Helium Accreting White Dwarfs as Progenitors of Explosive Stellar Transients

Abstract

Accretion induced transients, which are thought to include the important, yet little understood, supernovae (SNe) of the Type Ia, are among the most luminous events observable in the cosmos. However, SNe Ia are only one of a number of distinct classes of transients that may be the result of accretion induced ignition of white dwarfs stars (WD). Other types include the somewhat recently identified SNe Iax, theoretically predicted SNe .Ia, classical novae or fast, faint, calcium-rich SNe.<br /> Theoretical predictions allow for the possibility of producing thermonuclear detonations at masses less than the Chandrasekhar mass in systems with low accretion rates, but the proposed mechanism (the double-detonation mechanism) sensitively depends on the conditions at the point of nuclear ignition, and thus on the evolution of the WD during the accretion phase, as well as physical mechanisms included in the calculations.<br /> This work discusses the viability of He-accreting WDs as candidate progenitors of SNe Ia and Iax and other transients from a number of different perspectives, presenting results of numerical simulations, focussing on the effects of previously neglected mechanisms, such as angular momentum diffusion through magnetic dynamo action (Tayler-Spruit mechanism) and the time variability of the mass transfer rate expected in physical binary systems. The constraints placed on progenitor systems by the evolutionary behavior of the non-degenerate companion is remarked upon, as is the expected evolutionary properties of the progenitor systems.<br /> It is shown that the physics included in the numerical framework have a significant impact on the obtainable predictions, that the double detonation scenario is statistically unable to account for a large majority of observed SNe Ia, but might, depending on the included physics, account for some of the observed peculiar transients. It is further shown, that inclusion of magnetic dynamos increase the amount of helium necessary to induce detonations, which reduces the viability of these systems as possible progenitors of SNe Ia and Iax, but suggests a possible link to faint, fast, Ca-rich SNe.