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 1and 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.

Magnetically Driven Explosions of Rapidly Rotating White Dwarfs Following Accretion‐Induced Collapse

“…Hence, the second constraint set by the oxygen and 88 Sr solar abundances is not really relevant for the AIC we simulate, since we predict nearly all of the nuclei ejected have an electron fraction below 0.4. Overall, if the neutrino-or magnetically driven winds we witness in the B11 and B12 simulations persist over a number of seconds, the corresponding Galactic AIC rate, as limited by nucleosynthetic yields, should be at most 9 10 À6 yr À1 , thus from a factor of a few to up to 2 orders of magnitude lower than that obtained in the simulations of Woosley & