Astrophysics and Astronomy Commons^™

Probing The Physical Mechanisms Responsible For Brown Dwarf And Giant Planet Formation, Sarah Betti

Doctoral Dissertations

The disks that form around young stellar objects provide the essential material for their continued growth as well as the formation of planets, making them ideal laboratories to investigate the mechanisms and environments key for substellar and planetary formation. In this dissertation, I explore two main formation processes: the transportation of water necessary for giant planet formation, and the accretion and growth of young brown dwarfs. First, I study the water ice content in the circumstellar disk of AB Aurigae, a young Herbig Ae star. I detect and map icy grains on the disk surface using high contrast observations taken …

The Coeval Mass Assembly Of The Universe Via Supermassive Black Hole Accretion And Star Formation In Galaxies, Alyssa Sokol

Doctoral Dissertations

The possible co-evolution between galaxies and their central supermassive black holes is supported by the similarity in shape between the Star Formation Rate Density (SFRD) and Black Hole Accretion Rate Density (BHARD) out to z$\sim$ 3. This apparent connection between BH growth and star formation is only established globally; while both trends peak at z$\sim$ 2, the amount of stellar and black hole mass assembly occurring within the same galaxies is unknown. Computing these trends for the same galaxies will mitigate the present sample mismatch and can be accomplished with an IR-selected sample; however, the approach relies on a robust …

How Do Galaxies Form Their Stars Over Cosmic Time?, Jed H. Mckinney

Doctoral Dissertations

Galaxies in the past were forming more stars than those today, but the driving force behind this increase in activity remains uncertain. In this thesis I explore the origin of high star-formation rates today and in the past by studying the properties of gas and dust in the cold interstellar medium (ISM) of dusty galaxies over cosmic time. Critically, we do not yet understand how these galaxies could form so many stars. This work began with my discovery of unusual infrared (IR) emission line ratios in the class of dusty galaxies where most of the Universe’s stars were formed. To …

Study Of 134in Beta-Delayed Neutron Emission And Development Of A New Generation Neutron Detector, Joseph Heideman

Doctoral Dissertations

Beta-delayed neutron emission in very neutron-rich nuclei plays an essential role in nuclear structure and the understanding of the astrophysical r-process. A complete description of this process requires knowledge of both steps, beta decay and neutron emission. A leading theory poses the intermediate daughter nucleus to behave as a compound nucleus. The conditions for beta-delayed neutron emission of 134In are not well described by the assumptions in the neutron pandemonium hypothesis, therein providing a unique case to this process due to the proximity to 132Sn. Single-particle states in 133Sn obvserved after neutron emission have dissimilar shell occupancy compared to neutron-hole …

Probability Distribution Of Equations Of State For Astrophysical Simulations, Xingfu Du

Doctoral Dissertations

The detection of gravitational wave during the neutron star merger event GW170817 greatly enhanced our ability to probe the interiors of neutron stars. Future measurements of similar events will put further constraints to the equation of state (EOS) of nuclear matter. Also, uncertainties in the EOS create variations in the results of astrophysical simulations of core-collapse supernovae and neutron star mergers. In order to quantify the uncertainties, we construct a probability distribution of equations of state (EOSs). We create a new EOS which respects experimental, observational and theoretical constraints on the nature of matter in various density and temperature regimes. …

Observational Studies Of Fragmentation In Molecular Clouds, Riwaj Pokhrel

Doctoral Dissertations

In this dissertation, I explore fragmentation physics in multiple scales in nearby molecular clouds and discuss some implications of fragmentation for cloud structure formation and star formation, primarily by analyzing multi-wavelength observations of dust emission. First, I tested the complete thermal and combined thermal and nonthermal support mechanisms that balance gravitational contraction at multiple scales in the Perseus molecular cloud. I found that the observed multiscale structures in Perseus are consistent with an inefficient thermal Jeans fragmentation, where the Jeans efficiency increases from the largest scale ($\gtrsim$10s of pc) to the smallest scale ($\sim$10s of AU). Next, I studied the …

