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Articles 1 - 22 of 22
Full-Text Articles in Physical Sciences and Mathematics
An Investigation Of The Accretion Processes In T Tauri And Herbig Ae/Be Systems Using High Resolution Optical And Near-Infrared Spectroscopy, Joshua Kern
All Dissertations
Star and planet formation is intimately tied to the accretion of material from the environments in which they form. During the formation process, disks of gas and dust develop in young stellar objects through which material is facilitated to the star and forming planets. Theoretical models of these accretion processes invoke viscous spreading via hydrodynamics, as well as more complex interactions with magnetic fields be it from the stellar component or the formation environment in order to catalyze these mass flows. These accretion models predict various scenarios including magnetospheric accretion as well as supersonic accretion flows in the disk atmosphere …
Cosmic Diffuse Neutrino And Gamma-Ray Backgrounds In The Mev Regime, Ilukpitiye Samalka Anandagoda
Cosmic Diffuse Neutrino And Gamma-Ray Backgrounds In The Mev Regime, Ilukpitiye Samalka Anandagoda
All Dissertations
Cosmic Multi-Messenger backgrounds include relic diffuse components created in the early Universe and contributions from individual sources. In this dissertation, I present the work done in Anandagoda (2019); Anandagoda et al. (2020, 2023) where type Ia (SNe Ia) and core-collapse supernovae (CCSNe) contributions to the diffuse neutrino and gamma-ray backgrounds in the MeV regime are studied. These backgrounds are referred to as DSNB and DSGB respectively. Based on this work, the diffuse SN Ia background is ~106 times lower (for electron antineutrinos) than the CCSN background making it negligible. The predicted DSNB electron antineutrino flux at earth in the …
The Search For Heavily Obscured Active Galactic Nuclei In The Local Universe, Ross Silver
The Search For Heavily Obscured Active Galactic Nuclei In The Local Universe, Ross Silver
All Dissertations
Active galactic nuclei (AGN) are supermassive black holes (SMBHs) in the center of galaxies that accrete surrounding gas and emit across the entire electromagnetic spectrum. They are the most energetic persistent emitters in the Universe, capable of outshining their host galaxies despite their emission originating from a region smaller than our Solar System. AGN were some of the first sources discovered that helped teach us that there were galaxies outside of our own, and they proved the existence of black holes. Moreover, AGN can give us valuable insights into other branches of astrophysics. For example, they can be used to …
The Pursuit For Gamma-Ray Emitting Pulsar Wind Nebulae With The Fermi-Large Area Telescope, Jordan Eagle
The Pursuit For Gamma-Ray Emitting Pulsar Wind Nebulae With The Fermi-Large Area Telescope, Jordan Eagle
All Dissertations
Pulsar wind nebulae are highly magnetized particle winds, descending from core collapse supernovae (CC SNe), and each powered by an energetic, rapidly rotating neutron star. There are at least 125 Galactic pulsar wind nebulae (PWNe) that have been discovered from radio wavelengths to TeV gamma-rays, the majority of which were first identified in radio or X-ray surveys. An increasing number of PWNe are being identified in the TeV band by ground-based air Cherenkov Telescopes such as HESS, MAGIC, and VERITAS such that they constitute the dominant source class of Galactic TeV emitters. High-energy sources like PWNe may be responsible for …
Co Molecular Spectroscopy And Spectro-Astrometry Of Protoplanetary Disks, Stanley Jensen
Co Molecular Spectroscopy And Spectro-Astrometry Of Protoplanetary Disks, Stanley Jensen
All Dissertations
Until relatively recently, the only known planets were those in our own solar system (Mayor & Queloz, 1995). However modern instruments and techniques have revealed that planets are ubiquitous around main sequence stars over the last three decades. From this we have obtained a multitude of insights into planetary populations. For instance, while the terrestrial and gas giant planet types are both well accounted for in extra-solar systems, the arrangement of our own system with inner terrestrial planets and outer gas giants is not clearly the typical organization (van der Marel & Mulders, 2021). In fact we see that in …
