The strong radio emission from Sgr A*, an object located at the dynamical center of the Milky Way, has been attributed to accretion of interstellar gas by a supermassive compact object or dense cluster of objects. We show that any dynamically stable cluster of objects cannot compress the ambient magnetic field or heat the accreting gas sufficiently to reproduce the spectrum of Sgr A*, reaffirming the paradigm that Sgr A* is a single supermassive black hole. We investigate how such a black hole would interact with its surroundings and attempt to determine observational consequences of this interaction. The complexity of the gas, dust, and stellar dynamics of the central parsec of the Galaxy complicates this problem, however. Focusing our attention on the black hole itself but being constrained by observations of the surrounding gas and stars, we have constructed models of the accretion process. We examine two types of accretion models. The first, involving a cold, massive, fossilized accretion disk, is found to generate too much infrared radiation as infalling gas impacts the disk. The second model is spherical accretion, in which the radio emission from Sgr A* is dominated by magnetic bremsstrahlung. Such a model requires accurate emissivities for a wide range of temperatures and field strengths. In this work, we derive the magnetic bremsstrahlung emissivities and apply them to the spherical accretion model, yielding a spectrum that is fully consistent with the radio emission from Sgr A*. We empirically determine a magnetic field profile that suggests the presence of other phenomena, such as a central magnetic dynamo. In addition, the model predicts that the observed high energy emission from the Galactic Center region is not dominated by emission from the central black hole and its environs.

This dissertation clarifies two aspects of neutron-star formation and evolution: the formation of neutron stars through the accretion-induced collapse of white dwarfs and the common envelope evolution of neutron stars. In both cases, we utilize a 1-D lagrangean and 2-D SPH hydrodynamics codes equipped with a broad range of physics including neutrino emission, absorption, and transport, general relativity and dense equations of state. These results are then applied to a Monte-Carlo population synthesis code to study the effects of kicks placed upon neutron stars near the time of their formation. By comparing these results with the current observational data, we find that a bimodal kick distribution of neutron-star kicks is required.

The magnetic field dynamo in the inner part of accretion disks around supermassive black holes in AGNs may be an important mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We consider dynamo with the necessary helicity generation produced by star-disk collisions. Gas heated by a star passing through the disk is buoyant and form rising and expanding plume of plasma. Due to Coriolis forces the flow produced by plumes have coherent helicity. This helicity is the source of alpha effect in the alpha-O dynamo in differentially rotating accretion disk. We apply the mean field dynamo theory to the ensemble of plumes produced by star-disk collisions. We demonstrate the existence of the dynamo and evaluate the growth rate of magnetic field. The estimate of the nonlinear saturated state of the dynamo gives the magnetic field exceeding equipartition with the thermal energy in the accretion disk. Thus, star-disk collision dynamo can be important in generating dynamically significant magnetic fields, which could alter the disk structure and be the source of the energy flow in extragalactic jets. We present results of numerical simulations of the kinematic dynamo produced by star-disk collisions. It was found that for about one star-disk collision per period of rotation of the inner edge of an accretion disk, the typical value of the threshold magnetic Reynolds number is of the order of 100. The generated mean magnetic field has predominantly even parity. We also present theoretical consideration of magnetic dynamo in New Mexico dynamo experiment, which is currently under construction. The experiment utilizes Couette flow and driven jets of liquid sodium to simulate astrophysical alpha-O dynamos in the laboratory. We perform numerical simulations with ideally conducting boundary and evaluate the changes, which vacuum boundary conditions introduce in our numerical results. We also develop the theory of the MHD Ekman boundary layer in differentially rotating conducting fluid. The Ekman layer is formed at the end plates in the experiment. We show that the Ekman layer does not influence the generation of the large scale magnetic field in the experiment.

