Past Dissertations
Heather Jacobson, 2009
Title: A Study of Open Clusters in the Apparent Abundance Transition Zone Rgc~10 kpc in the Milky Way
Abstract: Detailed chemical abundance studies of open star clusters in the Milky Way Galaxy provide crucial information about the formation and evolution of our Galaxy and of other disk galaxies in general. Numerous studies have shown that the open cluster metallicity distribution decreases with increasing distance from the center of the Galaxy, in agreement with studies of other populations. Some recent studies in the last decade have uncovered evidence that the metallicity distribution may not be a linear gradient as originally thought, but rather a step function with a sharp break in the abundance distribution at a Galactocentri radius of Rgc~10-12 kiloparsecs (kpc). In this dissertation, I examine the nature of this transition from the inner disk to the outer disk of our Galaxy by determining detailed chemical abundances for nearly twenty open clusters evenly distributed throughout the supposed transition region. My sample more than doubles the number of clusters previously studied in this part of the Galaxy. Chemical abundances for several individual stars in each cluster are determined from high-resolution spectroscopy obtained with four different telescope/instrument combinations, including the Hydra multi-object spectrograph on the WIYN 3.5m telescope. Temperatures and surface gravities of individual stars in each cluster are determined using these spectra and available photometry, and stellar chemical abundances are averaged together to form a robust cluster mean abundance and dispersion. Cluster distances are also placed on a homogeneous scale.
For analysis of the open cluster abundance distribution across the full extent of the Milky Way disk, my cluster sample is combined with two other samples in the literature. Through visual examination of the resulting cluster abundance distributions as a function of Galactocentric distance along with the use of a statistical mixture modeling algorithm, it is found that the chemical abundance distribution in the region Rgc~10-12 kpc is well represented by a gradient, and is inconsistent with a sharp transition at any Galactocentric distance. These findings are consistent with the abundance distributions seen in other disk galaxies, and can be reproduced by theoretical models of galaxy formation and chemical evolution.
Aaron Boley, 2007
Title: The Three-Dimensional Behavior of Spiral Shocks in Protoplanetary Disks
Abstract: In this dissertation, I describe theoretical and numerical studies that address the three-dimensional behavior of spiral shocks in protoplanetary disks and the controversial topic of gas giant formation by disk instability. For this work, I discuss characteristics of gravitational instabilities (GIs) in bursting and asymptotic phase disks; outline a theory for the three-dimensional structure of spiral shocks, called shock bores, for isothermal and adiabatic gases; consider convection as a source of cooling for protoplanetary disks; investigate the effects of opacity on disk cooling; use multiple analyses to test for disk stability against fragmentation; test the sensitivity of GI behavior to radiation boundary conditions; measure shock strengths and frequencies in GI-bursting disks; evaluate temperature fluctuations in unstable disks; and investigate whether spiral shocks can form chondrules when GIs activate. The numerical methods developed for these studies are discussed, including a radiation transport routine that explicitly couples the low and high optical depth regimes and a routine that models ortho and parahydrogen. Finally, I explore the hypothesis that chondrule formation and the FU Ori phenomenon are driven by GI activation in dead zones.
Nick Mostek, 2007
Title: Calibration System Design and Determination of Filter Calibration Requirements for Snap
Abstract: The SuperNova/Acceleration Probe (SNAP) is a proposed space-based, wide-field telescope designed to measure the properties of dark energy in our universe. SNAP will measure ~2000 type Ia supernovae, and the reduction of systematic errors in the relative spectrophotometric measurements will be critical to the mission science. A stringent systematic error requirement of 2% in color photometry is driving the SNAP calibration methodology and system design into new areas of space-based, radiometric calibration for astronomical missions.
At the forefront of these new calibration techniques is the use of narrowband light and photodiodes to measure the precise irradiance incident on SNAP filters and detectors. Using these techniques, I have built the Monochromatic Illumination and Cryogenic Calibration System (MICCS) to address the SNAP calibration hardware requirements. With this system, I can transfer the NIST irradiance calibration of an InGaAs photodiode to transfer photodiodes operated at 140K as well as measure the transmission of interference filters at incident angles and temperature similar to that used on the SNAP focal plane.
Due to size and light efficiency constraints, I also investigated the use of light emitting diodes (LEDs) as calibrating light sources onboard SNAP. When coupled with calibrated photodiodes, a selection of LEDs could fly onboard SNAP and be used to track changes in the SNAP interference filters during the lifetime of the experiment. The error from this LED calibration technique will be propagated to the dark energy parameters to determine what design constraints are required of the onboard illumination system.
Steven J. Margheim, 2007
Title: Lithium in the Pleiades
Abstract: The light-element lithium (Li) has long been an invaluable tool in the study of the properties and evolution of stellar interiors. Since it was first observed in the late 80s, the observation of a large dispersion in the surface Li abundance in stars of similar mass in young Pleiades open cluster has defied understanding by standard stellar models. The observation of a similar dispersion in the young cluster NGC 2451 led to the discovery that a large portion of the anomalous LI dispersion is due to incorrect analysis on the most rapidly rotating stars. Using improved analysis techniques, a new survey of Li abundances in the Pleiades cluster has been completed from high quality data from the Hydra spectrograph on the WIYN telescope. The results of the survey are presented as well as a discussion of the primordial cluster Li abundance, formation of the lithium gap and the remaining Li dispersion in light of these new results.
Kai Cai, 2006
Title: 3D Hydrodynamic Simulations of Gravitational Instabilities in Embedded Protoplanetary Disks
Abstract: Planets are believed to form in the primordial disks of gas and dust around newborn stars. The discovery of giant planets outside our Solar System and new observations of protoplanetary disks pose new challenges to the formation theories of giant planets. Fragmentation of the disks due to severe gravitational instabilities (GIs) caused by rapid cooling has been suggested as one of the major formation mechanisms for these gas giants. It is generally thought that the disks embedded in thick gaseous envelopes are more massive and smaller in size and thus are more susceptible to GIs. On the basis of Mejía’s methods (2004 Ph.D. Dissertation, Indiana University), I developed a radiative scheme that incorporates the effect of envelope irradiation. 3D radiative hydrodynamics simulations for a disk of 0.07 Msun around a young star of 0.5 Msun using this scheme show that, the irradiation tends to suppress GIs and GIs become weaker in a protoplanetary disk with higher metallicity or larger grain size, due to higher disk opacity and therefore longer cooling times. The global mass transport induced by GIs seems to be dominated by the low-order modes and so displays a complex behaviour as the parameters vary. Preliminary results from a simulation of a massive embedded disk with physical characteristics similar to a disk in the embedded source L1551 IRS5 also suggest a long radiative cooling time. None of simulations produce dense clumps, even with high resolution, arguing against direct formation of giant planets by disk instability. In future simulations, I hope to include the effects of dust sedimentation and gas infall onto the disk from the envelope, to further explore the possibility of gravitational fragmentation in young embedded disks.
Allen Rogel, 2005
Title: The ChaMPlane Project: Spectroscopic Followup and Theoretical Modeling
Abstract: The ChaMPlane project is a multiwavelength survey designed to classify the serendipitous X-ray sources detected within the Galactic plane. The detection of CVs is of particular interest as to date nearly all CVs have been detected serendipitously during the course of other projects. The ChaMPlane project will result in a well-determined volume sample of CVs that will result in well-defined spatial parameters for the CV distribution in the Galaxy.
Targets for CV detection are determined by finding X-ray sources in archival Chandra data and finding H alpha-excess objects in images taken of the same fields. Classification of these targets is done using spectra acquired with Hydra at WIYN. To translate the observations into CV space density parameters, a Monte-Carlo model of the CV distribution of the Galaxy is used, with input parameters being the local CV space density and scale height. To date, with somewhat limited data (4 CV detections in anticenter fields), the observational results are consistent (within 2 sigma error limits) with the predictions made by the model using published values of the local CV space density and scale heights. The limitation imposed on the model by the question of CV luminosity variations results in the model placing an upper bound on the CV space density. For a CV distribution with a scale height of 160 pc, the ChaMPlane observations are best fit by a CV local space density of 1.4+2.1-0.8 x 10-5 pc-3.