Exploring The Glow Of The Universe In Gamma-Rays And Hunting Distant Agn, Changam Meenakshi Rajagopal
Exploring The Glow Of The Universe In Gamma-Rays And Hunting Distant Agn, Changam Meenakshi Rajagopal
All Dissertations
The entirety of the γ-ray radiation permeating our Universe is encoded in the extragalactic γ-ray background. This is a superposition of resolved sources, mostly powerful relativistic jets powered by supermassive black holes, i.e., blazars, and an unresolved isotropic component, aka, the diffuse isotropic gamma-ray background (IGRB). Studying the IGRB can help unveil its composition, as well as unearth multi-messenger relationships between the intensities of PeV neutrinos, ultra high energy cosmic rays (> 1018 eV), and sub-TeV γ-rays. The comparable energy budgets of these three phenomena (neutrinos, UHECR, and γ-rays) indicates a physical connection or a common source amongst them. On …
Defect Assisted Growth Of Copper-Silicide Nanostructures On Si(100) And Si(111), Endu Sekhar Srinadhu
Defect Assisted Growth Of Copper-Silicide Nanostructures On Si(100) And Si(111), Endu Sekhar Srinadhu
All Dissertations
As the dimensions of the smallest feature on the integrated circuit has minia-turized into the range of tens of nanometers, patterning of highly ordered nanos-tructures with tunable size and shape in large scale on the surface of substrate is highly demanding and increasingly challenges the limits of nanolithography. To over-come the conventional lithographic limitations, self assembled methods have been explored. While strain driven self assembly is widely viewed as a promising technique for patterning at the nanoscale, to follow this approach and create structures in a desired manner, a reliable means to engineer and characterize the shape and sizes of …
Scalable Synthesis And Energy Applications Of Defect Engineered Nano Materials, Mehmet Karakaya
Scalable Synthesis And Energy Applications Of Defect Engineered Nano Materials, Mehmet Karakaya
All Dissertations
Nanomaterials and nanotechnologies have attracted a great deal of attention in a few decades due to their novel physical properties such as, high aspect ratio, surface morphology, impurities, etc. which lead to unique chemical, optical and electronic properties. The awareness of importance of nanomaterials has motivated researchers to develop nanomaterial growth techniques to further control nanostructures properties such as, size, surface morphology, etc. that may alter their fundamental behavior. Carbon nanotubes (CNTs) are one of the most promising materials with their rigidity, strength, elasticity and electric conductivity for future applications. Despite their excellent properties explored by the abundant research works, …
A Spectroscopic Study Of Anomalous Stellar Populations In M67, Courtney Mcgahee
A Spectroscopic Study Of Anomalous Stellar Populations In M67, Courtney Mcgahee
All Dissertations
A population of stars exists in the old, open cluster M67, whose photometry, color magnitude diagram locations and associated evolutionary states cannot be explained by current, standard single star evolution theory. These stars are often referred to as ``yellow straggler' stars. Yellow stragglers have been identified in multiple star clusters suggesting that these stars constitute a real population. Additionally, according to independent studies, at least some of the yellow straggler stars in M67 are likely cluster members. Therefore, cluster non-membership is not a sufficient explanation for the observed anomalous photometry of these stars. It is possible that the yellow stragglers …
Multidimensional Simulations Of Non-Linear Cosmic Ray Production In Supernova Remnant Evolution, Joshua Wood
Multidimensional Simulations Of Non-Linear Cosmic Ray Production In Supernova Remnant Evolution, Joshua Wood
All Dissertations
When a high-mass star (& 4Msun) explodes at the end of its life, a supernova occurs, leaving its degenerate core and a fast-moving shell of matter, known as a supernova remnant (SNR). The SNR shell lasts for many thousands of years, generating emissions from low-frequency radio (~ 10-7 eV) up to γ-ray regime (~ 1015 eV). It is also believed that SNRs are the predominant source of galactic cosmic rays, accelerating a population of thermal ions, primarily protons, up to relativistic energies by means of the diffusive shock acceleration (DSA) mechanism. The small population of thermal (Boltzmann) particles, p ~ …