The Yarkovsky effect is a change in a body's orbit caused by its reaction to the momentum carried away by the thermal photons that it emits. This effect may play a key role in the orbital evolution of asteroids and near-Earth objects. To evaluate the Yarkovsky acceleration under a wide range of conditions, I have developed a three-dimensional finite-difference solution to the heat equation. This approach employs neither the linearized boundary conditions, the plane-parallel heat flow approximation, nor the assumption of fast rotation used in earlier approaches (Rubincam, 1998; Vokrouhlickỳ and Farinella, 1998). Thus it can be used to explore a wide range of orbital elements and physical properties that had not been previously accessible. I use the finite-difference approach to compute Yarkovsky perturbations for homogeneous, spherical stony bodies with 1-, 10- and 100-m diameters. For a 1-m scale body rotating with a 5-h period, the semimajor axis can change as much as 1 AU in 1 Myr and the eccentricity can change as much as 0.1 in 1 Myr. These rates are much faster than any found previously because those treatments were not valid for very eccentric orbits. For rotation periods expected to be more typical of such small bodies, these rates would be considerably slower. Nevertheless, there is no data concerning rotation rates for small bodies so these fast rates may be relevant. Yarkovsky drift rates are computed for models of specific near-Earth asteroids, demonstrating that the shape of a body is important in computing its precise Yarkovsky effect. Such calculations may be useful for assessing observable Yarkovsky perturbations and in predicting and mitigating NEA hazards. The approach presented in this dissertation is the only current one with the potential to rigorously treat bodies with arbitrary shapes.

We present the first results from the F̲aint I̲nfra-R̲ed E̲xtragalactic S̲urvey (FIRES) of the Rubble Deep Field (HDF) South. Using a combination of very deep ground-based near infrared (NIR) data with the WFPC2 Hubble Space Telescope data, we constructed a K(s)-band selected sample to K(s,AB) ≤ 26.0. To interpret this data, we developed a new photometric redshift technique and tested it using spectroscopic redshifts in the HDF-N and HDF-S. Our accuracy was Δz/(1 + z) ≈ 0.07 for z < 6. We derived realistic error estimates in z(phot) by accounting both for template mismatch and for the dependence of the redshift uncertainty on the photometric errors. We estimated the rest-frame optical luminosities from an initial NIR data set and found 90 times more galaxies at 2 < z < 3.5 and Lʳᵉˢᵗ(B) > 5 x 10¹⁰ h⁻² L(⊙,B) than are expected from local luminosity functions. This discrepancy can be explained if L*, B increases by a factor of 2.4-3.2 with respect to locally determined values. Using all available NIR data in the HDF-S, we then derived the rest-frame colors (U - B)(rest), (B - V)(rest), and (U - V)(rest) of all galaxies with K(s,AB) < 25. Eight of the 12 rest-frame optically reddest galaxies at 2 < z(phot) < 3.2 would have been missed by the U-dropout selection criteria. Three of the galaxies at z > 2 have strong rest-frame optical breaks with colors corresponding to those of present day Sbc's. Using theoretical relations between the color and stellar mass-to-light ratio M/L , we estimated the M/L and stellar mass M . Using these estimates, we found that the most massive galaxies at any redshift are those with the reddest rest-frame colors and those that would be missed by the U-dropout technique. We also found that the stellar mass budget at z < 3.2 has significant contributions from galaxies redder than local Scd's. There are, however, large uncertainties in the M/L analysis and we have a relatively small field. Confirmation of these results will require additional modeling, observations over a larger area, and extensive spectroscopic follow up. We found an intrinsically bright (Lʳᵉˢᵗ(V) =5.10x10¹⁰ h⁻² L(⊙,V)) U-dropout galaxy in the HDF-S with z(spec) = 2.793. This galaxy has an extended ring-like morphology, and a co-moving diameter of ≳ 9.4 h⁻¹ Kpc for a Ω(M) = 0.3, Ω(Λ) = 0.7 cosmology. The light profile appears more centrally concentrated and symmetric at longer wavelengths and this object may have an older population superimposed on a star-forming disk.

