The size distribution of main-belt of asteroids is determined primarily by collisional processes. Large asteroids break up and form smaller asteroids in a collisional cascade, with the outcome controlled by the strength-vs.-size relationship for asteroids. We develop an analytical model that incorporates size-dependent strength and is able to reproduce the general features of the main-belt size distribution. In addition to collisional processes, the non-collisional removal of asteroids from the main belt (and their insertion into the near-Earth asteroid (NEA) population) is critical, and involves several effects: Strong resonances increase the orbital eccentricity of asteroids and cause them to enter the inner planet region; Chaotic diffusion by numerous weak resonances causes a slow leak of asteroids into the Mars- and Earth-crossing populations; And the Yarkovsky effect, a radiation force on asteroids, is the primary process that drives asteroids into these resonant escape routes. Yarkovsky drift is size-dependent and can potentially modify the main-belt size distribution. The NEA size distribution is primarily determined by its source, the main belt population, and by the size-dependent processes that deliver bodies from the main belt. All of these processes are simulated in a numerical collisional evolution model that incorporates removal by non-collisional processes. This model yields the strength-vs.-size relationship for main-belt asteroids and the non-collisional removal rates from the main belt required for consistency with the observed main-belt and NEA size distributions. Our results are consistent with other estimates of strength and removal rates, and fit a wide range of constraints, such as the number of observed asteroid families, the preserved basaltic crust of Vesta, the cosmic ray exposure ages of meteorites, and the observed cratering records on asteroids. Finally, our analytical and numerical models are applied to the collisional evolution of the trans-Neptunian objects (TNOs). We show that the TNO population likely started with a shallow initial size distribution, and that bodies ≳ 10 km in diameter are likely not in a collisional steady state. In addition, we show that the population of bodies in the TNO region below the size range of recent observational surveys is likely large enough to explain the observed numbers of Jupiter-family comets.

Nulling interferometry is an important technique in the quest for direct detection of extrasolar planets. It is central to NASA's plans for a Terrestrial Planet Finder (TPF) mission to detect and characterize Earth-like planets. This thesis presents the first experiments to demonstrate that the technique is a useful tool for ground-based observations as well. It demonstrates the ability of the technique to study faint, circumstellar environments otherwise not easily observed. In addition the observations and experiments allow more confident estimation of expected sensitivity to planetary systems around nearby stars. The old MMT was used for the first telescope experiments of stellar suppression via nulling. The stellar suppression achieved was sufficient to observe thermal emission from cool dust in the outflows around late-type stars. Based on the original MMT prototype, which worked at ambient temperature, I have constructed a cryogenic nulling interferometer for use with the renovated 6.5 m MMT. Features include the capability of sensing and correcting the phase between the two arms of the interferometer, achromatic tuning of the null using a unique symmetric beam-splitter, and compatibility with the deformable secondary of the MMT. The instrument has been used in a laboratory setup with an artificial source to demonstrate a high level of suppression. Commissioning of the instrument took place at the MMT in June 2000 using the fixed f/9 secondary. The instrument was aligned, phased, and used for science observations of 17 stars over five nights. The future impact of nulling with the MMT and the Large Binocular Telescope is sketched out. These telescopes will be sensitive to very faint levels of zodiacal dust, indicative of planetary companions and giving us clues as to the make up of planetary systems. Substellar companions down to near Jupiter mass will be detectable around the nearest stars for the LBT, allowing direct imaging of long-period giant planets. The detection of such companions will be complementary to the Doppler velocity searches, currently so successful in verifying the existence of planets, thus giving a balanced view of the prevalence and range of separations possible for giant planets around nearby stars.