Chemical Evolution Of The Neutron-Rich Iron-Group Isotopes And Implications For The Formation Of Fun Cais, Tianhong Yu
Chemical Evolution Of The Neutron-Rich Iron-Group Isotopes And Implications For The Formation Of Fun Cais, Tianhong Yu
All Dissertations
Calcium-aluminum-rich inclusions (CAIs) are millimeter-sized refractory objects found in primitive meteorites. CAIs are considered as some of the first solids to form in the solar system, because they are refractory. FUN CAIs, those with Fractionated and Unknown Nuclear effects, are a small subset of CAIs. These FUN CAIs show correlated excesses and deficits in the neutron-rich iron-group isotopes such as 48 Ca and 50 Ti which regular CAIs do not. Interestingly, these isotopes are most likely produced infrequently but in huge quantities in a rare class of thermonuclear (Type Ia) supernovae. I propose that the isotopic effects in the neutron-rich …
Planet Formation In Transition Disks: Modeling, Spectroscopy, And Theory, Joseph Liskowsky
Planet Formation In Transition Disks: Modeling, Spectroscopy, And Theory, Joseph Liskowsky
All Dissertations
An important field of modern astronomy is the study of planets. Literally for millennia, careful observers of the night sky have tracked these 'wanderers', with their peculiar motions initiating avenues of inquiry not able to elucidated by a study of the stars alone: we have discovered that the planets (as well as Earth) orbit the sun and that the stars are so far away, even their relative positions do not seem to shift perceptibly when Earth's position moves hundreds of millions of miles. With the advent of the telescope, and subsequent improvements upon it over the course of centuries, accelerating …
The Power Of Thermonuclear Supernovae After One Year, Ginger Bryngelson
The Power Of Thermonuclear Supernovae After One Year, Ginger Bryngelson
All Dissertations
Type Ia supernovae (SNe Ia), the thermonuclear explosion of a white dwarf, shape our understanding of the expansion of the universe with the use of their uniformity in distance determinations. Powered by radioactivity synthesized in the explosion, they fade slowly over hundreds of days. Sometime after 200 days, the continually expanding ejecta allows γ-rays from 56 Ni and 56 Co decays to escape, and soon any radioactive power contributing to lighting up the SN comes from positrons formed in 19% of 56 Co
decays.
While at first it seemed that positrons escaped through the thinning ejecta, it has become apparent …
Measurements Of Mass Accretion Rates In Herbig Ae/Be Stars, Brian Donehew
Measurements Of Mass Accretion Rates In Herbig Ae/Be Stars, Brian Donehew
All Dissertations
Herbig Ae/Be stars(HAeBes) are young stellar objects of spectral class F2 through B0, with the central star often surrounded by a circumstellar disk of gas and dust. They are the higher mass analogs to T Tauri stars. The interaction between the star and the disk is not well understood, nor is the disk structure. The central star will often accrete mass from the disk, and the mass accretion rate is an important parameter for modeling the disk structure and evolution. The methods for measuring mass accretion rates of T Tauri stars are generally not applicable to HAeBe stars. As such, …
Modeling The Galactic 511 Kev Positron Annihilation Emission, Production, And Propagation, Bethany Johns
Modeling The Galactic 511 Kev Positron Annihilation Emission, Production, And Propagation, Bethany Johns
All Dissertations
The Galactic 511 keV positron annihilation emission has a strong bulge component to the emission and a weak disk component. The bulge emission is about 1.4 -- 6 times as the disk emission. The bulge emission is defined to be a diffuse emission centered at the Galactic center and extends to about a FWHM of 8 degrees in Galactic longitude and latitude. An asymmetry has also been observed in the disk emission, where the negative longitudes are brighter than the positive longitudes by a factor of 1.8.
