From the Earth to asteroids, numerous rocky bodies in our solar system are believed to have a metallic core at their center. However, due to the inaccessibility of these cores, fundamental issues, such as the composition of the cores or the processes of core formation and core evolution, are not well known. I have conducted both theoretical and experimental geochemical studies which have improved our understanding of the cores of terrestrial planetary bodies. The radioactive decay of K is an important planetary heat source, but the distribution of K in terrestrial planetary bodies has been debated. My experimental work, which examined the solubility of K in metal, shows no evidence for K to be an important heat source in metallic cores. The element pairs of Ag, Pd and Re, Os have been used to date core formation and core evolution events in our solar system. My experimental determination of the partitioning behavior of these important elements can be used to better understand their distribution in iron meteorites, our only samples of planetary cores. Simple fractional crystallization of a metallic core cannot explain the elemental trends observed within iron meteorite groups. I have developed a crystallization model which suggests slight inhomogeneities and mixing in the molten core were important during core evolution. As a metallic core crystallizes, liquid immiscibility may be encountered, which could significantly affect the subsequent evolution of the core. My experimental work suggests the role of liquid immiscibility during the crystallization of a metallic core is significantly smaller than the published phase diagram implies. These four topics, though each an independent project, together provide insight into the nature of the cores of terrestrial planetary bodies and the processes which affect those cores.

Observational evidence and theoretical arguments suggest that Jupiter's satellite Europa could be geologically active and possess an "ocean" of liquid water beneath its surface at the present time. We have searched for evidence of current geologic activity on Europa in the form of active plumes venting material above the surface and by comparison of Voyager and Galileo images to look for any changes on the surface. So far, we have observed no plumes and have detected no definitive changes. The lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km² y⁻¹ in the regions analyzed, assuming alterations will cover contiguous areas of at least 4 km² over a period of 20 years. Assuming this as a constant, globally uniform resurfacing rate leads to a minimum average surface age of 30 million years. Lava flows and plumes are the two main types of volcanic activity that resurface Io. We have used the Galileo Io dataset to observe the detailed sequences of interconnected plume activity, hotspot activity, and new surface deposits at a number of volcanic centers on Io. Red material has faded on a timescale of less than a year, and a green coating has formed on a caldera over a time period of about 3 months. Change detection maps can illustrate the percentage of the surface newly covered by plume deposits and lava flows, and constrain volume and mass resurfacing rates. Areal resurfacing is dominated by plume deposits, but volume resurfacing is dominated by lava flows. Estimates of resurfacing from these change maps range from 0.4 to 12.9 cm/year, assuming a flow thickness of 1 to 10 meters. The minimum resurfacing rate required for the lack of impact craters on Io's surface is about 0.02 cm/year. If high-magnesium (komatiitic) lavas dominate the observed Io heat flux, the maximum resurfacing rate is about 0.69 cm/year. Basaltic lavas could produce a rate of 1.3 cm/year. The komatiitic rate produces an average flow thickness of about half a meter. Thus, we suggest that the average resurfacing rate of lo is between 0.1 and 1 cm/year.

High-energy particle interactions have been of interest to scientists ever since the discovery of cosmic rays early this century. With the realization, almost half a century later, that the prodigious radio emission seen from outer-space is due in part to these particles gyrating in cosmic magnetic fields, a new field was born which joined physics and astronomy. By studying the interaction signatures of these particles, we can gain a better understanding of the microphysics of their motion and collisions, as well as the macrophysics governing the vast and distant astrophysical objects which host them. The Galactic center is very important for this burgeoning field. Because of its proximity, years of detailed observations in particularly the radio and infrared wavebands have provided us with a good picture of what the central environment is like. Therefore, when the gamma-ray telescope EGRET detected an excess of gamma-rays over the expected background coming from within 0.2° of the center, several candidates for the emission were suggested on the basis of their characteristics known from the low-frequency observations. Two promising sources, the massive black hole candidate Sgr A* and the extended shell structure Sgr A East, are considered here. We first investigate in detail the hadronic processes contributing to the gamma-ray emission, and then compare predicted spectra to the EGRET data. We conclude that Sgr A* cannot be the source but that Sgr A East is very promising, and suggest further observational tests. Understanding the high energy processes in our Galactic center is crucial for our modeling of the same processes throughout our Galaxy as well as in distant galaxies. Because we have so much more information about the Galactic center physical environment, we have the opportunity to test our theories in a familiar surrounding before attempting to apply our ideas to places we can never hope to resolve so well. The Galactic center may hold the key to our understanding of the high energy interactions in blazars, supernova remnants and by cosmic rays.

