High energy emission from supernovae provide a direct window into the quantity and distribution of radioactive elements produced in these explosions. Combining supernova explosion calculations with 3D Monte Carlo gamma-ray transport, I have studied the effect mixing and asymmetries have on the hard X-ray and gamma-ray spectra. Two types of asymmetries (bipolar and unipolar) are investigated, the parameters of which are motivated by the most recent findings from multi-dimensional core-collapse supernova simulations. These bipolar and unipolar asymmetries are imposed artificially on 1-dimensional stellar progenitor structures and their evolution is followed using a 3-dimensional smoothed particle hydrodynamics (SPH) code. Global asymmetries in the explosion enhance the outward mixing of heavy elements such as 56Ni, reducing the observable emergence time for the hard X-ray continuum and gamma-ray line emission over that of symmetrically mixed models. The details of the velocity asymmetry lead to very different nickel distributions in the outer envelope. The high energy spectra resulting from these models predict an angular variation for the correspondence between the emergence time of the hard X-ray continuum and the broadening of the gamma-line profiles. The unipolar explosion models, in particular, demonstrate that redshifted gamma-ray line profiles are attainable at epochs where gamma-ray emission arises predominantly from the outer extent of the nickel distribution. The departure from a symmetric explosion scenario manifests itself most clearly in the extended nickel, making gamma-ray line observations an ideal probe of the initial explosion asymmetry.

This work examines the relative chronology of geologic units within the Cerberus Plains of Mars with an emphasis on lava flows emplaced after the last Marte Valles fluvial episode. High resolution images show the bulk of the Cerberus Plains is covered by platy-ridged and inflated lavas, which are interpreted as insulated sheet flows. Eastern Cerberus Plains lavas originate at Cerberus Fossae fissures and shields. Some flows extend for >2000 km through Marte Valles into Amazonis Planitia. Athabasca Valles are both incised into pristine lavas and embayed by pristine lavas, indicating that Athabascan fluvial events were contemporaneous with volcanic eruptions. Deposits of the Medusae Fossae Formation lie both over and under lavas, suggesting the deposition of the Medusae Fossae Formation was contemporaneous with volcanism. Statistics of small craters indicate lavas in the Western Cerberus Plains may be less than a million years old, but the model isochrons may be unreliable if the small crater population is dominated by secondary craters. Images showing no large craters with diameters >500 m superimposed on Western Cerberus Plains lavas indicate the same surface is younger than 49 Ma. High resolution Mars Orbiter Camera (MOC) images have revealed the existence of small cones in the Cerberus Plains, Marte Valles, and Amazonis Planitia. These cones are similar in both morphology and planar dimensions to the larger Icelandic rootless cones, which form due to explosive interactions between surficial lavas and near-surface groundwater. If martian cones form in the same manner as terrestrial rootless cones, then equatorial ground-ice or ground water must have been present near the surface in geologically recent times. Evidence for a shallow lake in the Western Cerberus Plains during the Late Amazonian is also presented. High-resolution images show features interpreted as flood-eroded scarps and fluvial spillways exiting the lake. Based on present-day topography, a lake would have covered an area of 8.4 x 10⁴ km², had an average depth of 12 m, and have contained a volume of 1.0 x 10³ km³ of water. Lake waters were likely primarily lost to the atmosphere through sublimation, although some quantity of water likely spilled into the Eastern Cerberus Plains or infiltrated into the shallow crust.

Main sequence stars are commonly surrounded by debris disks, composed of cold dust continuously replenished by a reservoir of undetected dust-producing planetesimals. In the outer Solar System, Kuiper Belt (KB) objects produce dust by mutual or interstellar grain collisions. The orbital evolution of KB dust has been numerically modeled. Its equilibrium radial density distribution can be accurately estimated even though there are inherent uncertainties in the prediction of structure, owing to the chaotic dynamics of dust orbital evolution imposed by resonant gravitational perturbations of the planets. The particle size distribution of dust is greatly changed from the distribution at production, as a result of radiation forces and the perturbations of the planets. The contribution of KB dust to the population of interplanetary dust particles collected at Earth may be as low as a few percent. Gravitational scattering by giant planets creates an outflow of large grains. We quantify the characteristics of this large-particle outflow in different planetary architectures, discuss its implications for exo-planetary debris disks, and for the interpretation of in-situ dust detection experiments in space probes traveling in the outer Solar System. These outflows may contribute to the clearing of circumstellar debris in planetary systems, affecting the particle size distribution of their local ISM. In anticipation of future observations of unresolved debris disks with Spitzer, we are interested in studying how the structure carved by planets affects the shape of the disk's spectral energy distribution (SED), and consequently if the SED can be used to infer the presence of planets. We numerically calculate the equilibrium spatial density distributions and SEDs of dust disks originated by an outer belt of planetesimals (35-50 AU) in the presence of different planetary configurations, and for a representative sample of chemical compositions. The dynamical models are needed to estimate the enhancement of particles near the mean motion resonances with the planets, and to determine how many particles drift inside the planet's orbit. Based on the SEDs and predicted Spitzer colors we discuss what types of planetary systems can be distinguishable from one another.

