To examine the stellar populations of interacting or merging galaxies, a sample of 28 objects with disturbed morphology was selected. Integrated spectra of these galaxies were obtained, to study their global star formation histories and provide a database for comparison with morphologically disturbed galaxies at high redshift. Quantitative star formation histories were determined using evolutionary population synthesis models. Special emphasis was placed on observational and systematic uncertainties, e.g., IMF, metallicity, and reddening. The merger sample was divided into two subsamples for comparison with morphologically normal galaxies. The red subsample consists of galaxies whose spectra resemble those of early-type galaxies, while the blue subsample has moderate to strong Halpha emission. The model fits to the spectra of the red merger sample are indistinguishable from those in a control sample of S0 galaxies. Differences in the upper limits on recent star formation between these mergers and a sample of elliptical galaxies may be due to metallicity effects. The minimum amount of star formation required in the last Gyr is consistent with zero for the red merger and the E/S0 samples. The maximum amount of new star formation ranges from 0.2-3.2% by mass in the merger sample and 0.0-2.7% in the E/S0 control sample. Reddening contributes the largest source of uncertainty in determining the mass of a starburst in the blue merger subsample, while burst ages are relatively unaffected. We put limits on the quantitative star formations histories in these galaxies, although the uncertainties tend to be large. We find starbursts ranging in age from 10⁷ to 10⁹ years, and burst masses from 0 to more than 20% of the total stellar mass. We find higher recent rates of star formation in the merger sample based on far-infrared luminosities and Balmer absorption strengths, respectively. We cannot distinguish between truncated star formation followed by a starburst, and alternate star formation histories, such as those appropriate to spiral-type star formation, based on our model fits alone. A Salpeter IMF appears to be an adequate one to describe star formation in these galaxies.

Water is important in the solar nebula both because it is extremely abundant and because it condenses out at 5 AU, allowing all three phases of H₂O to play a role in the composition and evolution of the solar system. In this work, a thorough examination of the inward radial drift of ice particles from 5 AU is undertaken. Drift model results are then linked to the outward diffusion of vapor, in one overall model which is numerically evolved over the lifetime of the nebula. Results of the model indicate that while the inner nebula is generally depleted in water vapor, there is a zone in which the vapor is enhanced by ∼40-100%, depending on the choice of ice grain growth mechanisms and rates. This enhancement peaks in the region from 0.1-2 AU and gradually drops off out to 5 AU. Conversely, ice abundance is enhanced over 3-5 AU. Representative hot (early) and cool (later) conditions during the quiescent phase of nebular evolution are examined. Additionally, the effect of the radial dependence of water depletion on nebular chemistry is quantified using a chemical equilibrium code that computes abundances of nebular elements and major molecular C, N, S, etc. species over a range of temperatures. In particular, changes in the local C/O ratio and organics abundance due to the radially dependent decrease in oxygen fugacity are tracked and plotted. Generally, the diffusion-drift model results in a more complex water distribution than previous models, with both radial and temporal variations in the C/O ratio which produce both relatively oxidizing and reducing nebular conditions across 1-5 AU. Depending on the value assumed for the solar C/O ratio, modest to significant enhancements of CH₄ and other organics abundances are produced in the inner nebula. These results coupled with the revised ice distribution may explain the radial signatures of hydration detections and darkening in asteroids, and perhaps the oxidation states of enstatite chondrites. The results also indicate that the inner nebula could have supplied organics and water to the terrestrial planets, as well as possibly to Europa and beyond, via outward mixing processes.

