Ph.D. Thesis Defenses: 1998
Thermal Structure and Thermonuclear Flashes on Accreting Neutron Star Envelopes
May 26, 1998 | Donald Q. Lamb
Shanqun Zhan

We consider the thermal structure of accreting neutron star envelopes both with weak or no magnetic fields and with strong magnetic fields without assuming a steady state condition, for a variety of accretion rates and internal temperatures (or surface luminosities). We calculate numerically the thermal structure of accreting neutron star envelopes when the accreted matter is either pure helium or a mixture of hydrogen and helium, for different metallicities. We find that compressional heating, nuclear burning, compositions of accreted matter and magnetic field strongly influence the thermal structure of neutron star envelopes; in contrast, the metalicity has little impact to the thermal structure but changes dramatically the column depth at which hydrogen burning is exhausted. We show that the behavior of pure helium burning on accreting neutron stars depends on the column accretion rate s, the temperature Tb at the base of nuclear burning, and the column density of accreted matter σ. We survey the stability of helium burning in physical parameters (Tb, s ) plane and find an ignition regime. Our results show that previous global studies of stability of helium burning on accreting neutron stars, which assumed L=Lnuc , (i.e., which employed a steady-state assumption), correspond to a cut in our ignition region and miss essentially all of the parameter plane. Contrary to what was previously thought, we find that strong magnetic field destablizes nuclear burning and strongly magnetic neutron stars can produce X-ray bursts. We show that in strong magnetic field as in weak or no magnetic field case the behavior of pure helium burning on accreting neutron stars depends on the column accretion rate s , the temperature Tb at the base of nuclear burning, and the column density of accreted matter σ and there exists an ignition regime in physical parameters (Tb, s ) plane.

Design and Construction of Deformable Mirrors for a Facility Adaptive Optics System and a Morphological Study of Infrared Line Emission in Compact Star Forming Regions
June 17, 1998 | Edward J. Kibblewhite
Michael Smutko

I detail the design, construction, and performance of the deformable mirrors and their driving electronics built for Chicago Adaptive Optics System (ChAOS). Both the mirrors and the drivers are constructed at the University of Chicago using a combination of commercially available and custom-built components. Mirrors ranging in size from 7 to 201 actuators have been constructed and several of these mirrors have been in use on the Apache Point 3.5 m telescope since April of 1995. I discuss some theoretical design issues involved in building deformable mirrors and I demonstrate the functionality of these mirrors by presenting some images obtained with ChAOS. Near infrared broad and narrow band images are presented for eleven galactic H scII regions. Data were taken using the infrared camera on the Mount Palomar 60-inch telescope with the broad band filters, J, H, and K and with narrow band filters centered on H2 and Br/gamma. The observed morphologies of these regions are compared to ultraviolet fluorescence and shock front models of molecular hydrogen line emission. I find that the majority of these H scII regions can be described by ultraviolet fluorescence.

Galaxy Structural Parameters: Star Formation Rate and Evolution with Redshift
July 16, 1998 | Richard G. Kron
Marianne Takamiya

The evolution of the structure of galaxies as a function of redshift is studied in this thesis. Two structural parameters are considered: the metric radius of the galaxy (Rη) and the power at high spatial frequencies (/chi) in the disk of the galaxy. A direct comparison is made between nearby (z~ 0) and distant (0.2~5/pm1 kpc and the median radius of the HDF sample is < Rη>~6/pm2 kpc for qo=0.5/ Ho=65/ km/ s-1/ Mpc-1, however for qo=0.1/ < Rη>~7 kpc and for qo=1/ < Rη>~5 kpc. Among the HDF galaxies, the galaxies with redshifts larger than z>0.6 have flatter Rη distributions than galaxies with redshifts smaller than z<=0.6. However, the median Rη values of high and low redshift galaxies are consistent with each other. This result is consistent with the simulations of galaxy images at redshifts z=0.35,/ z=0.5 and z=0.9 which show that the metric sizes can be recovered within 2 kpc. The flocculency or power at high spatial frequencies is quantified using a simple method that is based on surface photometry in one band and that depends on the size of the star-forming regions and on the intensity profile of the galaxy. In nearby galaxies, the flocculency is found to trace the star formation rate as χ is correlated with optical colors (B-V) and the strength of the hydrogen recombination lines (Hα). For HDF galaxies at redshifts smaller than z~1 and with fluxes brighter than B = 25,/ /chi reaches values similar to what is measured in nearby galaxies and to what is expected from simulations of distant galaxy images. Among the HDF galaxies, I find that at most 4% can be identified as dwarf galaxies with rates of star formation similar to NGC 4449 or NGC 1569. Most HDF galaxies are giants with star formation rates similar to those in nearby giant galaxies. In summary, in this study I have introduced a method to measure the metric sizes and flocculency of the two-dimensional light distribution of galaxies. As a result, I find that the high spatial frequency power is related to the star formation rate. Further, I find that the sizes and power at high spatial frequencies of HDF galaxies remain largely unchanged at redshifts lower than z~1.

Energetics and Structure of Multispecies Solar Coronal Loops
September 17, 1998 | Robert Rosner
Dawn Lenz

The temperature and density profiles of multispecies quiescent solar coronal loops containing hydrogen, helium, and heavier species are investigated using a numerical model for steady-state force and energy balance. The model loop follows a semicircular magnetic field line anchored in the chromosphere and contains low-β plasma. The model allows for species-dependent heating. The electrons, protons, and helium ions are taken to be in thermal equilibrium and form the dominant plasma component. In nonisothermal regions (i.e., in the presence of steep transition-region temperature gradients), the outward thermal force induces an inward polarization electric field along the loop; in nearly isothermal (i.e., ∇ T small) coronal regions, the electric field is outward to counterbalance gravity. The pressure gradient is negative for the protons, though in many cases it is positive for heavier ions. The thermal force can induce local minor ion overdensities. Gravitational settling may deplete the heavy ion densities, especially in the longer loops, and can occur if the settling timescale is short compared with the loop lifetime and the turbulent mixing timescale. The calculated loop abundances vary with the loop parameters. For heavy ion temperatures of ~107-108 K, the collisional energy transfer rate per particle, and therefore the required heat input per particle, is ~10-8-10-9 ergs s-1.