Stella Kafka, 2005
Title: Exploring Stellar Activity on the Secondary Star of Cataclysmic Variables
Abstract: This thesis explores methods to infer stellar chromospheric activity on the secondary star of Cataclysmic Variable binary systems (CVs), which is usually masked by accretion effects. For such a complicated problem, both photometric and spectroscopic indicators of activity in CVs are sought, focusing on systems showing large (2-5 mag) drops in their optical light curves (VY Scl systems), presumably due to disruptions of the mass transfer from starspots near the L1 point. The shapes of the transitions to and from the low states of both disk and magnetic systems above the CV period gap, have been successfully related to starspots crossing L1. For systems below the gap, the characteristics of the transitions point to X-ray irradiation on the L1 point for the cause of their low states. This is in accord with the nature of the secondary star, which is expected to be fully convective for systems below the gap. Furthermore, an extended photometric monitoring campaign of the 2004 low state of the polar prototype AM Herculis resulted in resolving events attributed to activity (flares) on the secondary star, indicating the presence of two active regions on the secondary. Spectroscopic monitoring during the 2003 and 2004 low states reveals that the H alpha line is often triple-peaked. The central peak likely originates from the irradiated inner hemisphere of the donor star, whereas the variability, phasing and velocity of the satellite components indicate that they possibly arise from loop-like coronal structures connecting active regions on the secondary, perhaps triggered by the photometrically detected flaring events.
Brain Rebel, 2004
Title: Measurement of Neutrino Oscillation Parameters and Charge Separation in Upward-Going Muons Using the MINOS Far Detector
Abstract: The Main Injector Neutrino Oscillation Search (MINOS) is an experiment designed to probe the phenomenon of neutrino oscillations. When MINOS is completed it will consist of a neutrino beam and two detectors, which are separated by a distance of 735 km. The near detector measures the energy distribution and flux of a beam of muon neutrinos produced at Fermilab, while the far detector, located in Soudan, MN, measures these same neutrino properties 735 km away. The signal for a detection of neutrino oscillations is a deficit of neutrinos at the far detector compared to expectations based on the near detector measurements.
In addition to measuring beam neutrinos, the far detector can be used to measure neutrinos produced in cosmic ray interactions in the atmosphere. While waiting for the beam to begin running, the far detector was used in this mode. Several previous experiments, such as Super-K and MACRO, have suggested that the atmospheric neutrinos oscillate between different flavor states. This dissertation looks for an oscillation signal in the atmospheric neutrinos by using muons resulting from the interaction of the atmospheric neutrinos in the rock surrounding the MINOS far detector. MINOS has the advantage of a flat overburden allowing it to measure neutrino-induced muons coming from above the horizon. This advantage allows for the search for an oscillation signal to be extended into new regions of parameter space. In addition, MINOS is the first magnetized underground neutrino detector, an advantage that allows it to distinguish between muons created by neutrinos and those created by anti-neutrinos. A total of 50 neutrino-induced muon events were found in the data. A fit for the oscillation parameters sin2 2theta and Delta m2 shows that the data are consistent with both the null oscillation hypothesis and the observation of oscillations at the 90% confidence level. The data were also analyzed for evidence of CPT-violation in neutrino oscillations and the data are consistent with CPT being conserved at 68% confidence. The predicted confidence contours for the neutrino oscillation and CPT- violation analyses indicate that a 4 year exposure should allow MINOS to make a definite statement for both analyses.
Annie Mejia, 2004
Title: The thermal regulation of gravitational instabilities in disks around young stars
Abstract: The discovery of other planetary systems has led astronomers to revise the Solar Nebula theory, especially to question whether gas giant planets form by core accretion. It seems that Jupiter-size planets formed by pure runaway growth require more than the typical protoplanetary disk lifetime to collect their mass from the surrounding material. Gas giant planet formation by gravitational instabilities (GIs) does no have this problem because planets even several times more massive than Jupiter can be formed in just a few disk orbital periods (~ 1000 yr). In this scenario, the disk succumbs to its own gravity, develops spiral structures and finally fragments into stable, high-density planetary clumps. The efficiency of this planet formation mechanism depends on the detailed physics of protoplanetary disks, such as the thermal conditions of the gas, the role of stellar irradiation, the physical and chemical properties of dust grains and their distribution throughout the disk, to name a few.
