova feedback is added, 98% of the gas is used to create stars by 300 Myr. With ionization feedback instead, at 1 Gyr into the run, only 30% of the gas is in stars. Even with supernova feedback added to ionization feedback, the gas converted to stars is just 29% at 1 Gyr. Very strong supernovae take this fraction down to 25%. The star formation rates in the runs with supernova feedback are consistent with the low end of the Kennicutt-Schmidt relation, while the runs without ionization feedback have star formation rates that are an order of magnitude larger. Gas phase masses and volumes produced in the ionization runs are broadly consistent with observations.</p> / Master of Science (MSc)

<p>We present an observational protocol to observe and interpret asymmetries in stellar explosions using scattered light echoes. Spectroscopy of multiple light echoes are used to observe single astronomical sources from multiple viewing angles, allowing for direct observations of explosion asymmetries, when they exist. We present asymmetry detections for two famous historical supernovae: the ~25-year-old SN 1987A and the ~330-year-old Cassiopeia A. In both supernovae we find asymmetries in the first few hundred days of the explosion that appear to be correlated with the geometry of Fe-rich material in the remnant states.</p> <p>Spectroscopy of SN 1987A light echoes reveals a variation in the Hα line profile as a function of echo azimuth, with maximum asymmetry at position angles 16◦ and 186◦, in agreement with the major-axis of the elongated remnant ejecta. We interpret our asymmetry detection as evidence for a two-sided distribution of high-velocity 56Ni in the first few hundred days of SN 1987A, with the most dominant asymmetry redshifted in the south. For Cassiopeia A, we find evidence for a ~4000 km/s velocity excess in the first hundred days of the explosion, roughly aligned with an Fe-rich outflow in the supernova remnant and approximately opposite in direction to the motion of the compact object.</p> <p>Core-collapse supernovae have not yet been successfully modelled despite decades of progress in input physics and computing capability. Despite the significance of thermonuclear Type Ia supernovae to cosmology, the progenitor systems and explosion details also remain unclear. Both observational and theoretical work suggest that non-spherical effects are not only common in supernovae, but may in fact aid in generating successful explosions. In addition to offering a new technique for observing supernova asymmetries, spectroscopy of scattered light echoes allows a direct causal connection to be made between stellar explosions and their observed remnant states.</p> / Doctor of Philosophy (PhD)

<p>The formation of star clusters occurs in a complex environment and involve a large number of physical processes. One of the most important processes to consider is radiative feedback. The radiation released by forming stars heats the surrounding gas and suppresses the fragmentation of low mass objects. Ionizing radiation can also drive large scale outflows and disperse the surrounding gas. Owing to all this complexity, the use of numerical simulations to study cluster formation in molecular clouds has become commonplace. In order to study the effects of radiative feedback on cluster formation over larger spatial scales than previous studies, we present hydrodynamical simulations using the AMR code FLASH which make use of cluster particles. Unlike previous studies, these particles represent an entire star cluster rather than individual stars. We present a subgrid model for representing the radiative output of a star cluster which involves randomly sampling an IMF over time to populate the cluster. We show that our model is capable of reproducing the properties of observed clusters. The model was then incorporated into FLASH to examine the effects of radiative feedback on cluster formation in full hydrodynamical simulations. We find that the inclusion of radiative transfer can drive large scale outflows and decreases the overall star formation efficiency by a factor of 2. The inclusion of radiative feedback also increases the degree of subclustering. The use of cluster particles in hydrodynamical simulations represents a promising method for future studies of cluster formation and the large scale effects of radiative feedback.</p> / Master of Science (MSc)

