As the endpoints of stars less massive than roughly eight solar masses, the population of Galactic white dwarfs (WD) contain information about complex stellar evolution processes. Associated pairs of WDs add an extra degree of leverage; both WDs must have formed and evolved together. The work presented in this dissertation uses various populations of double WDs (DWD) to constrain evolution of both single and binary stars. One example is the set of low-mass WDs with unseen WD companions, which are formed through a dynamically-unstable mass loss process called the common envelope. To work toward a quantitative understanding of the common envelope, we develop and apply a Bayesian statistical technique to identify the masses of the unseen WD companions. We provide results which can be compared to evolutionary models and hence a deeper understanding of how binary stars evolve through a common envelope. The statistical technique we develop can be applied to any population of single-line spectroscopic binaries. Binaries widely separated enough that they avoid any significant interaction independently evolve into separate WDs that can be identified in photometric and astrometric surveys. We discuss techniques for finding these objects, known as wide DWDs. We present a catalog of 142 candidate wide DWDs, combining both previously detected systems and systems we identify in our searches in the Sloan Digital Sky Survey. Having been born at the same time, the masses and cooling ages of the WDs in wide DWDs, obtained with our spectroscopic follow-up campaign can be used to constrain the initial-final mass relation, which relates a main sequence star to the mass of the WD into which it will evolve. We develop a novel Bayesian technique to interpret our data and present our resulting constraints on this relation which are particularly strong for initial masses between two and four solar masses. During this process, we identified one wide DWD, HS 2220+2146, that was peculiar since the more massive WD in this system evolved second. We construct an evolutionary formation scenario in which the system began as a hierarchical triple in which the inner binary merged (possibly due to Kozai-Lidov oscillations) forming a post-blue straggler binary. The system then evolved into the DWD we observe today. We further discuss the potential for identifying more wide DWDs, including peculiar systems like HS 2220+2146, in future surveys such as Gaia.

This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries. First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ⪆10⁶ K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10⁻⁴ M ⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTₓ > 33 keV) X-rays. We additionally find roughly 10⁻⁵ M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles. Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10⁻⁷ M⊙ masses), and improve measurements of the period of jet precession (P=12013 +/- 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially be used as an indicator of whether a symbiotic is powered predominantly by shell burning on the surface of the WD or by accretion. We additionally make the first ever radio detections of seven of the targets in our survey. Our survey of seventeen radio bright symbiotics, comparing observations before and after the upgrades to the VLA, shows the technological feasibility to resolve the nebulae of nearby symbiotic binaries, opening the door for new lines of research. We spatially resolve extended structure in several symbiotic systems in radio for the first time. Additionally, our observations reveal extreme radio variability in symbiotic BF Cyg before and after the production of a jet from the system. Our results from our surveys of symbiotics provide some support for the model of radio emission where the red giant wind is photoionized by the WD, and suggests that there may be a greater population of radio faint, accretion driven symbiotic systems. This work emphasizes the powerful nature of radio observations as a tool for understanding eruptive WD binaries and their outflows.

Wong Ka Wah = 奇異星的特性 / 黃嘉華. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 98-101). / Text in English; abstracts in English and Chinese. / Wong Ka Wah = Qi yi xing de te xing / Huang Jiahua. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- History of Compact Stars --- p.1 / Chapter 1.2 --- The Proposal of Strange Quark Stars --- p.2 / Chapter 1.3 --- Outline of the Thesis --- p.3 / Chapter 2 --- Cold Equation of State from Perturbative QCD --- p.6 / Chapter 2.1 --- Description of Strange Quark Matter --- p.7 / Chapter 2.2 --- MIT Bag Model --- p.8 / Chapter 2.3 --- Perturbative QCD --- p.10 / Chapter 2.4 --- Comparison with MIT Bag Model --- p.11 / Chapter 3 --- Static Structure of Strange Stars --- p.16 / Chapter 3.1 --- Static Equilibrium --- p.16 / Chapter 3.2 --- Models --- p.18 / Chapter 3.3 --- Results of Global Properties and Discussions --- p.18 / Chapter 4 --- Stability of Strange Quark Matter --- p.25 / Chapter 4.1 --- Absolute Stable Condition --- p.25 / Chapter 4.2 --- Weak Stable Condition --- p.26 / Chapter 4.3 --- Stability Condition Compared to Neutron Stars --- p.27 / Chapter 4.4 --- Conclusion --- p.28 / Chapter 5 --- Effect of Massive Strange Quarks --- p.31 / Chapter 5.1 --- Numerical Analysis of the Effect of Strange Quark Mass on the EOS --- p.33 / Chapter 5.2 --- Structure of Strange Stars with Strange Quark Mass --- p.37 / Chapter 5.3 --- Conclusion --- p.38 / Chapter 6 --- QCD Phase Transition in a Compact Star --- p.46 / Chapter 6.1 --- Cooling Properties --- p.47 / Chapter 6.1.1 --- Heat capacity of quark stars --- p.49 / Chapter 6.1.2 --- Luminosity of quark stars --- p.50 / Chapter 6.1.3 --- Microphysics of the neutron star cooling --- p.54 / Chapter 6.2 --- Handling of the Phase Transition --- p.56 / Chapter 6.3 --- The Models --- p.59 / Chapter 6.4 --- Results --- p.60 / Chapter 6.4.1 --- Method 1 --- p.61 / Chapter 6.4.2 --- Method 2 --- p.66 / Chapter 6.5 --- Discussion and Conclusion --- p.66 / Chapter 7 --- Formation of a Strange Star --- p.73 / Chapter 7.1 --- Formalism of the Problem --- p.73 / Chapter 7.2 --- Lagrangian Hydrodynamics --- p.74 / Chapter 7.3 --- Hot Equation of State --- p.75 / Chapter 7.3.1 --- Nuclear Matter EOS --- p.75 / Chapter 7.3.2 --- Quark Matter EOS --- p.77 / Chapter 7.3.3 --- Mixed Phase --- p.78 / Chapter 7.4 --- Initial Models --- p.78 / Chapter 7.5 --- Results --- p.80 / Chapter 7.6 --- Discussion and Conclusion --- p.81 / Chapter 8 --- Conclusion --- p.95 / Bibliography --- p.98 / Chapter A --- Solving the EOS --- p.102 / Chapter B --- Solving C from Eq. (7.10) --- p.105

