This thesis examines the properties of star forming regions in galaxies across the Hubble sequence. It focuses on the stellar populations of giant extragalactic H scII regions and OB associations, addressing in particular the question of Initial Mass Function (IMF) variations with metallicity and/or galaxy morphology. This work is composed of three main sections: (a) 3650-10,000 Å spectroscopy of nearly 100 H scII regions in 20 spiral galaxies (Sa through Sm). Two indicators of the ionizing cluster effective temperature (T ͙) are analyzed: the intensity of the He I λ5876 line, and the 'radiation softness' parameter η = (O⁺O⁺⁺)/(S⁺S⁺⁺). The interpretation of the data relies on CLOUDY photoionization models. A positive T ͙ gradient of 7000-8000 K is found between 2 Z(⊙) and 0.2 Z(⊙). The diagnostic diagrams and the T ͙ - metallicity relation are consistent with an upper mass limit of the IMF of ∼100 M(⊙) and an age of ∼1 Myr, irrespective of chemical abundance or Hubble type. (b) An investigation of extragalactic OB associations, based on Hubble Space Telescope images. The size distribution of the associations (found with an automated search algorithm) is similar in all galaxies examined, with a mean size around 80 pc. An indication is found that the average number of bright blue stars depends on the parent galaxy Hubble type. The upper stellar V luminosity function is comparable among galaxies, with slope d log N/dMᵥ=0.61±0.03. A few star cluster candidates are identified. (c) UBVR and Hα photometry of 266 H II regions in 10 spiral galaxies (Sa through Sd). The Hα equivalent width is independent of Hubble type. The continuum and Hα luminosity functions show similar trends, namely a steeper slope and a smaller characteristic luminosity for early-type galaxies. These results lead to the conclusion that changes in the properties of H II regions and associations along the Hubble sequence are most likely due to variations in the number of stars per star forming region and in the number of regions per unit area, rather than the mass function.

A multi-pronged study aimed at disentangling the kinematical signatures of the earliest stages of star formation is presented. Radiative transfer calculations of millimeter and submillimeter molecular line emission from fully three-dimensional models of protostars are reported. These models are compared with detailed submillimeter molecular line observations of dynamical motions towards seven Class 0 protostellar objects. The radiative transfer calculations are performed for two classes of protostellar collapse solutions: (1) "self-consistent", nonspherical, hydrodynamic, collapsing, rotating protostellar systems (Boss 1993); (2) parameterized, semi-analytic, rotating collapse solutions of Terebey, Shu and Cassen (1984) The morphology of the gas and dust emission is found to be a strong function of collapse time and angular resolution. From model centroid velocity maps, a distinctive new infall signature called the "blue-bulge" infall signature is derived. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. This infall signature can be detected under a wide variety of source conditions, and should be easily observable using single-dish submillimeter telescopes. At high angular resolutions, models with moderate to high rotational rates exhibit the "polar blue-bulge" - a centroid velocity signature of underlying Keplerian rotation in an embedded cloud core. Submillimeter transitions of HCO+ and CS are found to be better than millimeter transitions in detecting infall, especially at early collapse times. Using new submillimeter observations in CS and HCO+ towards IRAS 16293-2422, the first detection of the "blue-bulge" signature towards a protostellar object is presented. The mass accretion rate through the infall region appears consistent with an inside-out collapse model for the source. Using new submillimeter HCO+ and CO observations, a detailed study was performed of six other nearby Class 0 objects. The blue-bulge signature of infall is detected in five sources. Among these, SMM4 and B335 are known infall candidates. VLA 1623, L483 and L1262 are new sources for which evidence for infall is derived in this work. SM1N, which does not exhibit a blue-bulge appears to be a pre-protostellar object. A low luminosity bipolar outflow was detected toward SM1N, suggesting that it may be in an extremely early stage of collapse. Of the six sources, only three, SMM4, B335 and L1262 exhibited the classic blue asymmetric line profile signature of infall, suggesting that the blue-bulge signature is more robust in detecting infall than traditional line profile techniques. Evolutionary trends are seen between observationally obtainable source parameters and model derived timescales for the Class 0 sources presented in this work. Such a study when extended to a larger sample of YSOs will help in the understanding of the evolution of YSOs from the embedded protostellar stage to revealed pre-main-sequence objects.