Magellan observed quasi-circular, apparently impact-related, radar albedo features on Venus. Pristine examples show dark regions centered within larger bright regions. Dark regions are interpreted as smooth and bright regions are interpreted as rough. Of the 518 features, 256 are centered on impact craters (crater haloes), 53 are centered upon small central disturbances (disrupted splotches), and the remaining 209 exhibit no central structure (craterless splotches). Most researchers interpret these features as airburst scars. Previous models of airburst formation only reproduced subsets of the observations. Models of splotch and halo formation were often mutually exclusive and no previous model connected them despite their similarities. I rectify this problem with a model that successfully reproduces 514 of the 518 patterns given appropriate airburst altitude and energy conditions combined with erosion. In my model, dark zones are pulverized rock, and bright zones are scoured surfaces. More complicated patterns are obtained with modification of the initial dark/bright pattern. Small impactors that do not penetrate the atmosphere cannot cause surface damage. Large impactors that penetrate the atmosphere without disruption create an impact crater without an associated airburst scar. Only intermediate-size impactors, partially or wholly disrupted in the near-surface zone, form airburst scars. Typical diameters for airburst scar-forming impactors are 200 meters--2 km for irons, 300 meters--3 km for stones, and >3 km for comets.

I report photometry and spectroscopy for 16 Type II supernovae (SNe) observed during the Calan/Tololo, SOIRS, and CTIO SN programs, a valuable resource for astrophysical studies. I perform a detailed assessment of the performance of the "expanding photosphere method" (EPM) in the determination of extragalactic distances. EPM proves very sensitive to the many steps involved in the analysis which can make it an art instead of an objective measurement tool. To minimize biases I implement objective procedures to compute synthetic magnitudes, measure true photospheric velocities, interpolate velocities, estimate dust extinction and realistic errors. While EPM performs well during the initial phases of SN evolution, I find distance residuals as large as 50% as the photosphere approaches the H recombination temperature. Despite the effort to lend credence to EPM, it proves necessary to exercise great care to avoid biasing the results. The main sources of uncertainties are observational errors (8%), dilution factors (11%), velocity interpolations (12%), and dust extinction (14%). The EPM Hubble diagram suggests the true error in an individual EPM distance is 20%. I find values of 63 ± 8 and 67 ± 7 km s⁻¹ Mpc⁻¹ for the Hubble constant, depending on the redshift sample chosen for the analysis. This result is independent of the extragalactic distance scale which yields 65 ± 5 from Cepheid/SNe la distances. From four objects the comparison of EPM and Tully-Fisher yields D(EPM)/D(TF) = 0.82 ± 0.12. I derive bolometric corrections for plateau SNe (SNe II-P) that permit me to obtain reliable bolometric luminosities from BVI photometry. Despite the great diversity displayed by SNe II-P, the duration of the plateau is approximately the same and the luminosities and expansion velocities measured in the middle of the plateau prove highly correlated. From the luminosity of the exponential tail I obtain ⁵⁶Co masses ranging between 0.02 and 0.28 M(⊙), and some evidence that SNe with brighter plateaus produce more Ni (and its daughter Co). The correlation between expansion velocity and luminosity permits me the use of SNe II-P as standard candles with a magnitude dispersion between 0.39-0.20 mag. Using SN 1987A to calibrate the Hubble diagram I get H₀ = 55 ± 12 and H₀ = 56 ± 9 from the V and I filters, respectively.