The absorption due to pairs of H₂ molecules is an important opacity source in the atmospheres of various types of planets and cool stars, such as late stars, low mass main sequence stars, brown dwarf stars, cool white dwarf stars, the ambers of the smaller, burnt out main sequence stars, exoplanets, etc., and therefore of special astronomical interest. Astronomers are interested in the outer planets as they still contain primordal matter. Furthermore, recent observations by the Hubble space telescope (in operation since 1990) have revealed several thousand cool white dwarf stars with temperatures of several thousand Kelvin. It is surprising that none of them has temperatures lower than roughly 4000 K. This means that the white dwarf stars have not had enough time to cool down to the temperature of the cosmic background radiation. Astrophysicists believe that this information can be used for an alternative and more accurate method of cosmochronology. However, the emission spectra of cool white dwarf stars differ significantly from the expected blackbody spectra of their cores, largely due to collision-induced absorption by collisional complexes of residual hydrogen and helium in the stellar atmospheres. In order to model the radiative processes in these atmospheres, which have temperatures of several thousand kelvin, one needs accurate knowledge of the induced dipole and potential energy surfaces of the absorbing collisional complexes, such as H₂--H₂, H₂--He, and H₂--H. These come from quantum-chemical calculations, which, for the high temperatures and high photon energies under consideration in this work, need to take into account that the H₂ bonds can be stretched or compressed far from equilibrium length. Since no laboratory measurements for these high temperatures and photon energies exist, one has to undertake \textit{ab initio} calculations which take into account the high vibrational and rotational excitation of the involved hydrogen molecules. However, before one attempts to proceed to higher temperatures and photon energies where no laboratory measurements exist it is good to check that the formalism is correct and reproduces the results at temperatures and photon energies where laboratory measurements exist, that is, at and below room temperature and for photon energies up to about 1.5 eV. In this work a formalism is developed to compute the binary collision-induced absorption of simple molecular complexes up to temperatures of thousands of kelvin and photon energies up to 2.5 eV, properly taking into account vibrational and rotational dependencies of the induced dipole and potential energy surfaces. In order to make the computational effort feasible, the isotropic potenial approximation is employed. The formalism is applied to collisional complexes of H₂--H₂, D₂--D₂, H₂--He, D₂--He, T₂--He, and H₂--H, and compared with existing laboratory measurements. / text

This dissertation is an observational exploration of the exciting physics that can be enabled by high-speed photometric monitoring of extremely low-mass (< 0.25 M[subscript sun symbol]) white dwarf stars, which are found in some of the most compact binaries known. It includes the cleanest indirect detection of gravitational waves at visible wavelengths, the discovery of pulsations in He-core WDs, the strongest evidence for excited p-mode pulsations in a WD, the discovery of the first tidally distorted WDs and their use to constrain the low-end of the WD mass-radius relationship, and the strongest cases of Doppler beaming observed in a binary system. It is the result of the more than 220 nights spent at McDonald Observatory doing high-speed photometry with the Argos instrument on the 2.1 m Otto Struve telescope, which has led to a number of additional exciting results, including the discovery of an intermediate timescale in the evolution of cooling DA WDs and the discovery of the most massive pulsating WD, which should have an ONe-core and should be highly crystallized. / text

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Thesis (Ph.D.)--Murdoch University, 2003. / Thesis submitted to the Division of Science and Engineering. Bibliography: leaves 255-267.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: D.E. Winget. Vita. Includes bibliographical references. Also available from UMI.