A total of six extragalactic objects have been detected so far at very high energies (VHE). They are BL Lac objects, a sub-group of active galactic nuclei characterized by intense nonthermal radiation. The VHE spectra of two of these, 1ES 1959 + 650 and 1ES 2344 + 514, were measured in this work. Similar to the other four BL Lacs detected, their VHE spectrum and flux level is highly variable and shows a broadband spectrum characterized by two emission peaks: one in X-ray, the other at GeV to TeV energies. For one of these, 1ES 1959 + 650, simultaneous observations were carried out at other wavelengths and for the first time, a VHE flare without increased X-ray flux level was recorded. For the other object, 1ES 2344 + 514, no simultaneous X-ray observations were taken, making further modeling impossible. VHE gamma-ray astronomy can establish important upper limits on the density of the extragalactic background light (EBL). If one can somehow guess what the source spectrum is, then one can infer the EBL density from the measured attenuation in the spectra. As the VHE spectra of BL Lac object are very similar, the zeroth order assumption was made (and justified) in this work that they are actually the same; differences in attenuation arising solely due to the different distances to the objects. The upper limits derived here are not very constraining, but they do question one particular set of EBL measurements that are very high in the near infrared waveband. Galaxy formation models are typically not able to reproduce this high density. The analysis of VHE spectra is still being developed; in part because of the new array of four telescopes, VERITAS, being built at the moment. Monte-Carlo simulations are used in this work and changes in the simulation software had not seen a comparison to the previous version until this work. Differences were identified that impact the energy reconstruction. A method was developed to calibrate the absolute energy scale by automatically identifying cosmic-ray muons recorded by the telescope.

We carried out extensive numerical orbit integrations to probe the long-term chaotic dynamics of the 2:3 (Plutinos) and 1:2 (Twotinos) mean motion resonances with Neptune. We derive maps of resonance stability measured both by time-averaged particle density and by mean dynamical diffusion rate, and investigate the effects of a massive perturber embedded in the resonance. We also investigate the population of Resonant Kuiper Belt Objects at 4 Gyr ago compared to the present, and discuss the implications for theories of Kuiper Belt origins. We have numerically investigated the long term dynamical behavior of known Centaurs. We find that their orbital evolution is characterized by frequent close encounters with the giant planets, with no significant long-term resonant behavior. Most of these Centaurs will escape from the inner solar system, while a fraction will enter the Jupiter-family comet (JFC) population and a few percent will impact a giant planet. We discuss the implications of our study for the spatial distribution of the actual Centaur population. Using numerical and analytical models, we investigate the ejection of water molecules from Europa's surface by sputtering, the subsequent evolution of their ballistic trajectories, and their re-deposition onto the surface as a water frost. We conclude that net deposition does occur under certain conditions, making sputtering erosion and re-deposition a plausible explanation for the observed color dichotomy between Europa's leading and trailing hemispheres. During Cassini's approach to Jupiter, a series of images was taken to search for any undiscovered satellites of Jupiter. Our analysis of these images indicates that no undiscovered satellites exist between 2.6 and 20 R(J) with inclination i < 1.6°, eccentricity e < 0.0002, diameter D > 15 km and albedo A > 0.1.