I test the predictive power of the stellar evolution code TYCHO. Systematic errors are present in the predictions for double-lined eclipsing binary stars when only standard physics common to the majority of stellar evolution codes is included. A mechanism for driving slow circulation and mixing in the radiative regions of stars is identified in numerical simulations of convection and a physical theory developed. Mixing is caused by dissipation of inertial waves driven by the interaction of convective fluid motions with the boundary of the convection zone. Evolutionary calculations incorporating this physics are tested in several observational regimes. Light element depletion in young clusters, turnoff ages of young clusters with brown dwarf Li depletion ages, and evolution of carbon stars on the asymptotic giant branch are all predicted satisfactorily. Tests of solar models yield good agreement with surface observables, chemical abundances, helioseismological data, and neutrino fluxes. The predictive accuracy of a non-calibrated, state-of-the-art stellar evolution code is ∼7% for surface observables. The main sequence sun is relatively easy to model, so this gives an estimate of our minimum predictive error. The solar models also highlight problems with uniqueness of evolutionary tracks converging on a given point and the potential for avoiding the effects of missing physics by calibration. A reanalysis of the binary sample with the more complete physics shows a dramatic improvement in the accuracy of the models. The potential for avoiding the effects of missing physics by calibration is explored. A TYCHO model for a late AGB star is used for the boundary conditions on a hydrodynamic simulation of proto-planetary nebula evolution as an illustration of the unified technique. NaCl and NaCn are observed at large radii in the Egg Nebula. These molecules require high densities to form, which are difficult to explain at large distances from the star. The 2-D simulation of a fast wind interacting with earlier mass loss produces clumps of material through a thermal instability with the necessary conditions for formation of the molecules. In conclusion, the effects of the more complete physics on the core size and abundance profiles of a massive star during core Si burning are examined as an example of future developments.

We present a study of the photometric and spectroscopic properties of galaxies in a sample of six nearby, rich galaxy clusters. We examine the variations of fundamental galaxy properties, such as luminosity, morphology, and star formation rates with environment, providing new constraints on the mechanisms that drive the evolution of galaxies. This study also introduces a new maximum likelihood algorithm to recover the true distribution function of galaxies from an incomplete sample. This algorithm is ideally suited for modern-day surveys that gather a large amount of information about each object. The R-band luminosity function (GLF) shows no variation among clusters or between the field and clusters, with the exception of an enhancement of the luminous tip of the GLF in clusters. However, the GLF of quiescent galaxies steepens significantly between the field and clusters and is not universal in clusters either, suggesting that star formation properties may be more strongly correlated than the luminosity function with environment. The U-band GLF in clusters is slightly steeper than the R-band GLF, indicating that cluster galaxies are bluer at fainter magnitudes and that the GLF is thus weakly sensitive to star formation, dust, or metallicity effects. To constrain the mechanisms that shape the morphologies of cluster galaxies, we have calculated separate R-band luminosity functions for galaxy bulges and disks. Their distribution as a function of morphology and environment indicates that intermediate- and early-type galaxies can be generated from late-type galaxies by increasing the luminosity of the bulge, but not by fading the disks alone, favoring galaxy-galaxy interactions or mergers as the primary morphological transformation mechanism. Finally, we find a residual correlation of star formation with environment even after accounting for environmental variations of morphology, stellar mass, and stellar age. Thus, the star formation gradient in clusters is not just another aspect of the morphology-density relation, and cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. These results thus constitute a true "smoking gun" pointing to the effect of environment on the later evolution of galaxies.

The concept of building a space based telescope capable of directly imaging extra-solar planetary systems has been in existence for more than a decade. While the basic ideas of how such an instrument might work have already been discussed in the literature, specific details of the design have not been addressed that will enable a telescope of this class to be functionally realized. A straw man configuration of the instrument is examined here for its ability to acquire data of sufficient informational content and quality to produce images and spectra of distant planetary systems and to find what technical problems arise from analyzing the interferograms it delivers. Computer programs that simulate the signals expected to be produced by a structurally connected instrument (SCI) version of Terrestrial Planet Finder (TPF) and reconstruct images from those signals will be presented along with programs that extract planetary parameters. An abbreviated radiometric performance analysis will also be provided that will assist astronomers in designing an appropriate mission.