I have used optical and near-infrared spectroscopy and imaging to measure spectral types and luminosities for young (τ < 10 Myr), embedded (Aᵥ = 0-50), low-mass (0.1-1 M(⊙)) stars in three nearby (d < 300 pc) clusters: L1495E, IC 348, and ρ Ophiuchi. In conjunction with theoretical evolutionary tracks, I have derived the star formation history and initial mass function for each stellar population. A large number of brown dwarf candidates have been identified in the photometry, several of which are confirmed through spectroscopy. Finally, I have measured the frequency and survival times of circumstellar disks and investigated the photometric and spectroscopic properties of protostars. In § 2, I apply observational tests to the available sets of evolutionary models for low-mass stars, concluding that the calculations of D'Antona & Mazzitelli are preferred for the range of masses and ages considered here. In § 3 and § 4, I examine in detail the spectroscopic characteristics and substellar nature of two brown dwarf candidates. The study then expands to include the populations within the clusters L1495E (§ 5), IC 348 (§ 6), and ρ Ophiuchi (§ 7). In § 8, I briefly discuss the past, present, and future of scientific research related to this thesis.

The images of the surface of Venus returned by the Magellan spacecraft show a cratering record unlike any other in the solar system. Multiple models of the geologic history of Venus have been proposed to explain this cratering record, including the "equilibrium resurfacing" model and the "global resurfacing" model. I use a two-dimensional Monte Carlo simulation of crater emplacement and volcanic resurfacing to determine what sorts of cratering records would in fact be produced by these models. The equilibrium resurfacing model fails to produce a cratering record resembling the observations. The global resurfacing model requires the specification of post-global resurfacing event history before it can be simulated by this program, but following appropriate specification, it did reproduce the observed cratering record. The global resurfacing model is thereby found to be a more satisfactory model than the equilibrium model. The length of the tail end of the global resurfacing event is found to be of the order of 100 million years, subject to uncertainty in the impactor flux at Venus. The fraction of the planet resurfaced after the end of the global resurfacing event is found to be roughly 15-20%.

This thesis derives and explores two solutions to the field equations of general relativity. These solutions are special cases of solutions already present in the literature. However, the present derivation is based on physical arguments which provides a basis for discussing the solutions as cosmological objects, rather than simply as a class of solutions with certain symmetries. This derivation naturally gives rise to the idea that the space-times indicated are generated by the passage of walls composed of gases of transcendant tachyons. After deriving the solutions, we explore the geodesics that these walls generate. In general, they tend to focus the paths of particles moving through the walls. It is also found that the walls generate blue-shifts in observed photons emitted from sources that pass through the walls. As applications of these solutions, numerical simulations demonstrate that a toy galaxy passing through the second type of tachyon wall develops spiral structures which persist for several rotations of the galaxy, dissolve, and then reform throughout the lifetime of the galaxy. A number of open problems associated with these walls are discussed.

In my thesis I present a study of the dynamics and observational characteristics of massive circumstellar disks in two dimensions (r, φ) using two complimentary hydro-dynamic codes: a 'Smoothed Particle Hydrodynamic' (SPH) code and a 'Piecewise Parabolic Method' (PPM) code. I also study the detection limits available to radial velocity searches for low mass companions to main sequence stars. This thesis is organized as a series of published or submitted papers, connected by introductory and concluding material. I strongly recommend that readers of this abstract obtain the published versions of each of these papers. I first outline the progress which has been made in the modeling of the structure and origins of the solar system, then in chapter 2 (The Astrophysical Journal v502, p342, with W. Benz, F. Adams and D. Arnett), I proceed with numerical simulations of circumstellar disks using both hydrodynamic codes assuming a 'locally isothermal' equation of state. The disks studied range in mass from 0.05M* to 1.0 M* and in initial minimum Toomre Q value from 1.1 to 3.0. Massive disks (M(D) > 0.2 M*) tend to form grand design spiral structure with 1-3 arms, while low mass disks (M(D) ≤ 0.2M*) tend to form filamentary, > 4 armed spiral structures. In chapter 4 (submitted to The Astrophysical Journal with W. Benz and T. Ruzmaikina), I relax the assumption the locally isothermal evolution assumption and instead include simple heating and cooling prescriptions for the system. Under these physical conditions, the spiral arm growth is suppressed in the inner 1/3 of the disks relative to the isothermal evolution and in the remainder, changes character to more diffuse spiral structures. I synthesize spectral energy distributions (SEDs) from the simulations and compare them to fiducial SEDs derived from observed systems. The size distribution of grains in the inner disk can have marked consequences on the near infrared portion of the SED. After being vaporized in a hot midplane region, the grains do not reform quickly into the size distribution on which most opacity calculations are based. In chapter 6 (The Astrophysical Journal v500, p940 with Roger Angel), I examine the limits which may be placed upon the detection of planets, brown dwarfs and low mass stellar companions using radial velocity measurements. I derive an analytic expression describing the amplitude limits for periodic signals which may be obtained from a set of data of known duration, number of measurements and precision. In chapter 7, I outline several problems which may be profitably addressed by building on this work. (Abstract shortened by UMI.)