The primary topics of this dissertation are (1) target selection for searches for extrasolar life, especially for the Search for Extraterrestrial Intelligence (SETI) and the Terrestrial Planet Finder (TPF) and (2) remote detection of biosignatures, especially with regard to TPF. Chapter 1 gives a brief introduction to the field of astrobiology, and to the search for life on other planets. Chapters 2 and 3 ask, "What are the best places in the Universe to search for Earth-like life?" A class of stars, "habstars," is defined as stellar systems that are potentially habitable to Earth-like complex life. The physical properties of habstars are derived from the biological requirement of habitable zone stability, and these properties are translated into observable characteristics. In Chapter 2, the Catalog of Nearby Habitable Stellar Systems (HabCat), containing ∼17,000 "habstars" within 300 parsecs, is presented for use as a new target list for the Search for Extraterrestrial Intelligence with the Allen Telescope Array. In Chapter 3, HabCat is augmented with other targets of interest, including a list of ∼250,000 stars within 1000 parsecs from the Tycho-2 Catalog that are likely to be main-sequence (based on their proper motions) F, G, K and M stars (based on their B-V colors), old open clusters, and the nearest 100 stars. This work is refined in Chapter 4 for the Terrestrial Planet Finder (TPF), a mission to image and spectroscopically analyze extrasolar terrestrial planets. The TPF Target List Database is presented, and it contains all Hipparcos stars within 30 parsecs plus data that are relevant to planetary habitability and detectability. From this database, a sample of targets is selected and recommended for observation based on suitability for life. Chapter 5 asks, "What are the spectral signatures of a habitable, or inhabited, planet?" The Earthshine spectrum, from 0.3 to 2.5 microns, is presented and used to illustrate the spatially unresolved spectrum of a planet with abundant water and life. Water vapor, oxygen, ozone, methane, and carbon dioxide are unambiguously detected, while the vegetation signature is less certain. Chapter 6 explores possibilities for extending the earthshine work and submits recommendations for improving the TPF database content and usability.

We present a data base of U,V photometric and structural properties for 642 spectroscopically confirmed members, V-band selected to Mᵥ = -18 mag, from three local Abell clusters: A85 (z = 0.055), A496 (z = 0.033) and A754 (z = 0.055). From our (U - V) half-light aperture colors and total model V-band magnitudes we construct precise color-magnitude diagrams for member galaxies out to ≳1 h⁻¹ Mpc. We measure well-defined color-magnitude relations (CMR) with low intrinsic scatter (σCMR ∼ 0.06-0.09 mag) in the cluster cores ( < 0.5h⁻¹ Mpc). We define three galaxy populations based on the relative color difference Δ(U - V) between the galaxies color and the best-fit CMR: (1) red sequence galaxies with Δ( U - V) ≥ -2σ(CMR); (2) intermediately blue (≡ bS0) galaxies with -2σ(CMR) > Δ(U - V) > -0.425 mag; and (3) blue (Butcher-Oemler ≡ B-O) galaxies with Δ( U - V) ≤ -0.425 mag. These color-magnitude cuts provide a rough galaxy age segregation assuming blueward deviations from the CMR represent mean stellar age differences. Red sequence members are the traditional cluster old, early-types (E/S0) and B-O galaxies have spiral-like colors; therefore, the bS0 population are assumed to be intermediate in age. We find a significant (∼10% in numbers) population of bS0 members in two local clusters (A85 and A754) . This is the first evidence for a quantitatively classified population of bS0 galaxies in clusters at < 1 Gyr look-back time. The bS0 populations exhibit the following characteristics: (1) Bulge-to-total morphologies intermediate between red sequence and B-O members. (2) Less morphological structure associated with star formation compared to normal, field spirals. (3) bS0 members are not found near the cluster cores which suggests more recent infall. (4) Lack of a significant color gradient which is different from both the cluster red sequence and field spirals. The observed bS0 properties are consistent with these galaxies being present-day examples midway through the predicted evolution of infalling, field spiral to red, cluster S0 via galaxy harassment and/or ram-pressure stripping. Therefore, the existence of these galaxies provides clear observational evidence for the present-day whereabouts of the blue galaxies once prevalent in rich clusters (the B-O effect) and for environmental based evolution of the cluster galaxy membership.

The possibility that some portion of the infrared (IR) radiation emanating from Active Galactic Nuclei (AGN's) may arise from disklike structures of ionized plasma accreting onto massive or supermassive black holes motivates the investigation of the effects on the observed radiation of the strong gravitational fields in the vicinity of the emitting particles. Numerous previous studies have been incomplete in several respects: (a) they have neglected to take into account the contribution to the observed specific power flux of radiation emitted from the underside of the disk and gravitationally lensed into the upper half-hemisphere; (b) they have considered only a limited range of observing positions and hole spins; (c) many have been restricted to examination of the steady state flux arising from homogeneous disks; (d) they have employed a methodology not readily extendible to the analysis of gravitational effects on radiation arising from more complicated physical systems (e.g., nonplanar ensembles of gaseous clouds). The present study develops, within the context of the optically thick, geometrically thin accretion disk model, a new method of complementary images. Fully taking into account the so-called first-orbit disk images, including the effects of disk self-occlusion, for the entire range of observing positions and hole spins, and for both homogeneous and thermally inhomogeneous disks, it applies this method to both steady state and time-dependent analyses in the paradigm case of the Galactic Center black hole candidate Sagittarius A*. Completely general results applicable to any similar black hole-accretion disk system are presented. An illustration is given of how the basic method, along with ancillary analytical devices such as that of extended images and computationally efficient techniques based on their properties, may be extended to the analysis of considerably more complicated physical systems, and tentative results for the case of the Broad Line Region (BLR) of quasar spectra are presented.

