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No description available.

No description available.

Many systems of physical interest are difficult to manage computationally because of the intrinsic nature of the equations that govern them. Many of these systems of equations are stiff, meaning that the standard approach to solving them is with implicit methods, because explicit methods either are unstable or require timesteps too small to be computationally efficient. Presented here is a study of explicit methods that decouple stability from accuracy under certain conditions, allowing for larger timesteps to be taken.

This study uses a combination of quantitative and qualitative enquiry to focus on determining the most salient factors that affect international students’ learning of introductory physics in Georgia State University. For purposes of the study, “international students” were defined as those who attended high school in a country other than the US. These students comprise a significant portion of the physics courses at Georgia State, and this study was motivated by the desire to support their success. The study involved a collaboration with the newly emerging Physics and Astronomy Education Research Group who has recently begun the routine collection of student learning data in all of its introductory physics courses. The factors considered in the research design were informed by the literature on student learning for all students while including the possibility of new factors emerging in interviews with international students. Factors probed included students’ previous study of mathematics, previous study of physics, language issues, pedagogical differences (i.e., style of teaching, classroom culture & environment) between GSU and the student’s country of origin. For international students who are proficient in English, classroom environment and culture (pedagogy) emerged as the most important factor. For International students who are not very proficient in English, language remains the most important factor. The effect of language issues on international students’ learning of physics turned out to be more complex than originally considered. Some students understood instructors differently depending on what country the students come from and on what country the instructor comes from. Instructor office hours and general accessibility for addressing questions emerged as especially important options for international students who felt uncomfortable asking questions in front of the whole class. An unanticipated outcome of the study was to discover how the vast differences in the structure of high school mathematics education in non-US countries has serious implications for the way we advise and query international students in physics vis-à-vis their academic background before entering Georgia State. Moreover, the study revealed that students who had taken a high school physics course generally scored no better than those who had not taken a high school course on a pre-test of conceptual knowledge in physics. However, students who had taken a physics class in high school had dramatically higher learning gains when given a post-test near the end of the Georgia State physics course. This phenomenon suggests that more consideration should be given to prior course-work in combination with a diagnostic pre-test to advise students about which math and physics courses to take when they arrive at Georgia State.

<p> Dwarf galaxies present interesting observational challenges for the studies of various galaxy properties: despite their abundance and proximity to the Milky Way, they typically have very low surface brightnesses and small physical sizes. Until now, only the extreme variety of dwarfs &mdash; those undergoing strong bouts of star formation &mdash; have been observed in the FIR, due to observational difficulties. However, this population does not represent the majority of dwarfs, which have only moderate star formation rates and extremely low metallicity (the fraction of heavy elements to hydrogen). The advent of the Herschel Space Telescope, with its superior resolution and sensitivity over previous generations of telescopes, has made it possible to measure FIR spectral lines and broadband continuum in normal dwarf galaxies, expanding the scope of studies beyond the brighter, but more extreme, varieties. </p><p> The general goal of my research was to study the conditions in the interstellar media (ISM) of typical dwarf galaxies. The LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, TheHI Nearby Galaxy Survey) project aims to unravel many mysteries of nearby dwarfs using a suite of multi-wavelength data, and the new additions from <i>Herschel</i> help provide insight into the physics of these systems. I reduced and analyzed FIR fine-structure spectral line data for the LITTLE THINGS sample to study the different phases of the ISM, as well as FIR photometry data to access the dust properties and infrared continuum emission in these systems. The FIR spectral lines are diagnostics for the conditions in the ISM of galaxies, telling us about heating efficiency, the fraction of gas that resides in photodissociation regions (PDRs), abundance of highly ionized gas from massive stars, and other physical descriptions. The photometric continuum observations enable the modeling of interstellar dust properties &ndash; dust plays an important role in shielding and cooling molecular clouds which form stars, as well as heating via the photoelectric effect. I also utilized neutral hydrogen data to probe the neutral medium in relation to the FIR, as well as optical and UV data to characterize star formation and the emission of starlight.</p>

<p> Mergers of a neutron star and a black hole are interesting because of the dual complexity of the black hole's strong gravity and the neutron star's nuclear-density fluid. Mergers can yield short-lived nuclear accretion disks, emitting copious neutrinos. This radiation may change the thermodynamic state of the disk itself, may drive an ultrarelativistic jet of electrons and positrons, may oscillate in its flavor content, may irradiate surrounding matter, playing a role in nucleosynthesis, and may be detected directly. </p><p> In this thesis I present a model of such a merger, its remnant accretion disk, and its neutrino emission. In particular, we evolve a neutron star&mdash;black hole merger through &sim;100 ms, solving the full general relativistic hydrodynamics equations, from inspiral through merger and accretion epochs. We treat the neutrinos approximately, using a leakage framework, which accounts for local energy losses and composition drift in the fluid due to escaping neutrinos. We use geodesic ray tracing on a late time slice of the model to calculate the full spatial-, angular-, and energy-dependence of the neutrino distribution function around the accretion disk. This distribution then serves in a computation of the energy available to form a jet via neutrino-antineutrino annihilation in the disk funnel. In this scenario, we find that enough energy is deposited to drive a jet of short-gamma-ray-burst-energy by neutrino processes alone.</p>

