Indiana University
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Past Dissertations

Alec Hirschauer, 2016

Title: Metallicities of Galaxies in the Local Universe

Abstract: The degree of heavy-element enrichment for star-forming galaxies in the universe is a fundamental astrophysical characteristic which traces the amount of stellar nucleosynthesis undertaken by the constituent populations of stars. Estimating this quantity via the so-called "direct-method" is observationally challenging and requires measurement of intrinsically weak temperature-sensitive nebular emission lines, however these are typically not found for galaxies unless their emission lines are exceptionally bright. Metal abundances ("metallicities") must then therefore be estimated by empirical means utilizing ratios of strong emission lines, calibrated to sources of known abundance and/or theoretical models, which are measurable in essentially any nebular spectrum of a star-forming system. Relationships concerning metallicities in galaxies such as luminosity-metallicity and mass-metallicity are critically dependent upon reliable estimations of abundances. Therefore, having a reliable observational constraint is paramount to developing models which accurately reflect the universe. This dissertation explores metallicities for galaxies in the local universe through a variety of means. First, an attempt is made to improve calibrations of empirical relationships for estimating abundances for star-forming galaxies at high-metallicities, finding some intrinsic shortcomings but also revealing some interesting new findings regarding the computation of temperatures of the electron gas of star-forming systems, as well as detecting some anomalously under-abundant, overly-luminous galaxies. Second, a discovery is made of an extremely metal-poor star-forming galaxy, which opens the possibility to find more similar systems and to better understand star-formation in exceptionally low-abundance environments. Finally, the development of a self-consistent scale for estimating metallicities allows for the creation of luminosity-metallicity and mass-metallicity relations for a statistically representative sample of star-forming galaxies in the local universe.

Emily Richards, 2016

Title: Baryonic Distributions in Galaxy Dark Matter Halos

Abstract: In our best current understanding of the growth of structure in the Universe, visibly complex distributions of gas and stars form and evolve into a wide range of galaxies inside over densities of dark matter. Re-creating the observed diversity in the organization of baryonic mass within dark matter halos represents a key challenge for galaxy formation models. In this dissertation, I constrain the distribution of baryonic and non-baryonic matter in a statistically representative sample of 44 nearby galaxies defined from the Extended Disk Galaxy Exploration Science (EDGES) survey to address the growth of galaxy disks in dark matter halos. I trace the gravitational potentials of each galaxy using rotation curves derived from new and archival radio synthesis observations of neutral hydrogen (HI). The measured rotation curves are decomposed into baryonic and dark matter halo components using 3.6 micron images for the stellar content, the HI observations for the atomic gas component, and, when available, CO data from the literature for the molecular gas component. The HI kinematics are supplements with optical integral field spectroscopic (IFS) observations to measure the central ionized gas kinematics in 26 galaxies. Distributions of baryonic-to-toal mass ratios are determined from the rotation curve decompositions under different assumptions about the contribution of the stellar component, and are compared to global and radial properties of the dominant stellar populations extracted from optical and near-infrared photometry.Galaxies are grouped into clusters of similar baryonic-to-total mass distributions to examine whether they also exhibit similar star and gas properties. The radial distribution of baryonic-to-total mass in a galaxy does not appear to correlate with any characteristics of its star formation history. This result encapsulates the challenge facing simulations to create galaxies which evolve with different star formation histories but similar distributions of mass.