A survey of unidentified 100 MeV γ-ray sources is undertaken, with the Whipple 10m telescope, with the objective of detecting very high energy (>350 GeV) γ-ray emission. The survey consists of nineteen sets of observations of sources detected by the EGRET instrument on the Compton Gamma-Ray Observatory between 1991 and 1995. Results for 21 EGRET sources are reported; in some cases two EGRET sources are close enough to be viewed in a single observation. For each EGRET source, candidate associations are listed and the implications of each candidate for VHE emission discussed. Finally, a study of the performance of a next-generation ground based instrument, VERITAS, using simulations is presented. The implications of the increased sensitivity of such an instrument for suture gamma-ray surveys is briefly discussed.

Post-T Tauri stars (PTTSs) are low-mass, pre-MS stars which have ceased accreting, and are not necessarily near star-forming molecular clouds. Historically, they have been difficult to identify due to their benign spectroscopic signatures. With recent all-sky X-ray surveys and proper motion catalogs, it is now possible to find PTTSs in large numbers. The nearest PTTSs will be important targets for future imaging surveys characterizing dust disks and planetary systems around young solar analogs. The goal of this work is to systematically identify samples of PTTSs, investigate the evolution of circumstellar disks, to infer the fossil star-formation history of molecular clouds, and to estimate kinematic distances to young stars lacking trigonometric parallaxes. We present the results of a spectroscopic survey which identified 110 PTTS members of the nearest OB association (Sco-Cen). We find that 2/3rds of the low-mass star-formation in each OB subgroup occurred in <5 Myr, and that only ∼1% of solar-type stars with mean age ∼13 Myr shows signs of accretion from a circumstellar disk. In order to assess how long circumstellar material is detectable around PTTSs, we conducted a 10 μm imaging survey of post-T Tauri members of the ∼30-Myr-old Tuc-Hor association. The goal was to find evidence of either remnant accretion disks or dusty debris disks with orbital radii of ≲10 AU. Combined with data from other surveys, we conclude that mid-IR emission from warm dust grains in the terrestrial planet zones around young stars become undetectable compared to the stellar photosphere for nearly all stars by age ∼20 Myr. Lastly, we present a technique for calculating distances isolated young field stars that currently lack trigonometric parallax measurements. The technique is a generalization of the classical cluster parallax method, but can handle anisotropic velocity dispersions and non-zero Oort parameters. Distances and isochronal ages are estimated for a subsample of PTTSs included in the Formation and Evolution of Planetary Systems (FEPS) Spitzer Space Telescope (SST) Legacy Science program. The techniques developed in this thesis will allow one to efficiently conduct a systematic survey to identify the nearest, youngest stars to the Sun using existing databases.

The heliosphere, the volume inflated by the solar wind, is impenetrable to inter-stellar plasma, except for high-energy galactic cosmic rays. Neutral components of the local interstellar medium (LISM), however, do enter at the speed of Sun's relative motion to LISM. On their journey through the heliosphere, interstellar neutrals are subject to ionization by solar wind protons and electrons, solar photons, and also the gravitational pull of the Sun. Once ionized, the newly created ions, called "pickup ions" (PUIs), are swept out by the solar wind toward the termination shock (TS). Helium atoms, having a higher ionization potential, penetrate deeper into the solar system, with their trajectories gravitational focused to form a cone of high concentration of LISM He in the wake of the Sun's motion through LISM. This He cone provides a rich source of He⁺ PUIs. Recent theories suggest that PUIs are the seed particles for the detected anomalous cosmic rays (ACRs)--singly-charged ions highly enriched in He, N, O, and Ne, and with energies < 50 MeV/nuc after solar modulation. To reach such high energies, it would be necessary that the PUIs be pre-accelerated before reaching the TS, where the ions are finally accelerated to become ACRs. This thesis investigates two aspects of the PUIs life cycle by using the data gathered by Solar Wind Composition Spectrometer (SWICS) on the Advance Composition Explorer (ACE). The first is a study of pre-acceleration of He⁺ PUIs up to 100 KeV, by shocks generated by coronal mass ejecta. Our result implies that quasi-parallel shocks are more efficient at accelerating He⁺ PUIs in this energy interval than quasi-perpendicular shocks. The second is a study of time-variability of the gravitationally focused He cone. Our results suggest that the most likely cause of observed variability is due to the changing ionization rate of He over the solar cycle.

