No description available.

No description available.

This dissertation explores the non-Doppler shifting of Helium lines in the high pressure conditions of a white dwarf photosphere. In particular, this dissertation seeks to mathematically quantify the shift in a way that is simple to reproduce and account for in future studies without requiring prior knowledge of the star’s bulk properties (mass, radius, temperature, etc.). Two main methods will be used in this analysis. First, the spectral line will be quantified with a continuous wavelet transformation, and the components will be used in a χ^2 minimizing linear regression to predict the shift. Second, the position of the lines will be calculated using a best-fit Levy-alpha line function. These techniques stand in contrast to traditional methods of quantifying the center of often broad spectral lines, which usually assume symmetry on the parts of the lines.

As the remnants of stars with initial masses < 8 M⊙, white dwarfs contain valuable information regarding the formation histories of stellar populations. This dissertation focuses on using white dwarfs as tracers of Galactic evolution by first creating a self-consistent model of the Milky Way’s white dwarf population and comparing the results of various inputs to observational white dwarf catalogues. The model is applied to data from the Canada France Imaging Survey to derive the star formation histories of the thin disk, thick disk, and stellar halo. The results show that the Milky Way disk began forming stars (11.3 ± 0.5)Gyr ago, with a peak rate of (8.8 ± 1.4)M⊙ yr−1 at (9.8 ± 0.4)Gyr, before a slow decline to a constant rate until the present day — consistent with recent results suggesting a merging event with a satellite galaxy. Studying the residuals between the data and best-fit model shows evidence for a slight increase in star formation over the past 3 Gyr. The halo star formation history is relatively unconstrained owing to the relative rarity of halo white dwarfs. A complementary method to determine the age and star formation history is to obtain masses and temperatures to derive individual ages for a sample of halo objects. Using a sample of 18 spectra obtained at the Gemini Observatories the age of the inner halo is determined to be 9.3 ± 1.4 Gyr using the Cummings et al. (2018) IFMR and MIST isochrones, or 10.8 ± 0.6 Gyr using the relation from Kalirai (2012), however, the study determined that a bias is present in the mass determinations at low signal-to-noise and suggests that a larger, high signal-to-noise follow-up will be required to more accurately characterize this population. Finally, the future of white dwarf astronomy will be in good hands with the imminent start of the Legacy Survey for Space and Time (LSST) on the Vera C. Rubin Observatory, as well as several new space telescopes expected to begin operations later in this decade. The white dwarf population synthesis model is modified to simulate the WD populations in four upcoming wide-field surveys (i.e., LSST, Euclid, the Roman Space Telescope and CASTOR) and use the resulting samples to explore some representative WD science cases. The results confirm that LSST will provide a wealth of information for Galactic WDs, detecting more than 150 million WDs at the final depth of its stacked, 10-year survey. Within this sample, nearly 300,000 objects will have 5σ parallax measurements and nearly 7 million will have 5σ proper motion measurements. This sample will be used to detect the turn-off in the halo WD luminosity function for the first time, allowing for an accurate determination of the age and star formation history of the Milky Way at its earliest epoch. / Graduate

A new type of astronomical polarimeter is presented that incorporates a fused silica stress-birefringent modulator which simultaneously allows the selection of chopping frequency (≤ 5 Hz) and spectral tuning (1 ≤ λ ≤ 2.5 μm) with very high modulation efficiency. Two integrating detector packages are discussed. The first uses a single germanium photodiode and HR polaroid analyzer and achieves σᵥ = 1% for a J = 14.5 object in 0.5 hr with the MMT. The second package uses a 12 x 30 pixel HgCdTe array and MgF₂ Wollaston prism to provide four simultaneous polarimetric channels that properly eliminate the systematic errors resulting from "staring" mode operation. The instrumental polarizations of the MMT are calculated using Mueller calculus and experimentally verified. It is found that the hexagonal symmetry of the telescope cancels the large amplitude spurious polarizations that arise from the single "arms" and results in a polarimetric efficiency term that is a function only of spectral bandpass and not sky position. Therefore, the MMT behaves essentially like a clean Cassegrain telescope. The continuum polarizations of five isolated highly magnetic white dwarfs are investigated both observationally and theoretically. The most complete broad-band polarimetric survey throughout the spectral region 0.35 ≤ λ ≤ 1.65 μm is performed. An apparent correlation in the linear polarizations of Grw + 70°8247, GD229, and G240-72 is discovered. In addition, no evidence for time-dependent rotation of position angle is found for any of the stars. Hydrogen Balmer photoionization occurring from the magnetically-perturbed bound states into the Coulomb-Lorentz mixed quasi-Landau continuum is investigated with the aid of recent high field calculations. Finally, the continuum polarization of Grw + 70°8247 is compared to models for cyclotron and inverse magnetobremsstrahlung absorptions in a dipolar field. The conclusion is that the continuum polarizations of these objects still eludes an exact description. New observations of BG C Mi reveal the first definitive discovery of polarized cyclotron emission in any intermediate polar and confirms that the long-held basic model of a magnetically accreting white dwarf is correct. The wavelength dependence of circular polarization is found to increase rapidly into the near-infrared, from V(I) = -0.25±0.06% to V(J) = -1.74±0.26% and suggests a field strength in the cyclotron emission region near 5-10 MG if the system scales directly with the AM Her stars.

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 188-196). Available also in a digital version from Dissertation Abstracts.

White dwarfs are particularly interesting due to their broad application to the field of astronomy (cosmochronology, SN Ia progenitors, asteroseismology). Examining distributions of white dwarf masses and temperatures, it is evident that there is some flaw in our ability to make physical atmosphere models. Tremblay and Bergeron (2009) used an {\it ad hoc} treatment of line broadening and derived significantly different surface gravity and temperatures for white dwarfs, demonstrating the importance of the line broadneing treatment in determining stellar parameters for high surface gravity stars. This thesis presents a new line broadening program, Xenomorph, based on simulation techniques. Xenomorph is used to examine various approximations used in line broadening calculations used in white dwarf atmospheres. Some approximations, like including fine structure and lower state perturbations, have small, if detectable effects. Ion motions during a transition can make features commonly seen in Stark profiles less pronounced and leads to an increase in the FWHM of the lines. Including a more complete basis set at higher densities will result in extra features, including asymmetries that has been observed in many experiments. / text

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