The evolution of tidal debris within the Galactic halo has been simulated to determine its detectability within the constraints imposed by currently available telescopes and instrumentation. Observations of the simulations show that there is a very high probability of detecting and quantifying the presence of tidal debris with a pencil beam survey of 100 square degrees. The debris is readily detectable via the presence of kinematic substructure in the radial velocities. The detection probabilities show surprisingly little change with the age of the debris. Accretion events that occurred up to ≳ 10 Gyr ago can be detected. In the limiting case of a single 10⁷ M(⊙) satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg² field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. Accurate CMDs in the Washington photometric system have been derived for four fields spanning the range of Galactocentric distances from 1.5 to 5.5 kpc. The differential reddening variations within each field were corrected by a new technique optimized for the highly variable reddening variations found in bulge fields. Abundance distributions in the four fields were derived from color-color diagrams in the Washington system. The quality of the photometry which yields photometric abundances with σ[Fe/H] ≲ 0.25 dex (including reddening errors) supplemented by the luminosity information from observations in the 51 filter allows contamination by foreground and background stars to be eliminated from the bulge sample. A clear abundance gradient is seen which is consistent with the change in morphology of the CMDs. The abundance gradient is predominantly due to a decrease in the fraction of stars in the metal-rich shoulder of the abundance distributions. The modal abundance changes little. Relative to Baade's window the magnitude distribution of clump stars in the L354 B-06 field implies a bar angle of ≃ 40 deg.

We study the physics of core-collapse supernovae and the neutron stars they create. We study the microphysics of neutrino interactions and demonstrate the importance of two processes previously ignored in full supernova simulations: inelastic neutrino-nucleon scattering and nucleon-nucleon bremsstrahlung. We show that these processes dominate neutrino-electron scattering and electron-positron annihilation as thermalization and production mechanisms, respectively, for mu- and tau-neutrinos in regimes vital to emergent spectrum formation. In addition, we solve the general-relativistic steady-state eigenvalue problem of neutrino-driven protoneutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and composition profiles and explore the systematics and structures generic to such a wind for a variety of protoneutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and compositions essential in assessing this site as a candidate for r-process nucleosynthesis. Finally, we construct dynamical models of core-collapse supernovae. We employ a full solution to the transport equation for each neutrino species, a realistic high-density nuclear equation of state, and explicit hydrodynamics. We present results from a set of different supernova progenitors. We vary the microphysics and nuclear equation of state and compare our results to those of other groups. We examine the electron-neutrino breakout phenomenon and address the importance of nucleon-nucleon bremsstrahlung and inelastic neutrino-electron scattering in mu and tau neutrino spectrum formation. We convolve the emergent spectra obtained in these models with terrestrial neutrino detectors and find that the electron-neutrino breakout burst can likely be observed and identified uniquely.