This research examines the morphology of the Galactic 511 keV positron annihilation emission …
Gamma Ray Bursts As Probes Of Dust In The Evolving Universe, Adria Updike
Gamma Ray Bursts As Probes Of Dust In The Evolving Universe, Adria Updike
All Dissertations
Dust is ubiquitous in the universe. Understanding where it comes from and where we observe it can have major implications to all astronomical observations. In this study, we investigate how gamma ray bursts (GRBs) can be used as probes of dust in the evolving universe. Making the simplification that silicate dust comes from core collapse supernovae and that graphite dust is produced in the winds of low- to intermediate-mass stars, we present numerical simulations of the resulting dust evolution in GRB hosts and show how the SEDs evolve. Dust extinction laws are re-derived from scattering theory of small particles and …
Internal Transitions In Nuclei And Applications To Astrophysics And Cosmochemistry, Joseph Johnson
Internal Transitions In Nuclei And Applications To Astrophysics And Cosmochemistry, Joseph Johnson
All Dissertations
In this dissertation I analyze internal transitions in nuclei and their potential effects on s-process branching and on accelerated β-decay in cosmochemical samples. To accomplish my analysis, I have constructed a number of computational tools, which I have publicly released or make available through this thesis. I apply my analysis to the question of s-process branching at 186Re and study the implications for the osmium isotopes in mainstream presolar SiC grains. My essential conclusion is that the problem of the large neutron flux inferred from the SiC osmium isotopic abundances cannot be solved by branching through an out-of-equilibrium isomer of …
Observing Co In Circumstellar Disks, Matthew Troutman
Observing Co In Circumstellar Disks, Matthew Troutman
All Dissertations
This dissertation includes high-resolution, near-infrared spectroscopy to study CO in circumstellar disks around Herbig Ae/Be stars. The velocity-resolved spectra was used to measure the distribution of gas in the circumstellar disk and thus determine the evolutionary state of the system. Near-infrared spectra were obtained on over 30 young circumstellar disks around Herbig Ae/Be stars to study the physical processes in the disk. The radial location of the CO emitting gas bears directly on the evolutionary state of the transition objects in the sample. This study included a detailed study of the debris disk star β Pictoris where the disk mass …
Branchings And Time Evolution Of Reaction Networks, Changyuan Wang
Branchings And Time Evolution Of Reaction Networks, Changyuan Wang
All Dissertations
In this thesis I analyze flows in reaction networks in terms of branchings
in a digraph. If the coupled differential equations governing the rate
of change of probabilities X of a state or species are finite-differenced in time, a matrix equation (I + Adt)X(t+dt) = X(t) results, where X(t) is a vector giving the probabilities at time t and X(t+dt) is a vector giving the probabilities at time t + dt. I demonstrate that the matrix (I + Adt) may be written as the product of an incidence matrix and a weight matrix for a directed graph (digraph) representing the …
Observations Of Very High Energy Gamma-Ray Emission From Supernova Remnants With Veritas, Mark Theiling
Observations Of Very High Energy Gamma-Ray Emission From Supernova Remnants With Veritas, Mark Theiling
All Dissertations
The nature and source of cosmic rays has been at the core of particle astrophysics since their discovery almost a century ago. The cosmic ray spectrum is best described by a broken power law, and can be better understood as three distinct parts. Theory holds that cosmic rays up to ∼1015 eV – those below the “knee” or steepening in the spectrum – are produced in the shocks of supernova remnants. Direct detection of cosmic rays produced in supernova remnant shocks is impossible, however, as cosmic rays below ∼1018 eV are deflected by the Galactic magnetic field and cannot be …
The Reality Of The Wolf 630 Moving Group, Eric Bubar
The Reality Of The Wolf 630 Moving Group, Eric Bubar
All Dissertations
The concept of relic kinematic assemblages from dispersed stellar clusters has
remained contentious since Eggen's initial formulation of moving groups in the
1960's. However, the availability of high quality parallaxes from the Hipparcos
space astrometry mission has resulted in distance measurements for thousands of
nearby, seemingly isolated stars. With these newly determined distances, a
high resolution spectroscopic abundance analysis can be brought to bear on many of the
alleged members of these relic associations. If a structure is a relic of
an open cluster, the members can be expected to be monolithic in age and abundance
inasmuch as homogeneity is …
Supernova Injection Of Short-Lived Radionuclides Into The Presolar Cloud: A Feasibility Study, Keith Davis
Supernova Injection Of Short-Lived Radionuclides Into The Presolar Cloud: A Feasibility Study, Keith Davis
All Dissertations
Meteorite inclusions show that the early solar system was radioactive with species of short lifetimes compared to the formation time of the solar system. Transporting the radioactive material from the creation site to the formation site of the sun was expected to take enough time that these species should have decayed to nonexistence. Some special series of events seems necessary to speed the process along. Cameron & Truran (1977) suggested that the source of these short-lived radionuclides could have been a supernova. Numerical hydrodynamic studies have shown that slow shockwaves can inject material into a small, dense cloud core. Most …