The Impact Of Protostellar Feedback On Astrochemistry, Brandt Gaches

Doctoral Dissertations

Star formation is the lynch pin that lies in between the scales of galaxy and planet formation. Observational studies of molecular clouds, the sites of star formation, primarly use molecular line emission, providing dynamical and chemical information. Two of the key parameters of astrochemical models are far-ultraviolet (FUV) flux and the cosmic ray ionization rate. We use analytic accretion histories to predict the bolometric and FUV luminosities of protostar clusters and compare different histories with observed bolometric luminosities. We find that the Tapered Turbulent Core model best represents the observed luminosities and their dispersion. We extend the models to calculate …

Aztec Survey Of The Central Molecular Zone: Modeling Dust Seds With Hierarchical Bayesian Analysis, Yuping Tang

Doctoral Dissertations

In this dissertation, we present a study based on the AzTEC/Large Millimeter Telescope (LMT) survey of dust continuum at 1.1mm on the central 200 parsecs (The Central Molecular Zone (CMZ)) of our Galaxy. Owing to its unusually high gas density and turbulence, strong magnetic field, and high cosmic ray flux, the CMZ represents an initial condition for star-formation typical of starburst galaxies in the distant universe. In order to understand dust properties in such an extreme environment. We perform a joint SED analysis of existing dust continuum surveys on the CMZ, from a wavelength of 160 μm to 1.1 mm. …

Environmentally Driven Galaxy Evolution And Quenching: Insights From The Low-Redshift Circumgalactic Medium, Joseph Burchett

Doctoral Dissertations

The gaseous halos of galaxies -- the circumgalactic medium (CGM) -- serve as interfaces playing host to the fueling and feedback processes that sustain and regulate star formation. Furthermore, interactions between galaxies one with another and with larger scale structure, such as galaxy cluster halos, must necessarily act through the CGM. This dissertation examines the CGM as traced by H I, C IV, and O VI absorption lines across wide range of halo environments, from isolated dwarf galaxies with M_* < 10⁸ M_sun to galaxy clusters with M_halo > 10¹⁴ M_sun. By first conducting a blind …

Astrophysical Accretion And Feedback: The Bayesian Linchpin Of Theory And Observation, Shawn Roberts

Doctoral Dissertations

Despite being a major pillar of galaxy evolution, galactic feedback from stars and supermassive black holes (SMBHs) is subject to very little observational constraint. This is particularly true of the hot component, as viewed in X-rays. Yet, the hot component is directly linked to much of the energetic feedback released from these compact objects. X-ray observations suffer from several challenges that make placing this constraint a difficult task. In the face of considerable model uncertainty, these challenges underscore the need for novel X-ray data analysis techniques. In this dissertation, I seek to lend a unique perspective to X-ray data analysis …

Intrinsic Characteristics Of Galaxies In The Distant Universe: The Correlation Between Galaxy Morphology And Star Formation Activity, Bomee Lee

Doctoral Dissertations

One of the major questions in observational cosmology is how galaxies formed and how they evolved. In particular, understanding the assembly history of galaxies at the peak epoch of the star formation activity, z=1-3, is a key to understanding the whole picture of the Universe, but remains uncertain. Galaxies with various physical properties and morphologies have different formation and evolution histories. As such, we seek insight into galaxy formation and evolution at z=1-3 using galaxies selected from Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) in this dissertation. First, we investigate the relationship between spectral types and morphologies using various …

The Cosmic Web, And The Role Of Environment In Galaxy Evolution, Ryan Cybulski

Doctoral Dissertations

The Universe, on extra-galactic scales, is composed of a vast network of structures dubbed the “cosmic web”. One of the most fundamental discoveries about the evolution of galaxies is that their properties have a dependence on their location relative to this cosmic web (i.e., their environment). However, detailed studies of the environmental dependence on galaxy evolution have been extremely challenging due to the inherent complexity of the structures on the largest scales, a plethora of techniques being used to try to map the cosmic web, and other confounding factors, such as the masses of galaxies, that also affect their evolution. …

Solar Modulation Of The Cosmic Ray Intensity And The Measurement Of The Cerenkov Reemission In Nova’S Liquid Scintillator, Philip James Mason

Doctoral Dissertations

The NOνA (NuMI Off-axis electron neutrino Appearance) experiment is a long baseline neutrino oscillation experiment at Fermi National Accelerator Laboratory. Its purpose is to observe the oscillation of ν_μ (muon neutrino) to ν_e (electron neutrino) and to investigate the neutrino mass hierarchy and CP violation in the neutrino sector. Two detectors have been built for this purpose, a Near Detector 300 feet underground at Fermilab, and a Far Detector, on the surface at Ash River, Minnesota.