<p>Stars form in clusters amidst complex and coupled physical phenomena. Among the most important of these is radiative feedback, which heats the surrounding gas to suppress the formation of many low-mass stars. In simulations of star formation, pre-main-sequence modeling has often been neglected and stars are assumed to have the radii and luminosities of zero-age main sequence stars. We challenge this approach by developing and integrating a one-zone protostellar evolution model for FLASH and using it to regulate the radiation output of forming stars. The impact of accurate pre-main-sequence models is less ionizing radiation and less heating during the early stages of star formation. For stars modeled in isolation, the effect of protostellar modeling resulted in ultracompact HII regions that formed slower than in the ZAMS case, but also responded to transitions in the star itself. The HII region was seen to collapse and subsequently be rebuilt as the star underwent a swelling of its radius in response to changes in stellar structure and nuclear burning. This is an important effect that has been missed in previous simulations. It implies that observed variations in HII regions may signal changes in the stars themselves, if these variation can be disentangled from other environmental effects seen in the chaotic cluster environment.</p> / Master of Science (MSc)

We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emission from high ionization lines that will not be emitted by the Crab’s filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The NIR [Ne VI] λ 7.652 mm line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV λ1549Å doublet is predicted to be the strongest absorption line from the shell, which is in agreement with HST observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually “remember” its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion. Infrared observations have discovered a variety of objects, including filaments in the Crab Nebula and cool-core clusters of galaxies, where the H2 1-0 S(1) line is stronger than the infrared H I lines. A variety of processes could be responsible for this emission. Although many complete shock or PDR calculations of H2 emission have been published, we know of no previous simple calculation that shows the emission spectrum and level populations of thermally excited low-density H2. We present a range of purely thermal collisional simulations, corresponding to constant gas kinetic temperature at different densities. We consider the cases where the collisions affecting H2 are predominantly with atomic or molecular hydrogen. The resulting level population (often called “excitation”) diagrams show that excitation temperatures are sometimes lower than the gas kinetic temperature when the density is too low for the level populations to go to LTE. The atomic case goes to LTE at much lower densities than the molecular case due to larger collision rates. At low densities for the v=1 and 2 vibrational manifolds level populations are quasi-thermal, which could be misinterpreted as showing the gas is in LTE at high density. At low densities for the molecular case the level population diagrams are discontinuous between v=0 and 1 vibrational manifolds and between v=2, J=0, 1 and other higher J levels within the same vibrational manifold. These jumps could be used as density diagnostics. We show how much the H2 mass would be underestimated using the H2 1-0 S(1) line strength if the density is below that required for LTE. We give diagnostic diagrams showing level populations over a range of density and temperature. The density where the level populations are given by a Boltzmann distribution relative to the total molecular abundance (required to get the correct H2 mass), is shown for various cases. We discuss the implications of these results for the interpretation of H2 observations of the Crab Nebula and filaments in cool-core clusters of galaxies.

We present distances and kinematics and look at the possible populations for the blue plume (BP) stars in the Canis Major Overdensity (CMa). We conducted a medium resolution spectral survey on the BP stars (N=303) in CMa (centered at l = 238° ; b = -8°) using the data from AAOmega Spectrograph. We used a modified version of the Statistics-sensitive Non-linear Iterative Peak-clipping (SNIP) algorithm to normalize our fluxed absorption spectra. After determining the radial velocities from measurements of strong absorption features for the stars we use a Bayesian analysis of spectral feature strengths and photometric colors to determine Teff, Logg and [Fe/H]. Our procedure makes use of grid for model synthetic spectra computed using SPECTRUM with Atlas9 model atmospheres and Kurucz model colors. We determine the absolute magnitude using the stellar parameters and BaSTI isochrones and compute distances and ages for the BP stars. Our analysis of the BP stars indicates Teff ranging from 6500K to 8000K, metallicity ranging from 0.0 to -1.0 with an average of -0.5. We found for this temperature range that the surface gravity of the stars could not be well constrained. From the spatial and kinematics results we found that most of the stars are thick disk stars with a small mixture of thin disk stars. The stars are most likely a mixture of thick disk blue stragglers and normal A-type stars preferentially seen to greater depths due to the low dust extinction in this location of the Galaxy.