Jaime Overbeek, 2016

Title: The Evolution of Neutron-Capture Elements in the Milky Way

Abstract: Neutron-capture elements (those with Z > 30) are formed in two ways: slow neutron-capture (the s-process) and rapid neutron-capture (the r-process). The s-process is thought to mainly occur in low and intermediate-mass asymptotic giant branch (AGB) stars; the r-process site has not been conclusively identified but probably involves core collapse supernovae or neutron star mergers. A conflict has recently arisen between s-process models and observations, as [Ba/Fe] ratios appear to increase dramatically with decreasing age in open clusters to a degree not predicted by standard s-process models. Other s-process elements do not show the degree of enhancement in young clusters that Ba does. Various solutions have been proposed, including an intermediate process which may disproportionally create Ba, and an increased neutron source in low-mass (M < 1.5Msolar) AGB stars. We have assembled and analyzed a sample of 75 stars in 24 open clusters to measure abundances of three light s-process elements (Sr, Y, Zr), three heavy s-process elements (Ba, La, Ce), three r-process elements (Eu, Dy, and Gd), and three elements with significant contributions from both processes (`mixed' elements Mo, Pr, and Nd). Examining [s-process/Fe] trends with age confirms that Ba is unique in its behavior, as it has a large trend with age of -0.05 dex per Gyr but the other s-process elements have more modest trends with cluster age of -0.01 to -0.02 dex per Gyr. All of the s-process elements except Ba fit models based on an enhanced neutron source in low-mass AGB stars; we also do not see in our data the heavy s-process element trends with metallicity predicted by standard models. Problems we encountered with measuring Ba features combined with the uncertainty in predictions of i-process models require caution in interpreting the i-process as a solution to the Ba discrepancy. We do not find strong trends in s-process elements with Galactocentric radius, although there is some evidence of a break in the s-process gradient at ~11 kpc. Open cluster [Eu/Fe] abundances appear to fit with models accounting for r-process contributions from neutron star mergers and jet supernovae, but [r-/H] ratios do not appear to change with Galactocentric radius.

Michael Young, 2016

Title: Giant galaxies and their globular cluster populations: Analysis and results from a wide-field imaging survey and archive

Abstract: The globular cluster (GC) systems of giant galaxies are valuable and intriguing tools for a number of reasons, both in terms of the properties of the overall system as well as the properties of the individual GCs that make up the system. GCs are old: their ages range from a few Gyrs up to ~12 Gyrs, and they apparently form during galaxy mergers and major star formation events. The ensemble properties (including the color, metallicity, and spatial distributions) of the GC system constrain theoretical models of galaxy formation. For several years we have been carrying out a wide-field imaging survey of the GC populations of a sample of giant spiral, S0, and elliptical galaxies with distances of ~10-30 Mpc.
In this dissertation I present results and analysis of the GC systems of eight giant galaxies, representing a significant addition to the survey dataset. I also describe how the survey data and metadata was collected, homogenized, and ingested into a custom database and archive, and how a web portal was created to disseminate the survey products to the wider scientific community. I have developed and tested a probability factor to quantify the likelihood that a given GC candidate is in actuality a GC. I explored enhanced statistical methods to detect subpopulations in GC systems, and found that six of the GC systems in our survey presented with three GC subpopulations. I explored how the spatial and azimuthal distributions of these subpopulations differ in each host galaxy. I have supplemented our survey results with select GC system studies from the literature, and tested how different host galaxy properties correlate with the total number of globular clusters in a given system, finding that the combination of the dynamical mass of the galaxy and the K-band luminosity of the galaxy offered the best correlation with the number of GCs. Lastly, I applied this combination of predictors to a published catalog of GC system studies and found that the predictions were in good agreement with existing observations.

Cameron Pace, 2015

Title: Star Formation and Feedback From Radio Galaxies: Insights from Large Multiwavelength Surveys

Abstract: Active galactic nuclei (AGN) are believed to play an important role in the evolution of their host galaxies by influencing the galaxy's gas reservoirs. This may affect the growth of the galaxy's massive black hole and star formation in the host galaxy. I address two unanswered questions central to our understanding of AGN: what triggers AGN, and how and to what extent do they affect their host and neighboring galaxies? I study radio galaxies, which are a subset of AGN, because their radio jets may provide a natural feedback mechanism between the AGN and the host and neighboring galaxies. Previous studies, which were limited to small samples, produce conflicting results as to whether mergers or environmental effects lead to triggering. It is also uncertain whether radio galaxies have a net positive (via gas cloud collapse) or negative (via gas heating) effect on star formation. I use a large (~7,200) statistically significant sample of radio galaxies, for which I extract photometric information from several large-scale, multiwavelength surveys. The radio galaxies are compared to a sample of control galaxies whose properties match those of the radio galaxies, except for their lack of radio activity. This approach allows me to determine the frequency of feedback events and whether radio galaxies are responsible. I derive and compare composite spectral energy distributions (SEDs) for the radio galaxies and control sample, and find a deficit of ultraviolet and infrared emission for slow accreting radio galaxies, suggesting that they may suppress star formation in their hosts. Fast accreting radio galaxies are found to have an infrared excess, which is characteristic of their high accretion rate and not a result of AGN feedback on star formation. I compare the populations of neighbor galaxies of the two samples and find that radio galaxies have an excess of neighbors within 100 kpc, which must play a role in triggering. My results also show that radio galaxies rarely (<2%) affect star formation in neighboring galaxies. I also investigate the possibility of AGN jets triggering diffuse star formation in external gas clouds and find that such triggered star formation is uncommon.

