Talks & Events
Ph.D. Thesis Defenses: 1998
Thermal Structure and Thermonuclear Flashes on Accreting Neutron Star Envelopes
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
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
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
Energetics and Structure of Multispecies Solar Coronal Loops
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.