Many asteroids show absorption features diagnostic of hydrated minerals in the 3-μm spectral region. Reflectance studies in this region can determine the hydration state of surface minerals, and by inference, the thermal histories of bodies. Observations of M-class asteroids from 1.25-3.5 μm show that many of these asteroids have water of hydration, and those that do cannot be interpreted as the cores of differentiated parent bodies. Because of this, it is suggested that the hydrated M asteroids should be split off into their own class--the W class. Simple spectral mixing models of these asteroids show they are consistent with enstatite chondritic material mixed with talc, suggesting the W asteroids may be the result of aqueous alteration of enstatite chondrites, though other models may also hold merit. The E asteroids are also found to have hydrated members, inconsistent with their interpretation as purely igneous bodies. A trend for large E and M asteroids to be hydrated is found. A compilation of S-class asteroid data at 3 μm has been performed, supporting the finding that some S asteroids have spectra consistent with a mixture of ordinary chondrite and metal. There is some evidence for a trend altering the spectra of near-Earth asteroids to look like main-belt asteroids, but no simple trend can also include the ordinary chondrite meteorites. Variation in asteroids at 3 mu m was studied, and while no clear evidence of rotational variation is found, there is circumstantial evidence for latitudinal variation on several asteroids, perhaps as interior layers of an aqueously altered body are excavated. Finally, high-resolution studies of C-class asteroids were performed. A finding that 1 Ceres' spectrum matches that of an ammoniated phyllosilicate is supported over an extended wavelength range. Observations of other CBG-class asteroids find no ammoniated minerals. The CBG-class asteroids, other than Ceres, all share very similar spectra, suggesting similar hydrated minerals on their surfaces.

I present the data for an optical/near-infrared study of radio-loud quasar environments from z = 0.6-2.0, and the analysis of the data from z = 1.0-2.0. I thoroughly discuss the sample selection, observing, data reduction, and object cataloging. Even accounting for possible systematic offsets, I find a significant excess of K ≳ 19 galaxies in the fields of z = 1-2 RLQs, on two spatial scales. One component is at θ <40'' from the quasars and is significant compared to the galaxy surface density at θ >40'' in the same fields. The other component appears roughly uniform across the fields (to θ∼100'') and is significant compared to the galaxy surface density seen in random-field surveys in the literature. The r-K color distributions of the excess galaxy populations are indistinguishable, and are significantly redder than the color distribution of the field population. The excess galaxy population is thus consistent with being predominantly early-type galaxies at the quasar redshifts. The average excess within 0.5h⁻¹₇₅ Mpc $(∼65'') of the quasars corresponds to Abell richness class ∼0 compared to the galaxy surface density at >0.5h⁻¹₇₅ Mpc from the quasars, and to Abell richness class ∼1 compared to that from the literature. I estimate -0.65⁺⁰·⁴¹₋₀.₅₅ magnitudes of evolution in M*(K) to z̄ = 1.67 by assuming the excess galaxies are at the quasar redshifts. I discuss the spectral energy distributions (SEDs) of galaxies in fields with data in several passbands. Most candidate quasar-associated galaxies are consistent with being 2-3 Gyr old early-types at the quasar redshifts of z∼1.5. However, some objects have SEDs similar to extremely late-type stars; others have SEDs consistent with being 4-5 Gyr old at z∼1.5 and others are consistent with old but dust-reddened galaxies at the quasar redshifts. These potentially different galaxy types suggest there may be considerable dispersion in the properties of early-type cluster galaxies at z∼1.5. There is also a population of galaxies whose SEDs are best modelled by background galaxies at z≳2.5, Many of these are dusty or have composite stellar populations, or both, and some may be ≳2 Gyr old at z≳ 2.5. Confirmation of old galaxies at high redshift would constrain the cosmology by requiring a relatively old universe at large lookback times.

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.)