Angela Parker Van Sistine, 2015

Title:The ALFALFA Hα Survey

Abstract:The ALFALFA Hα survey utilizes a large sample of HI-selected galaxies from the ALFALFA survey to study star formation in the local universe. ALFALFA Hα contains 1555 galaxies with distances between ~20 and ~100 Mpc. We have obtained continuum-subtracted narrowband Hα images and broadband R images for each galaxy, creating one of the largest homogeneous sets of Hα images ever assembled. Our procedures were designed to minimize the uncertainties related to the calculation of the local star formation rate density (SFRD). The galaxy sample we constructed is as close to volume-limited as possible, is a robust statistical sample, and spans a wide range of galaxy environments. In this dissertation, we discuss the properties of our galaxy sample, our procedure for deriving individual galaxy star formation rates (SFRs), and our method for calculating the local SFRD. We derive a value of log(SFRD [Msun yr-1 Mpc-3] = -1.723 +0.015 -0.017 (random) +/- 0.05 (systematic) based on the entire ALFALFA Hα survey. We also compare our Hα-based SFRs to those based on ultraviolet (UV) and infrared (IR) emission. We find that SFRs derived from a combination of UV and IR fluxes agree best with our Hα-derived SFRs.

Steven Janowiecki, 2015

Title:The Evolutionary Status of High and Extremely Low Surface Brightness Dwarf Galaxies: BCDs and Almost Dark Galaxies

Abstract: Studying dwarf galaxies can shed light on the original building blocks of galaxy formation. Most large galaxies are thought to be built up over billions of years through the collisions and mergers of smaller galaxies. The dwarf galaxies we see today are the evolved remnants of those building bloacks, and by understanding their nature and evolution, we can study the raw ingredients of galaxy formation.

Blue Compact Dwarf Galaxies (BCDs) and Almost Dark galaxies are at opposite extremes of today's population of dwarf galaxies. BCDs are exceptionally compact and host very intense starbursts, while Almost Dark galaxies are much more diffuse and have weak stellar populations.

This work studies the evolutionary context of BCDs by using deep, high-resolution images to study the detailed structure of their components, and by fitting our multi-wavelength observations with models to describe the properties of their stars, gas, and dust. BCDs appear to have exceptionally compact old stellar populations and unusually large star formation rates, when compared to typical dwarf galaxies.

By contrast, the optically faint, gas-dominated Almost Dark galaxies have extremely low star formation rates and weak stellar populations. In particular, one of the Almost Darks studied in this work has very unusual properties and is in disagreement with widely-studied scaling relations for large samples of galaxies. It appears to have too little stellar mass, a distribution of HI that is too extended to be supported by its modest rotation, and the highest well-measured gas mass-to-light ratio ever observed.

These two extreme classes may represent evolutionary stages that all galaxies pass through, and appear to be extreme ends of the broad continuum of dwarf galaxy properties. In order to use today's dwarf galaxies as windows into the building blocks of early galaxy formation, these unusual states and evolutionary pathways must be understood.

Jessica Windschitl Dowell, 2015

Title: Photometric and Kinematic Studies of Extragalactic Globular Cluster Systems

Abstract:Globular clusters (GCs) are old, luminous, compact collections of stars found in galaxy halos that formed during the early stages of galaxy formation. Because of this, GCs serve as excellent tracers of the formation, structure, and merger history of their host galaxies. My dissertation will examine both the photometric and kinematic properties of GC systems and their relationship to their host galaxies. In the first section, I will present the analysis of the GC systems of two spiral galaxies, NGC 891 and NGC 1055. I will discuss the photometric methods used to detect GCs using wide-field BVR imaging and to quantify the global properties of the system such as the total number of GCs and their radial distribution. My results for these two GCs systems were compared to other galaxies. I will also present the results of spectroscopic follow-up for two giant galaxies: the S0 galaxy NGC 4594 (M104), and the elliptical galaxy NGC 3379 (M105). I measured the radial velocities of GCs in these two galaxies, and combined them with published results to determine the mass distribution and mass-to-light (M/L) ratio profile for each galaxy out to large effective radius (7-9 Re). For both galaxies, I found that the M/L profiles increase with radius and do not flatten, which suggests that the dark matter halos in these galaxies extend to the edge of my data. I also looked for evidence of rotation in the GC systems, and found that neither system exhibits significant rotation around the host galaxy. I examined the velocity dispersion profile of each GC system and found kinematic differences between the red and blue GC subpopulations. Finally, I compared my results to mass estimates for these galaxies from other kinematic tracers and considered them in the context of galaxy formation models.

