What Can the Accretion Induced Collapse of White Dwarfs Really Explain?

Abstract

The accretion induced collapse (AIC) of a white dwarf into a neutron star has been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of problematic neutron star populations and specific binary systems. The ejecta from this collapse has also been claimed as a source of r-process nucleosynthesis. So far, most AIC studies have focussed on determining the event rates from binary evolution models and less attention has been directed toward understanding the collapse itself. However, the collapse of a white dwarf into a neutron star is followed by the ejection of rare neutron-rich isotopes. The observed abundance of these chemical elements may set a more reliable limit on the rate at which AICs have taken place over the history of the galaxy. In this paper, we present a thorough study of the collapse of a massive white dwarf in 1- and 2-dimensions and determine the amount and composition of the ejected material. We discuss the importance of the input physics (equation of state, neutrino transport, rotation) in determining these quantities. These simulations affirm that AICs are too baryon rich to produce gamm-ray bursts and do not eject enough nickel to explain Type Ia supernovae (with the possible exception of a small subclass of extremely low-luminosity Type Ias). Although nucleosynthesis constraints limit the number of neutron stars formed via AICs to <0.1% of the total galactic neutron star population, AICs remain a viable scenario for forming systems of neutron stars which are difficult to explain with Type II core-collapse supernovae.