The hard X-ray spectrum from black hole candidates, such as 1E1740.7-2942 and Cygnus X-1, has been attributed to an inner hot (Tₑ ≈ 10⁹ K) two-temperature disk which Comptonizes externally produced soft photons from the outer disk. We developed a natural extension of this model, wherein the innermost region of the two-temperature disk, is much hotter (Tₑ ≈ 5 x 10⁹ K) since it is shielded from the external photons and is forced to cool via bremsstrahlung self-comptonization. The emission from this region can account for the long term γ-ray variability in Cygnus X-1. The e⁺e⁻ pairs produced above the hot plasma give rise to the annihilation line observed in 1E1740.7-2942 and the residual pairs form the extended radio jets observed in this source. These early successes called for more detailed modeling of the hot disk. The effects of e⁺e⁻ pairs produced inside the disk were investigated using a better technique for the Comptonization process than what had been reported previously in the literature. This has important quantitative (but no qualitative implications) on the model. Another crucial effect is that of the proton thermal energy being advected to smaller radii. This makes the disk sensitive to the outer boundary conditions (i.e the structure of the transition zone between the outer cold disk and the inner hot region). To determine the physics of the transition region we have developed a scheme for the radiative cooling which is valid at all optical depths. The application of this scheme revealed that the transition region is an extended one. The disk has a hot two-temperature configuration even without the assumption that instabilities in the disk drive the cold disk to this stage (which has been doubted in the past). Moreover, the spectrum from the transition zone matches well with the observed X-ray spectrum of Cygnus X-1 and variations in the magnetic field can account for the two X-ray states. We conclude from this new self-consistent model that the soft X-ray photons observed arise from the cold disk, the X-ray continuum is produced in the transition zone, while the γ-rays and associated phenomena (like the e⁺e⁻ line and the radio jets) are due to the inner hot disk.

This thesis is an observational study of the impact of star formation on the interstellar medium. Emission from the ionized component of the interstellar gas is used to measure both the kinematics and the physical properties of the gas in 14 dwarf galaxies. The galaxies examined show emission from ionized gas outside the HII regions. This warm ionized medium has a substantial power requirement and often shows arcs and filaments on scales exceeding 100 pc. To determine the mix of physical processes exciting the gas, I measure optical emission-line ratios averaged over scales of 30-150 parsecs at thousands of locations within the galaxies. I find that, relative to HII regions, the spectrum of the diffuse ionized gas has stronger lines from low-ionization states of oxygen, nitrogen, and sulfur and weaker lines from highly ionized atoms. The HII-DIG spectral transition defines a narrow sequence in diagnostic-line-ratio diagrams which is distinct from the conventional HII-region excitation sequence. Photoionization modeling demonstrates that the HII-DIG sequence is driven primarily by a decrease in the relative density of ionizing photons to atoms--consistent with ionization by distant stellar clusters. The strength of the line emission from ionized He relative to that from ionized hydrogen implies stars of mass greater than 35 M(⊙) contribute to the ionizing continuum. A second excitation process, shocks with speeds from 60-100 km s⁻¹, is shown to contribute up to 50% of the emission from the lowest surface brightness gas. High-resolution, longslit spectra reveal organized gas flows on scales ranging from the resolution limit, about 20-100 km s⁻¹, to galactic-scale dimensions, roughly 1 kpc. Many of the expanding shells detected kinematically are coincident with arcs or filaments in the galaxy images, and the geometry of the larger bubbles is often polar rather than spherical. The simplest dynamical models for their growth imply ages from 1-20 Myr and require energies from 1 to more than 6000 supernovae. The extent, mass, and rotation of the neutral gas are compiled from the literature and used to argue that many of these bubbles breakthrough the HI disk and form galactic winds. A detailed study of I Zw 18 suggests such winds could have a strong influence on the chemical evolution of dwarfs. The total mass of gas escaping the galaxies is, however, not yet well-constrained and may be quite small.