The completion of NOνA’s Far Detector in October 2014 enabled not only the recent measurement of neutrino oscillations, but an array of …

Nucleosynthesis In Self-Consistent Core-Collapse Supernova Models Using Multidimensional Chimera Simulations, James Austin Harris

Doctoral Dissertations

Observations of nuclear abundances in core-collapse supernova (CCSN) ejecta, highlighted by γ-ray [gamma-ray] observations of the ⁴⁴Ti [titanium-44] spatial distribution in the nearby supernova remnants Cassiopeia A and SN 1987A, allow nucleosynthesis calculations to place powerful constraints on conditions deep in the interiors of supernovae and their progenitor stars. This ability to probe where direct observations cannot makes such calculations an invaluable tool for understanding the CCSN mechanism. Unfortunately, despite knowing for two decades that supernovae are intrinsically multi-dimensional events, discussions of CCSN nucleosynthesis have been predominantly based on spherically symmetric (1D) models, which employ a contrived energy source …

Dusty Star Formation In Extreme Environments: Galaxies And Galaxy Clusters In The Distant Universe, Stacey Alberts

Doctoral Dissertations

In this thesis, we present a comprehensive study of the dust-obscured star formation (SF) activity in galaxy clusters out to high redshift using infrared (IR) imaging. Using hundreds of galaxy clusters and wide-field far-IR imaging across nine square degrees, we quantify the average star formation rates (SFRs) out to the distant Universe for mass-limited cluster galaxy samples using stacking. We compare the evolution of this SF activity to field galaxies, finding that the evolution in clusters occurs more rapidly than in the field and clusters have field-like SF approximately nine billion years ago, during an epoch before SF quenching becomes …

The Study Of Nuclear Structure Of Neutron-Rich 81ge And Its Contribution In The R-Process Via The Neutron Transfer Reaction 80ge(D,P), Sunghoon Ahn

Doctoral Dissertations

The study of low-lying levels of nuclei near closed shells not only elucidates the evolution of nuclear shell structure far from stability, but also affects estimates of heavy element nucleosynthesis in supernova explosions. Especially, the properties of the low-lying levels in ⁸¹Ge[Germanium 81] are important because the sensitivity study of the r-process pointed out that the properties of the nucleus can affect the final bundance pattern. Also, the spins and parities measurements of the states are essential to understand the shape coexistence in odd-mass N = 49 isotones.

This work describes the study of the odd-mass N = …

Analysis Of 26Al + P Elastic And Inelastic Scattering Reactions And Galactic Abundances Of 26Al, Stephen Todd Pittman

Doctoral Dissertations

²⁶Al(p,p)²⁶Al and ²⁶Al(p,p’)²⁶Al^* scattering reactions were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) at the Oak Ridge National Laboratory (ORNL). The purpose of the elastic scattering study was to determine properties of previously uncharacterized ²⁷Si levels above the proton threshold in the energy range E(c.m.) ~ 0.5 - 1.5 MeV and to calculate reaction rates for the ²⁶Al(p,γ[gamma])²⁷Si reaction that destroys ²⁶Al. The inelastic scattering reaction was also evaluated to investigate the reaction that produces the metastable state of ²⁶Al at E(c.m.) = 228 keV, …

Towards Simulations Of Binary Neutron Star Mergers And Core-Collapse Supernovae With Genasis, Reuben Donald Budiardja

Doctoral Dissertations

This dissertation describes the current version of GenASiS and reports recent progress in its development. GenASiS is a new computational astrophysics code built for large-scale and multi-dimensional computer simulations of astrophysical phenomena, with primary emphasis on the simulations of neutron star mergers and core-collapse supernovae. Neutron star mergers are of high interest to the astrophysics community because they should be the prodigious source of gravitation waves and the most promising candidates for gravitational wave detection. Neutron star mergers are also thought to be associated with the production of short-duration, hard-spectral gamma-ray bursts, though the mechanism is not well understood. In …