Supernova (SN) 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. Initial results were reported in J. Kane et al., Astrophys. J. 478, L75 (1997). The Nova laser is used to shock two-layer targets, producing Richtmyer-Meshkov (RM) and Rayleigh-Taylor (RT) instabilities at the interfaces between the layers, analogous to instabilities seen at the interfaces of SN 1987A. Because the hydrodynamics in the laser experiments at intermediate times (3 ns-40 ns) and in SN 1987A at intermediate times (5 s-10⁴ s) are well described by the Euler equations, the hydrodynamics scale between the two regimes. The experiments are modeled using the hydrodynamics codes HYADES and CALE, and the supernova code PROMETHEUS, thus serving as a benchmark for PROMETHEUS. Results of the experiments and simulations are presented. Analysis of the spike and bubble velocities in the experiment using potential flow theory and a modified Ott thin shell theory is presented. A numerical study of 2D vs. 3D differences in instability growth at the O-He and He-H interfaces of SN 1987A, and the design for analogous laser experiments are presented. We discuss further work to incorporate more features of the SN in the experiments, including spherical geometry, multiple layers and density gradients. Past and ongoing work in laboratory and laser astrophysics is reviewed, including experimental work on supernova remnants (SNRs). A numerical study of RM instability in SNRs is presented.

The recent discoveries of extrasolar giant planets (planets like Jupiter orbiting other stars like our Sun) at small distances from their central stars have revitalized the fields of planet and planetary system formation. The discoveries have overturned the former paradigm for planetary system formation which suggested that all planetary systems would look like our Solar System: these decidedly do not. The new view is that the early solar system was not nearly the quiescent place previously thought, but rather a dynamic environment in which planets are both easily created and easily destroyed. I have participated in the building of a new paradigm of planetary system formation, and this thesis describes theoretical and observational work which have contributed to this field. My theoretical work on the migration of giant planets from their formation location to distances close to their central star is described. I show that giant planets can reside at a range of heliocentric distances and masses, and we reproduce the distribution of observed giant planets, as well Jupiter. Using this model, I have predicted what the rate of planet formation must be, and what the initial, mass function for forming planets must be, in order to reproduce the observed planets. I also place some constraints on the mass and viscosity of the circumstellar disk out of which planets form. I show that giant planets close to their central stars are tidally stable, and stable against atmospheric loss, contrary to intuition. I predict that tidally stripped rocky cores, the remnants of giant planets, should exist at small heliocentric distances, a byproduct of the migration and mass loss that a majority of giant planets go through. Lastly, as an outgrowth of our migration work, I designed an observing program to search for circumstellar disks around stars with known extrasolar planets. I have detected three such disks, analogs to our Solar System's Kuiper Belt, and failed to detect disks around three other stars with extrasolar planets. I discuss my observing results, and the implications of detecting disks around some, but not all, of the stars with extrasolar planets I have looked at. In the conclusions and future work, I describe how this work forms a coherent part of a larger goal of understanding how, where, and how often planets and planetary systems form, answering the question of the origin, nature, and uniqueness of our Solar System.