Maria Cordero, 2014

Title: Multiple Populations in More Metal-Rich Galactic Globular Clusters

Abstract: In this thesis I present chemical abundances for bright stars in the intermediate
metallicity globular cluster (GC) M5, and the relatively metal-rich GCs M71 and 47 Tuc with the goal of improving the understanding of chemical evolution in the metallicity regime sampled by these three GCs. One of the main goals of this thesis is to characterize the multiple populations hosted by these clusters using Na abundances, since Na is considered the most suitable element to separate the primordial population from younger generations. When available, oxygen abundances are used to separate the secondary generation into an intermediate and extreme populations. This chemical tagging allows the determination of population ratios in each cluster and to explore the radial distributions of the different populations, which are observational characteristics that GC formation models aim to either reproduce or predict. Moreover, we assess whether the Al scatter in these three clusters is sensitive to metallicity and compare our findings with predicted yields from intermediate-mass asymptotic giant branch stars. Furthermore, one of the predictions of mulitple pouplations GC formation models is that younger generations rotate faster than the primordial population in the early stages of the cluster.To test this idea, we selected the GC M13 since the cluster is know to host multiple populations and exhibit significant internal rotation.

Nathalie Haurberg, 2013

Title: Chemical Abundances and Star-Formation Histories of Low-Mass Dwarf Galaxiess

Abstract: The study of the nature and evolution of the lowest-mass galaxies is important to our overall understanding of the formation and evolution of galaxies over time. Low-mass systems are a key to many lingering issues concerning galaxy formation including the apparent lack of very low-mass dwarf galaxies in the local universe. Additionally, low-mass dwarf galaxies are very metal-poor and thus provide possible analogs to processes of star-formation and evolution in the early universe. Low-mass galaxies are very low-luminosity and thus observing the lowest-masssystems is a difficult task. Until recently, nearly all studies of low-mass galaxies were performed with optically selected samples. These samples have an unavoidable bias toward higher surface brightness systems and it is unclear whether they can accurately characterize the dwarf galaxy population. In this work, we present studies performed on samples of gas-rich low-mass dwarf irregular galaxies chosen from the catalogs of two blind H I - surveys, the Arecibo Dual Beam Survey and Arecibo Legacy Fast ALFA Survey. These catalogs are free of optical biases and thus may provide a more comprehensive view of the gas-rich dwarf galaxy population. We find that optical properties of our samples are broadly consistent with other samples of dwarf irregulars, however our sample tends toward extremely low surface brightness in some cases. A small number of our galaxies have properties which indicate they may be nearly undetectable in typical optical surveys if in a more faceon orientation. The star-formation rates for our sample are roughly consistent with expectations for low surface brightness dwarf irregulars and distinctly lower than that seen in blue compact dwarf galaxies. We perform a spectral analysis of both samples and discover two new extremely-metal deficient (XMD) galaxies; this is particularly important as XMD galaxies have been largely elusive despite signifcant effort to find them. Our spectral analysis provides evidence that H I - selected samples may probe a chemically distinct population and that the luminosity-metallicity relationship may not be universal for low-mass systems. Understanding the deviations from this trend may help reveal the underlying physical mechanisms that cause the relationship to exist.

Jonathan Hargis, 2013

Title: New Results from a Wide-Field Imaging Survey of Globular Cluster Systems: Clues to the Formation and Evolution of Giant Galaxies

