The Origin of Carbon-Atmosphere White Dwarfs with Implications for Type Ia Supernovae

This study weaves together two strands that at first do not seem to be intertwined. Type Ia supernovae are the class of explosions crucial to measuring out distances on cosmological scales. In the classical theory of their origin, a white dwarf amasses enough material from a main sequence or giant companion star that its interior carbon ignites and explodes. However, these kinds of binary systems may be too sparse to account for the observed rate of type Ia supernova explosions, and much recent work has gone into exploring alternatives, especially the merger of two white dwarfs (the double degenerate scenario). In merger simulations this either results in a type Ia supernova or, depending on initial conditions, produces a massive white dwarf with a magnetic field. In this context the discovery of a new class of variable white dwarf stars in our solar neighborhood would not seem relevant, but it is. We present the discovery of the 2nd, 3rd, and 4th variables among the white dwarfs with carbon-dominated atmospheres (the hot DQs), which establishes them as a class of variables. We show that the properties of this class are consistent with their being massive white dwarfs that are rotating rapidly with magnetic spots that account for the variability. Furthermore, we show that the existence of these stars at their observed temperatures represents a conundrum. They have the space motions of an older stellar population but the temperatures and masses of a younger one. This is best explained if they are the reheated remnants of a binary white dwarf merger. This double degenerate scenario for the origin of the hot DQs neatly explains all the other curious features of the class, namely their odd carbon-dominated atmospheres, their magnetic fields, their high masses, and their variability. As white dwarf merger products that did not explode, the hot DQs are "failed" type Ia supernovae. Their properties thus become the best observational endpoints against which to calibrate simulations of type Ia supernovae in the double degenerate scenario, and their formation rate becomes an important piece in computing the fraction of double degenerate binaries that remain as candidates for explosion as type Ia supernovae.Doctor of Philosoph