One method of discriminating between the many Type Ia progenitor scenarios is searching for contaminating hydrogen stripped from the companion star. However, this requires understanding the effect of the impact on different companion stars to predict the amount of hydrogen stripped and its distribution in velocity and solid angle for the types of binary scenarios have been proposed as progenitor models. We present several 2-D numerical simulations of the impact of a Type Ia supernova explosion with low-mass main sequence, subgiant, and red giant companions. The binary parameters were chosen to represent several classes of single-degenerate, hydrogen-rich Type Ia progenitor models that have been suggested in the literature. We find that the main sequence and subgiant companions lose ∼15% of their mass as a result of the impact of the supernova shell. The red giant companions lose 96%-98% of their envelopes. The main sequence companion receives a kick of 86 km s⁻¹, the subgiant 49 km s⁻¹. In all cases, the kick received by the remnant is smaller than the original orbital velocity. Because it is too small to intercept more than a negligible amount of momentum, the red giant core will not receive a kick. The characteristic velocity of the stripped hydrogen is less than 10³ km s⁻¹ for all the scenarios: 420-590 km s⁻¹ for the red giant companions (depending on the scenario), 850 km s⁻¹ for the main sequence companion, and 900 km s⁻¹ for the subgiant companion. The stripped hydrogen contaminates a wide solid angle behind the companion: 115° from the downstream axis for the red giant, 66° for the main sequence star, and 72° for the subgiant. We find that the bulk of the stripped hydrogen is embedded within the low-velocity iron of the supernova ejecta and may be visible as narrow emission lines months after maximum light. However, to make any definitive predictions requires non-LTE radiative transfer calculations using the low-velocity distribution of the stripped hydrogen to determine the effect of hydrogen contamination on the late-time supernova spectrum.

The discrete wavelet transform (DWT) is developed as a general statistical tool for the study of large scale structures (LSS) in astrophysics. The DWT is used in all aspects of structure identification including cluster analysis, spectrum and two-point correlation studies, scale-scale correlation analysis and to measure deviations from Gaussian behavior. The techniques developed are demonstrated on "academic" signals, on simulated models of the Lymanα (Lyα) forests, and on observational data of the Lyα forests. This technique can detect clustering in the Ly-α clouds where traditional techniques such as the two-point correlation function have failed. The position and strength of these clusters in both real and simulated data is determined and it is shown that clusters exist on scales as large as at least 20 h⁻¹ Mpc at significance levels of 2-4 σ. Furthermore, it is found that the strength distribution of the clusters can be used to distinguish between real data and simulated samples even where other traditional methods have failed to detect differences. Second, a method for measuring the power spectrum of a density field using the DWT is developed. All common features determined by the usual Fourier power spectrum can be calculated by the DWT. These features, such as the index of a power law or typical scales, can be detected even when the samples are geometrically complex, the samples are incomplete, or the mean density on larger scales is not known (the infrared uncertainty). Using this method the spectra of Ly-α forests in both simulated and real samples is calculated. Third, a method for measuring hierarchical clustering is introduced. Because hierarchical evolution is characterized by a set of rules of how larger dark matter halos are formed by the merging of smaller halos, scale-scale correlations of the density field should be one of the most sensitive quantities in determining the merging history. We show that these correlations can be completely determined by the correlations between discrete wavelet coefficients on adjacent scales and at nearly the same spatial position, Cs Scale-scale correlations on two samples of the QSO Ly-α forests absorption spectra are computed. Lastly, higher order statistics are developed to detect deviations from Gaussian behavior. These higher order statistics are necessary to fully characterize the Ly-α forests because the usual 2nd order statistics, such as the two-point correlation function or power spectrum, give inconclusive results. It is shown how this technique takes advantage of the locality of the DWT to circumvent the central limit theorem. A non-Gaussian spectrum is defined and this spectrum reveals not only the magnitude, but the scales of non-Gaussianity. When applied to simulated and observational samples of the Ly-α clouds, it is found that different popular models of structure formation have different spectra while two, independent observational data sets, have the same spectra. Moreover, the non-Gaussian spectra of real data sets are significantly different from the spectra of various possible random samples. (Abstract shortened by UMI.)