Abstract: As some of the oldest and most luminous stellar populations in galaxies, globular clusters (GCs) provide an observable record of the formation and evolutionary history of their host galaxy. Theories for the origin and evolution of galaxies need to be able to explain the properties of both the host galaxy and the ensemble properties of the GC system. Therefore quantifying the global properties of galaxy GC systems over a range of galaxy luminosities, masses, and environments is an important step towards a comprehensive picture of the formation of giant galaxies. In this dissertation, I present new measurements of the ensemble properties of six early-type giant galaxy GC systems: total numbers of GCs, specific frequencies, their spatial distributions, color distributions, and color gradients. I combine ground-based, wide-field optical imaging with analyses of archival Hubble Space Telescope data to study the GC populations from the galaxy's inner regions out to several tens of kpc around the host galaxies. I perform surface photometry to derive surface brightness profiles and color gradients for the host galaxies in order to directly compare the galaxy and GC system properties. For example, my wide-field imaging study of the field S0 galaxy NGC 7457 shows a highly elliptical GC spatial distribution and evidence that unequal mass mergers may have played an important role in the galaxy's formation. In my wide-field CCD study of the GC systems of five moderate-luminosity giant galaxies, I find that the GC system of the elliptical galaxy NGC 5813 shows a significant color gradient that results from the differing spatial distributions of the metal-poor and metal-rich GC subpopulations. I use a subsample of galaxies from our ongoing GC system survey to explore previously under-studied aspects of extragalactic GC populations. I quantify the azimuthal spatial distributions of four galaxy GC systems (including the metal-rich and metal-poor subpopulations) and find elliptical shapes consistent with the host galaxy light. Lastly, I compare the results of the azimuthal distribution analyses to galaxy formation scenarios and discuss my dissertation results in the context of our ongoing GC system survey.

Brian Baptista, 2013

Title: Photodiode Radiation Hardness, Lyman-α Emitting Galaxies and Photon Detection in Liquid Argon Neutrino Detectors

Abstract: My dissertation is comprised of three projects: 1) studies of Lyman-α Emitting galaxies (LAEs), 2) radiation hardness studies of InGaAs photodiodes (PDs), and 3) scintillation photon detection in liquid argon (LAr) neutrino detectors. I began work on the project that has now become WFIRST, developing a science scaes that would use WFIRST after launch for the observation of LAEs. The radiation hardness of PDs was as an effort to support the WFIRST calibration team. When WFIRST was significantly delayed, I joined an R&D effort that applied my skills to work on photon detection in LAr neutrino detectors.

I report results on a broadband selection method developed to detect high equivalent width (EW) LAEs. Using photometry from the CFHT-Legacy Survey Deep 2 and 3 fields, I have spectroscopically confirmed 63 z = 2.5 - 3.5 LAEs using the WIYN/Hydra spectrograph. Using UV continuum-fitting techniques I computed properties such as EWs, internal reddening and star formation rates. 62 of my LAEs show evidence to be normal dust-free LAEs.

Second, I present an investigation into the effects of ionizing proton radiation on commercial off-the-shelf InGaAs PDs. I developed a monochromator-based test apparatus that utilized NIST-calibrated reference PDs. I tested the PDs for changes to their dark current, relative responsivity as a function of wavelength, and absolute responsivity. I irradiated the test PDs using 30, 52, and 98 MeV protons at the IU Cyclotron Facility. I found the InGaAs PDs showed increased dark current as the fluence increased with no evidence of broadband response degradation at the fluences expected at an L2 orbit and a 10-year mission lifetime.

Finally, I detail my efforts on technology development of both optical detector technologies and waveshifting light guide construction for LAr vacuum UV scintillation light. Cryogenic neutrino detectors use photon detection for both accelerator based science and for SNe neutrino detection and proton decay. I have developed waveshifter doped cast acrylic light guides that convert scintillation light and guide the waveshifted light to SiPMs detectors.

Joe Hughto, 2012

Title: How Stars Freeze

Abstract: While plasmas are generally thought of as extremely hot environments, there are places in the Universe where these plasmas can be dense enough so they can freeze. In this work, we investigate the frozen plasmas of white dwarf cores and neutron star crusts. We use Molecular Dynamics (MD) simulations to model small regions of these environments and compute various transport properties. We find that the composition of the cores of white dwarfs can give rise to observable signals that help set limits on astrophysically important nuclear reaction rates. We also find that the neutron star crust should be remarkably defect-free due to the fast diffusion of ions through the crystal lattice.

Randy Hamper, 2012

Title: Observational Exploration of M dwarf Dynamos

Abstract: I have measured the degree of variability and searched for periodicities in the stellar flux of 224 M dwarfs in the Praesepe open cluster, using data over 3 observing seasons. My targets have H alpha emission line measurements at epochs similar to that of our photometry. I am particularly interested in possible changes in the character of chromospheric activity of M dwarfs near M3-M4, where the stars are expected to transition to a fully convective interior. I find that Praesepe M dwarfs earlier than M4 often have significant photometric variations, while no stars in our sample later than M4 show statistically significant variability. Periods of variability were found for 14% of our targets, with periods between 20 and 45 days. This range of periods is longer than for earlier investigations, which we attribute to differing observing cadences among the studies.