I present moderate resolution spectra for 39 Quasi-Stellar Objects (QSOs) at z ≈ 2 obtained at the Multiple Mirror Telescope (MMT). These are combined with spectra of comparable resolution of 60 QSOs from the literature with z > 1.7 to investigate the distribution of Lyman α (Ly-α) forest absorption lines in redshift and equivalent width. I find γ = 1.88 ± 0.22 for lines stronger than a rest equivalent width of 0.32 Å, where γ is the line redshift distribution parameter, in good agreement with some previous studies. These spectra are used to measure J(ν₀), the mean intensity of the extragalactic background radiation at the Lyman limit, using the proximity effect signature. I find J(ν₀) = 7.0⁺³·⁴₋₄.₄ x 10⁻²² ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at 1.7 < z < 3.8. A sample of 151 QSO spectra from the Faint Object Spectrograph on the Hubble Space Telescope are used to measure J(ν₀) at low redshift. I find J(ν₀) = 6.5⁺³⁸₋₁.₆ x 10⁻²³ ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at z < 1, and J(ν₀) = 1.0⁺³·⁸₋₀.₂ x 10⁻²² ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at z > 1, indicating that J(ν₀) is evolving over 0.03 < z < 3.8. This work confirms that the evolution of the number density of Ly-alpha lines is driven by a decrease in the ionizing background from z ∼ 2 to z ∼ 0 as well as by the growth of structure in the intergalactic medium and the formation of galaxies from intergalactic gas. These measurements of J(ν₀) are in reasonable agreement with the predictions of models based on the integrated quasar luminosity function. I present simulated Ly-α forest spectra created using the lognormal approximation to the linear and mildly non-linear evolution of the density and velocity fields. The model spectra give a mean Ly-α forest flux decrement of 0.128 at < z >= 2.07, while the MMT data show < D >= 0.129. The photoionization effects of quasars placed in the simulated density fields on the surrounding intergalactic medium are incorporated into the synthetic spectra. This reasonably reproduces the proximity effect signature seen in the data, a 2-3σ deficit of absorption lines within 2h⁻¹ Mpc of quasars. I find that maximum likelihood methods reliably estimate the ionization rate from the UV background radiation if quasars do not preferentially occupy regions of high overdensity. I analyze the extent to which the clustering of mass around quasars and uncertainty in quasar redshifts will bias the measurement of the ionizing background. In both cases, the ionization rates are overestimated by a factor of ∼3.

This work is a detailed study of extrasolar giant planet (EGP) atmospheres and spectra. Models representative of the full range of systems known today are included, from the extreme close-in EGPs to Jovian-like planets at large orbital radii. Using a self-consistent planar atmosphere code along with the latest atomic and molecular cross sections, cloud models, Mie theory treatment of grain scattering and absorption, and incident stellar fluxes, I produce an extensive set of theoretical EGP atmosphere models and emergent spectra. The emergent spectra of EGPs strongly depend upon their outer atmospheric chemical compositions, which in turn depend upon the run of temperature and pressure with atmospheric depth. Because of qualitative similarities in the compositions and spectra of objects within several broad temperature ranges, EGPs fall naturally into five groups, or composition classes. Such a classification scheme, however preliminary, brings a degree of order to the rich variety of EGP systems known to exist today. Generic models that represent the EGP classes, as well as a set of specific models for a number of important systems that have been detected, are provided. Furthermore, the effects on emergent EGP spectra of varying key parameters such as surface gravity, cloud particle sizes, orbital distance, etc. are modeled. A discussion of current and future ground-based and space-based missions to detect and characterize EGPs in light of theoretical spectral models is included to facilitate an understanding of which systems are most likely to be studied successfully.

