The Origins and Impacts of Magnetic Fields in White Dwarfs

Abstract

White dwarfs are the final product of stellar evolution for the overwhelming majority of stars in our Galaxy. These "dead stars'' radiate their residual thermal energy into space and cool down over billions of years, as they can no longer produce energy via nuclear fusion to sustain themselves. Despite this lack of an internal energy source, multiple physical processes arise on the white dwarf cooling sequence that complicate the otherwise static, relatively unchanging picture of white dwarf evolution. The first is spectral evolution, in which the elements that make up a white dwarf's atmosphere completely change as the object cools due to internal motion of material. The second is magnetism, which requires a dynamo to be spun up and similarly implies some sort of internal mechanism to generate the magnetic field. The source of these fields, and their impacts on the processes that lead to spectral evolution, have been poorly understood until now. Three primary channels have been proposed as the source of magnetic fields in white dwarfs: a fossil origin, crystallization, and binary evolution. All three channels have their own specific problems, with no one channel able to explain the occurrence of all magnetic white dwarfs. Recent work has discovered two distinct populations of magnetic white dwarfs that form through two unique channels based on their physical properties. In order to further test this hypothesis, we conducted a survey of all white dwarfs in the Sloan Digital Sky Survey (SDSS) footprint and within 100 pc of the Sun, and identified 163 magnetic white dwarfs in this sample, 87 of which are new discoveries. We analyzed trends in field strength, mass, and cooling age, and provided statistical evidence of the existence of two populations using Kolmogorov-Smirnov tests and a Gaussian mixture model. The high mass, young, high field strength group is likely dominated by binary evolution products, while the low mass, old, weak field group is dominated by fields that form via crystallization or on the main-sequence. However, when accounting for the breakout timescales of a crystallization-induced dynamo, we find that a main-sequence dynamo can account for the incidence of magnetism in many more of our old, low mass population. We discovered a hot mixed-atmosphere white dwarf that shows strong variations in its spectral line profiles as it rotates. We do not detect radial velocity shifts, hence this object is not in a binary system, and the object has a confirmed magnetic field based on the Zeeman-splitting of Hα. Homogeneous atmosphere models, in which the distribution of hydrogen and helium in the atmosphere is assumed to be constant across the surface, completely fail to reproduce our optical spectra. Instead, we employ an inhomogeneous model with hydrogen polar caps and a helium equatorial belt, which yields excellent fits to our time-resolved spectra. Following this discovery, we detected similarly significant variations in a second hot mixed-atmosphere white dwarf, and conducted extensive follow-up on a third object with known variability and a confirmed magnetic field. Our polar cap model successfully reproduces our spectra for these targets as well, whereas homogeneous models once again fail. These three targets are part of the class of eight double-faced white dwarfs that show spectral variations as they rotate. We defined this class for the first time, and linked the inhomogeneities to the effects of magnetism on convective dilution. This dilution process normally causes the switch from a hydrogen to helium atmosphere during spectral evolution, but the fields in these objects greatly inhibit convective motion at the poles. This results in the hydrogen atmosphere remaining intact at the poles, while the equatorial regions switch to helium as convective dilution is not as inhibited in these regions. This dilution process occurs at high effective temperatures, while a second process called convective mixing occurs at cooler temperatures. The mixing process similarly swaps a hydrogen atmosphere to helium, hence we should be able to detect double-faced targets at cool temperatures in which homogeneous atmosphere models fail to reproduce their spectra. We led an observing program to obtain time-resolved spectroscopy on seven cool magnetic white dwarfs that were initially proposed to be in binary systems due to the inability to fit their spectra with homogeneous models. All seven targets are strongly magnetic, and none of them show radial velocity shifts. Once again, our polar cap model successfully yields excellent fits to our spectra. Four of the seven objects also show variations in the position of their Zeeman-split absorption profiles due to the rotation of the strong magnetic fields. We use an offset dipole model to determine the geometry and field strength of each system, and we obtain rotation periods on the order of minutes for three of the four objects that show line position variations. The rapid rotation and/or low masses in six of our targets point to binary evolution as the source of their magnetic fields.