Supermassive black holes are ubiquitous in nearby galaxies. The strong correlations between black hole masses and their host galaxy bulges suggest they are intimately connected. To understand their coevolution we study quasars where both quantities can be probed out to high redshifts. To overcome the well known obstacles in studying quasar hosts at z > 1, we study 28 gravitationally lensed host galaxies, located at 1 ≤ zs ≤ 4.5, which are stretched out into arcs and Einstein rings. Applying two new algorithms, GALFIT and LENSFIT, to images obtained in the HST NICMOS F160W filter, we clearly resolve the host galaxies. Many have evidences of multiple components, interaction, offset galaxy components, or bulges and disks. The host galaxies at z > 1 are mostly brighter than L*V galaxies today, but would become fainter than L*V today after accounting for passive evolution. Furthermore, they have modest sizes (Re ≲ 6 kpc), and the profiles of the hosts are roughly equally split between bulge dominated and disk dominated. Due to these evidences, the quasar hosts may not be fully evolved early-type galaxies undergoing passive evolution if they evolve into L*V galaxies today. Moreover, comparing the hosts of radio-loud quasars and radio-quiet quasars, there is not a significant difference in their luminosities. Finally, we study the bulge luminosities (L(bulge)) and black hole masses (M(BH)) at z ≈ 1 and z ≈ 2, finding that the hosts at z > 2 already lie near the same L(bulge) vs. M(BH) relationship as for z = 0 normal galaxies . Accounting for an early-type galaxy evolution, they would fade below the relationship at present day. Therefore, the hosts at z ≈ 2 must undergo a stellar mass buildup by a factor of 3-5, if they evolve into early-type galaxies. This implies their M(BH)/M(bulge) ratio at z ≈ 2 is a factor of 3 higher than today. On the other hand, the L(bulge)-M(BH) relation for hosts at z ≈ 1 is consistent with early-type galaxies undergoing fading. Thus the parent population of the hosts at z = 2 may not be not the same as those at z = 1; the z = 2 hosts may either be progenitors of more typical late-type galaxies today, or must undergo significant stellar mass buildup.

In this thesis, we study the relationship between the angular momentum and shape of galactic disks, satellite galaxies, dark matter halos, and large scale structure using N-body simulations in the context of current models of galaxy formation. In warped galactic disks, the angular momenta of the inner and outer disk are misaligned. We have calculated the torques a misaligned halo imparts on an embedded galactic disk. N-body simulations of disks subject to torques of this strength indicate that the disk tilts in response and develops a trailing warp of the same magnitude as the Milky Way warp. We have investigated whether the warp of the Milky Way's disk is caused by nearby satellite galaxies. The misaligned warp angular momentum is anti-aligned with the orbital angular momentum of the Sgr dSph, and is of the same magnitude. This suggests that Sgr is responsible for the warp. However, N-body simulations of such disk-satellite interactions indicate that the warps excited by Sgr with its current mass and orbit are much smaller than the warp of the Milky Way. The alignment of the shapes and angular momenta of dark matter halos and the large scale environment has been studied in cosmological dark matter simulations. We have analyzed several late snapshots of such a simulation and found rotation of the triaxial figure of the halos. The figure rotates about the minor axis in most cases, at a rate that follows a log-normal distribution centred on Ωp = 0.148 h km s⁻¹ kpc⁻¹. Halos have triaxial shapes that become more spherical at larger radii. The principal axes of individual halos show strong internal alignment, as does the angular momentum, which is usually oriented along the minor axis. This alignment is not perfect, and the median misalignment is large enough to cause galactic warps. The minor axes of halos tend to point perpendicular to filaments and sheets. Major axes show a weaker tendency to point along filaments. These alignments are much stronger for higher mass halos. The angular momenta of galaxy mass halos tend to point along filaments and sheets, while those of group mass halos point perpendicular to the surrounding mass distribution.