GIs are spiral distortions in a self-gravitating disk that appear wherever the local surface density and temperature become favorable for their growth. The restructuring of the disk as it becomes unstable, the thermal processes that sustain the instabilities, and their effect on the long-term evolution of the disk are the studied using 3-D hydrodynamic simulations. These show that the cooling and heating processes balance each other, and as a result the disk asymptotes to quasi-equilibrium within a few orbital periods after the GI onset. The final values of the internal energy and the Toomre Q are independent of cooling time, while the asymptotic mass transport rates are inversely proportional to the cooling time. The formation of dense rings is common in these simulations. GIs fragment into clumps when the cooling time is on the order of the local orbital time, but the clumps are short-lived.
External radiation can affect the evolution considerably. GIs are an effective angular momentum and mass transport mechanism, and during their onset, they can reproduce the mass transport rates necessary to trigger FU Ori outbursts.
Aaron Steinhauer, 2003
Title: Formation and Evolution of the Open Cluster Lithium Gap
Abstract: Lithium plays an important role at the intersection of many vital fields in astronomy, including cosmology and stellar structure and evolution.
In this thesis I explore the evolution of the ``lithium gap,'' a dramatic over-depletion of lithium seen in F stars of the Hyades cluster which strongly contradicts the strictures of the Standard Theory. This issue is examined from the heretofore unexplored angle of the timing of its formation, with proposed solutions such as mass loss, diffusion and slow mixing predicting different epochs for the depletion. Four clusters were chosen with ages intermediate to the Hyades and the Pleiades (whose F stars do not show such depletion): M34, M35, M39, and NGC 3532. High resolution spectra were obtained of the lithium region using the Hydra and Hydra II multi-object spectrographs on the WIYN 3.5m and CTIO 4m telescopes. These same spectra, and independent photometry, were used to determine fundamental parameters for these clusters as a whole (age, metal abundance, and interstellar reddening) and for the individual stars (effective temperature and surface gravity).
Results include a clear anti-correlation between rotational velocity and depletion in the gap whose slope becomes steeper with age, and the determination of an early epoch of formation, with older clusters exhibiting greater depletion in the gap. It is also discovered that the gap covers a greater range in effective temperature than had previously been known. All of these results are consistent with the predictions from stellar evolution theory that includes treatments of rotationally-induced mixing, and they also argue strongly against diffusion and mass loss.
Combined with a litany of evidence from the literature, there can be little doubt that stellar rotation plays a major role in determining the lithium abundance evolution of F stars. Possible implications for cosmology are also discussed.
Adam Rengstorf, 2003
Title: Quasar Detection via Variability in a High Galactic Latitude Drift Scan Survey
Abstract:
As part of the QUEST (QUasar Equatorial Survey Team) collaboration, I have been studying the intrinsic variability of quasars as a means of quasar detection. I first give a brief review of the quasar central engine that suggests the mechanism driving quasar variability, then report on the current status of the QUEST project as a whole. I discuss the data collection, reduction and analysis program, and the findings over the course of three observing seasons (February 1999 - April 2001) at the 1-m Schmidt telescope at the Venezuelan National Observatory. The method of incomplete ensemble photometry was used to compile a list of variable objects from these data. I then describe the subsequent spectroscopic confirmation work carried out at the WIYN 3.5-m telescope in April 2001 and February 2002. (The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory.) Between the two spectroscopic observing campaigns, 30 of a selected subset of variable objects were identified as quasars by means of their distinctive spectral emission lines, 50% of which are newly discovered quasars and 50% of which are independent rediscoveries of previously cataloged quasars.
The number of previously cataloged quasars found in the data is used as a benchmark for statistical analysis and to predict what percentage of variability candidates should be expected to be confirmed as quasars. The percentage of known quasars that vary is seen to increase when comparing data from two observing seasons (38%) with data from all three observing seasons (61%). In addition, the colors of objects in the variability list and those of the previously cataloged quasars are studied as a possible means of increasing the efficiency of future spectroscopic observing campaigns.