<p> Solar flares can release ~1e33 ergs of power, accelerate particles to relativistic speeds, heat plasma to ~15 million K and catastrophically reorganize 1e5 km long field structures in 100s&ndash;1000s of seconds. Magnetic reconnection of large-scale field structures in the corona are thought to power flares, but the precise mechanisms that convert the stored magnetic energy into particle kinetic energy are poorly understood. </p><p> Flare spectra in the 20 keV&ndash;10 MeV energy range are rich with information that provide a window into the underlying physics of flare particle acceleration. This hard X-ray (HXR)/gamma-ray emission can be used to understand electron and ion dynamics, particle abundances and the ambient plasma conditions in solar flares. Enhanced imaging, spectroscopy and polarimetry of flare emissions in this energy range are needed to address the current era of particle acceleration and transport questions, including: What causes the spatial separation between HXR emission generated by relativistic electrons and the gamma-ray line emission from energetic ions? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? </p><p> The <i>Reuven Ramaty High Energy Solar Spectroscopic Imager</i> (RHESSI) instrument, launched in 2002, provided the first combined imaging and high-resolution spectroscopy in the HXR/gamma-ray range. RHESSI's volumes of detailed study on electron-associated emission &lt; 1 MeV is in contrast to comparatively few ion-associated gamma-ray observations. Over the past two solar cycles RHESSI has imaged only five flares at the 2.2 MeV neutron-capture line and has been able to resolve ion lines in ~30 events. My research aims to expand this small set gamma-ray flare observations by (1) using new techniques to study flares obscured by high-background counts in the existing RHESSI dataset and (2) providing new observations through the development and flight of the <i>Gamma-Ray Imager/Polarimeter for Solar Flares</i> (GRIPS) instrument.</p><p>

<p> Currently, we lack a means of identifying the type of matter at the core of compact stars, but in the future, we may be able to use gravitational wave signals produced by fluid oscillations inside compact stars to discover new phases of dense matter. To this end, we study the fluid perturbations inside compact stars such as Neutron Stars (NS) and Strange Quark Stars (SQS), focusing on modes that couple to gravitational waves (GWs). Using a modern equation of state for quark matter that incorporates interactions at moderately high densities, we implement an efficient computational scheme to solve the oscillation equations in the framework of General Relativity, and determine the complex eigenfrequencies that describe the oscillation and damping of the non-radial fluid modes. We find that the <i>f</i>- mode frequency only weakly distinguishes NS from SQS. However, we do find that the <i> p</i>- mode has a strong discriminating signature between the two models. In addition we study the impact of parameters of the SQS equation of state on the oscillation spectra. Finally, we discuss the significance of our results for future detection of these modes through gravitational waves.</p><p>

<p> Through their connection with dark matter structures, galaxies act as tracers of the underlying matter distribution in the Universe. Their observed spatial distribution allows us to precisely measure large scale structure and effectively test cosmological models that explain the content, geometry, and history of the Universe. Current observations from galaxy surveys such as the Baryon Oscillation Spectroscopic Survey have already probed vast cosmic volumes with millions of galaxies and ushered in an era of precision cosmology. The next surveys will probe over an order of magnitude more. With this unprecedented statistical power, the bottleneck of scientific discovery is in the methodology. </p><p> In this dissertation, I address major methodological challenges in constraining cosmology with the large-scale distribution of galaxies. I develop a robust framework for treating systematic effects, which significantly bias galaxy clustering measurements. I apply new innovative approaches to probabilistic parameter inference that challenge and test the in- correct assumptions of the standard approach. Furthermore, I use precise predictions of structure formation from cosmology and observations of galaxies during the last eight billion years to develop detailed models of how galaxies are impacted by their host dark matter structures. These models provide key insight into the galaxy-halo connection, which bridges the gap between cosmology theory and observations. They also answer crucial questions of how galaxies form and evolve. The developments in this dissertation will help unlock the full potential of future observations and allow us to precisely test cosmological models, General Relativity and modified gravity scenarios, and even particle physics theory beyond the Standard Model.</p><p>