Active Galactic Nuclei (AGNs) are believed to be powered by accretion onto supermassive black holes (BHs). With the development in high resolution observations over a broad frequency range, it is now tenable to study the corresponding physical processes in detail. We find that the emission from the closest supermassive BH candidate, Sagittarius A*, a compact radio source presumably accreting from stellar winds prevailing at the Galactic Center, can be explained as due to a quasi-spherical accretion flow, which circularizes to form a small magnetized accretion disk near the BH's event horizon. The mm/sub-mm and X-ray emissions are produced via thermal synchrotron processes and their self-Comptonization, respectively, in the inner ten Schwarzschild radii of the resultant Keplerian structure. The cm radio emission, however, appears to be produced by non-thermal synchrotron processes in the circularization zone. The recently detected X-ray flare seems to indicate a transient enhancement of mass accretion rate through the inner accretion disk. The 106-day cycle seen at 2.0 cm and 1.3 cm, on the other hand, suggests that the disk is precessing around a spinning BH, whose spin may be determined by timing observation of Sgr A* at mm/sub-mm wavelengths. Our tentative observational result is consistent with this magnetized disk model. The supermassive BH M31*, a compact radio source in the nucleus of M31, has many features in common with Sgr A*, yet their differences are significant. We show that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the capture radius. Sgr A* is presumably a 'hot' BH. While M31* seems to be a member of the 'cold' BH family. The study of the nuclei in radio galaxies reveals many new characteristics of the large scale accretion flows. In NGC 4261, we show that a turbulence-dominated disk, illuminated by its AGN, can not only account for the observed sub-parsec scale radio gap in the core, but also produce the optical broad lines emitted from the region. However, the prominent radio jets distinguish such BHs from those in the compact radio sources. The relativistic jets are probably driven by the action of supermassive, fast spinning BHs. Our study on NGC 6251* indicates that the initial ejection of matter can be associated with the thermal expansion of the accreted gas, which is heated by a spinning BH near its even horizon.

I study the weak nonlinear regime of structure formation using high resolution and high signal-to-noise ratio (S/N) samples of Quasi Stellar Objects' (QSOs) Lyα transmission spectra. Using a space-scale decomposition, the Discrete Wavelet Transform (DWT), I show that the field traced by Lyα transmission flux is intermittent on scales less than 2000 km/s. The distribution of the local power of fluctuations is spiky with almost no power between the spikes. This spike-gap-spike feature gets more pronounced on smaller scales (128-16 km/s). This feature contradicts the predictions of the correlation hierarchy model on small scales ( < 64 km/s). Intermittency renders lower order statistics, like the power spectrum of fluctuations, ineffective in describing an intermittent field and discriminating between various structure formation models. I show that the structure functions and the intermittent exponent are not only able to quantitatively differentiate between different dark matter models but also qualitatively describe the nature of non-Gaussianity. The structure functions and the intermittent exponent are powerful tools for describing an intermittent field. Intermittency opens a new window in the study of the nonlinear evolution of structure in the universe.

This work explores how certain properties of galactic nuclei can be understood on the basis of the available data. Current evidence for the presence of large central masses in these regions, believed to be supermassive black holes, is first reviewed. Methods for estimating the mass are discussed, and a new algorithm is presented for implementing reverberation techniques with time-variable broad line data from active nuclei. The effectiveness of this new algorithm is first tested on sample data sets; it is then applied to actual data. Next, a model is presented for the formation of the cool, dense clouds responsible for the broad line emission, involving the rapid cooling of shocked gas embedded in a quasi-spherical, turbulent accretion flow. As an illustrative example, fitting of the model (with simplify assumptions) is performed on data pertaining to the Seyfert nucleus NGC 5548. Accretion flows in two specific objects are then discussed. First, a cool, spherical accretion flow is argued for the non-active nucleus of M31 on the basis of the observed broad-band spectrum. In addition to comparisons of the model with the currently available data, we provide detailed predictions of the UV and optical line spectra, correcting for extinction due to intervening dust and cold gas. Then, a turbulent disk structure is argued for the weakly-active nucleus in the radio galaxy NGC 4261. This structure is capable of producing both the observed broad line spectrum and radio absorption, and may have application to the nuclei of other radio galaxies. Finally, the iron Kalpha emission from Sgr B2, a giant molecular cloud located in the Galactic Center region, is reviewed. While many argue that this suggests recent activity associated with the radio source Sgr A*, our modeling indicates that the data are also consistent with a time-variable illuminator embedded within the cloud. The observations and modeling suggest that turbulence may be a key component to accretion in active nuclei, facilitating the transfer of angular momentum and allowing the greater accretion rates needed to fuel the central engines. Future 3D-hydrodynamical simulations are required to test this assertion.