Kate Barnes, 2012

Title: Star Formation in the Outer Disks of Spiral Galaxies

Abstract: I present results from a multi-wavelength study of star formation and the gaseous content in the outer disks of a sample of eight nearby spiral galaxies. In particular, the study focuses on galaxies with typical HI-to-optical sizes of ~1-2, to provide a comparison to studies of galaxies with star formation occuring in extended gas disks. The study features new, ultra-deep ground-based Halpha imaging and deep ultraviolet (UV) imaging from the GALEX space telescope to trace the recent star formation. I find that star formation typically extends through most (>85%) of the gas disk, with an outermost star forming regime characterized by low covering fractions and low star formation rate surface densities. The result that star formation extends through most of the gas disk regardless of the HI-to-optical size implies that it is important to further our understanding of the formation of extended gas disks to fully understand the implications of extended star forming disks. I find that the outer gaseous disks are gravitationally stable, which is in agreement with the lower level of star formation. I use ultraviolet and Halpha colors to probe the recent star formation in the outer disks and find significant variations between colors of young star clusters. I run stellar population synthesis models to show how episodic star formation histories (SFHs) with periods of 100-250 Myr could cause similar color variations as seen in outer disks. An episodic SFH would have implications for the gas depletion time and chemical evolution of spiral galaxies. In addition to an episodic SFH, the observed ultraviolet and Halpha colors of young stellar clusters in the outer disks of galaxies in our sample are also in agreement with recently published models of a stochastically sampled initial mass function (IMF). Therefore, there remains some uncertainty for the possible cause of this observational result. Finally, we present a pilot study of deep, near infrared (NIR) imaging of the outer stellar disk and halo of M83 to determine the mass of the tidal stream and discuss formation scenarios for the stellar, gaseous, and extended ultraviolet (XUV) star forming disk.

Jeffrey Cummings, 2011

Title: Correlation of Initial Lithium and Iron in Young Open Clusters

Abstract: Lithium (Li) is a valuable element for testing models of a) chemical evolution in our Galaxy, b) stellar interiors, and c) our early universe when elements first formed. This is because Li is created in a variety of processes in our Galaxy and is also destroyed in the interiors of stars. Studying Li in open star clusters gives us a large sample of depleted Li abundances for different types of stars, but because of the Galactic production of Li, we do not know what Li abundance these star clusters began with. Therefore, we have difficulty evaluating the degree of Li depletion in the cluster stars. In this dissertation I analyze the Li abundances in seven young star clusters and discover that we can measure their initial Li abundance directly. These star clusters also have a range of iron (Fe) abundance, with Fe being the standard tracer of the production of elements, and I find that our initial Li measurements show a positive correlation with Fe abundance. Comparing this Li-Fe correlation to the initial Li abundance of our Sun (from meteoritic data) shows remarkable agreement and suggests that we can apply this correlation to learn the initial Li abundance of older star clusters based on their Fe abundance. This application to older clusters can be used to test models of stellar Li depletion, and the correlation itself can be used directly to test models of Galactic Li production.