The present surface composition of primitive objects provides clues to understanding the conditions under which the solar system formed. Characterization of the inner solar nebula progresses rapidly due to remote studies of asteroid and planetary surfaces as well as laboratory analyses of meteorites and lunar samples. Because of their atypical orbital position at 5.2 AU, the Trojan asteroids hold the potential to help resolve several problems in planetary science, including conditions in the early solar nebula. The study presented herein was undertaken in order to uncover the nature of these poorly understood asteroids. Near-infrared reflectance spectra are presented over the wavelength range 0.8-4.0 μm. These observations nearly double the number of published 0.8-2.5 μm spectra of Trojans and provide the first systematic study of the L-band (2.8-4.0 μm) region for these distant asteroids. The spectra do not contain any definitive absorption features characteristic of surface composition (e.g. H₂O, organics, silicates) as seen on main-belt asteroids and several Centaur and Kuiper Belt objects. These data are combined with previously published data to construct spectra covering the visible and near-IR (0.3-4.0 μm) for as many objects as possible. The composite spectra are analyzed quantitatively using the formulation for scattering in a particulate medium developed by Hapke. Under this rigorous examination, it is found to be unlikely that the red spectral slope is a result of organics on the surfaces, due mainly to the lack of absorptions in the L-band. These surfaces are compatible with mixtures of anhydrous silicates and carbonaceous material. Upper limits are placed on the amount of water ice and hydrated silicates present on the surfaces. Similar analysis is performed for several other groups of dark solar system objects. Comparison of these results with those for Trojan asteroids indicates that it is likely that the Trojans formed in the solar nebula near 5 AU. If this is true, then the determination that the red slope is probably not due to organic material does not fit with the generally accepted view of trends of composition with heliocentric distance. Implications and possible alternative explanations are discussed.

We carried out an extensive infrared study of galaxies with Seyfert and starburst activity, to constrain the impact of these phenomena on the host galaxy evolution in a local galaxy sample. We developed new tools for stellar population analysis in the infrared, using stellar absorption features. These tools are ideally suited for studies of dusty galaxies and obscured stars in the Milky Way. We used the CO band at 2.3μm to estimate the contribution from red supergiants to the total K-band flux in the central regions of 46 Seyferts from the CfA redshift survey, based on new infrared spectroscopy (R ∼ 700). We removed the non-stellar contribution for 16 Seyferts, and found no evidence for strong starbursts in the majority of them. A comparison to starbursts suggests that the [FeII]¹˙²⁶μm/Paβ¹˙²⁸μm versus H₂²˙¹²¹μm/Brγ²˙¹⁶μm diagram is the best available Seyfert-starburst separator. We created a unique infrared evolutionary population synthesis model for starbursts based on empirical spectral library (R≈2000-3000) of HK spectra of 218 red stars, spanning a range of [Fe/H] from -2.2 to +0.3. We analyzed the behavior of 19 indices, and developed diagnostics for the stellar parameters. Finally, we incorporated the library into Starburst99 (Leitherer et al. 1999), and compared the synthetic spectra with observational data, to constrain the metallicity of starburst and "normal" galaxies. Our new infrared magnesium-based [Fe/H] indicator is in excellent agreement with the strength of the optical iron lines. The correlation between alpha-element and iron-peak element suggests multiple bursts of star formation in the galaxies of our sample.

The sub-mm wave portion of the electromagnetic spectrum is on the frontier of both scientific and technical research in astrophysics. Being a relatively young field, scientific advancement is driven by advancements in detector technology. In this thesis, I discuss the design, construction, testing and deployment of two sub-mm wave heterodyne array receivers. Polestar is a 4 pixel (2 x 2) heterodyne array built for operation in the 810 GHz atmospheric window. It is in operation at the AST/RO telescope at the South Pole. This receiver has increased imaging speed in this band at AST/RO by a factor of ∼20 compared to previous receiver systems. DesertStar is a 7 pixel, hexagonally close packed heterodyne array receiver built to operate in the 345 GHz atmospheric window at the Heinrich Hertz Telescope in Arizona. This system will be a facility instrument at the telescope, and will increase mapping speed over the existing dual polarization single beam receiver at the telescope now by a factor of ∼16. Both these receiver systems enable scientific projects requiring large area imaging that were previously impossible. I also discuss two scientific applications of sub-mm wave receiver systems. We have used multiple telescopes to observe several mm, sub-mm transitions and continuum emission towards the R CrA molecular cloud core. Originally thought to be associated with high mass star formation, we find that the driving source behind the mm-wave emission is a low mass protostar. The close proximity of R CrA allows us to achieve high spatial resolution even with single dish mm-wave and sub-mm wave telescopes. With this resolution, we are able to disentangle the effects of infall, rotation and outflow motions. We also use vibrationally excited HCN emission to probe the protostellar accretion disk in a sample of nearby high and low mass protostars of varying ages. While these observations are difficult with single dish telescopes, we show the promise of the technique, and report results on 4 sources.