Ryan Maderak, 2011

Title: The Evolution of Galactic Oxygen as Traced by Open Cluster Dwarfs

Abstract: Over two decades ago, Oxygen was proposed as a superior tracer for Galactic chemical evolution, and has, furthermore, been suggested to exhibit a correlation with age for open clusters (hereafter OCs). This dissertation presents the results to date of our long-term, spectroscopic study of O in OC dwarfs. Our study seeks to explore the potential of O as a chemical tracer by testing the [O/H]-versus-age correlation previously found for OCs, and by examining other important chemical evolution relationships, such as [O/Fe]-versus-[Fe/H]. Whereas the reliability of the various stellar O abundance indicators has been debated, we use recent developments for analysis of the oxygen triplet at 7774A , in solar-type dwarfs, to eliminate various complications, thus yielding reliable cluster O abundances. Our major conclusions are as follows: (1) We do not find the robust [O/H]-versus-age correlation previously implied. However, the young clusters that have age < 1 Gyr are completely distinguished from the old clusters by their on-average-super-solar [O/H] values. These high O abundances contradict the [O/H]-versus-age relationship predicted by chemical evolution models which incorporate infall of metal-poor gas onto the Galactic Disk. We argue that such an increase in O would result if infall ceased about 1 - 2 Gyr ago. (2) [O/H]-versus-age and [Fe/H]-versus-age both exhibit scatter in the 4 - 7 Gyr age range, which may be chemical evidence of an infall event. The reality of this scatter is supported by a constant, near-solar [O/Fe] for the entire age > 1 Gyr range, implying a physical cause which preserves the abundance ratios. (3) The [O/Fe]-versus-[Fe/H] relationship for OCs exhibits substantial scatter, and is in contradiction to the trend observed in field stars, suggesting that the chemical history of OCs is, in fact, different than that of field stars. (4) The OCs exhibit a clear, roughly linear trend in [Fe/O]-versus-[O/H], but shockingly, this trend contradicts the well-defined, roughly linear trend exhibit by field stars, which has opposite slope. This is strong evidence that the chemical history of OCs is different than that of field stars, and specifically, may be clear evidence for the effects of self-enrichment within cluster-forming regions. (5) The super-metal-rich OC NGC 6253 appears to reflect a different chemical history than that of the other OCs in the Galactic Disk, and its combination of high abundances and old age is more similar to stars of the Galactic bulge. We suggest that its abundance patterns may be characteristic of supernova enrichment in a high-metallicity regime.

TalaWanda Monroe, 2011

Title: The Effects of Stellar Metallicity on the Frequency of Debris Disks in Young Star Clusters

Abstract: Debris disks are the dusty, observational signatures of planet formation. Dust in these later-stage circumstellar disks is generally thought to have been generated during collisions between planetesimals that have been gravitationally stirred by massive planets. A robust correlation has been demonstrated between the occurrence of massive planets and their host star's metallicity, an observational result expected from the core-accretion model of planet formation. Given the association between planets and debris disks and the massive planet-metallicity correlation, I have investigated whether a correlation is present between debris disk frequency and stellar metallicity for young early-type stars. I have determined spectroscopic chemical abundances of six 10-100 Myr open clusters observed with Spitzer Space Telescope, from moderate-resolution WIYN-Hydra and CTIO-Hydra spectra of F and G stars. These results were complemented with Spitzer-MIPS 24 micron photometry to examine the fraction of B and A stars with excess mid-infrared emission, indicative of a debris disk, as a function of metallicity. When the abundances and MIPS photometry for the six clusters are augmented with additional young clusters from the literature, no strong correlation between debris disk frequency and metallicity is found. The result suggests one of the following three scenarios. (1) Massive planets formed by gravitational disk instabilities on wide orbits may be responsible for the perturbations giving rise to the debris disks, since their formation is not as sensitive to metallicity. (2) Self-stirring of planetesimals by recently formed Pluto-sized planetary cores may be responsible for the collisions that produced the dust in lower metallicity debris disks, thus not requiring massive planets to be present. (3) Similar fractions of debris disks at all metallicities may result from a distribution of debris disk masses and sizes, which may obscure metallicity's possible influence on debris disk occurrence. These scenarios and implications for the efficiency of planetesimal production are discussed.

Scott Michael, 2011

Title: Planet Migration Induced by Gravitational Instabilities

Abstract:The study of gravitational instabilities (GIs) and their effects in protoplanetary disks has been an area of active research for over a decade. Although some studies have indicated that GIs cannot form gas giant planets directly, it is clear that they can have a significant effect on a protoplanetary disk. In this dissertation I present several areas where GIs may play a key role in the evolution of a protoplanetary disk. These studies are carried out using three dimensional numerical simulations. I have carried out and analyzed nearly twenty simulations with varying initial conditions, resolutions, and physical effects. Although all indications from these simulations are that GIs cannot form gas giant planets directly at radii smaller than 40 AU, they have shown that GIs can have a dramatic effect on protoplanetary disk structure and planets embedded in a protoplanetary disk. I present several key results including: the effects of a varied initial surface density profile azimuthal resolution, the amplitude of the initial random perturbation, and the adiabatic index used on the onset, strength and general evolution of GIs in protoplanetary disks. Additionally, I present results on studies of the interaction of the instabilities with the central star when it is allowed to move freely in response to the action of the GIs. Finally, I present several results regarding the interaction of embedded massive planets and GI active disks. I find that the presence of massive planets can have a dramatic effect on the evolution of GIs in an active disk, and the GIs can also dramatically effect them migration of the embedded planet. In fact, the action of the GIs may planets to migrate outward, contrary to the standard theory of the planet migration in laminar disks.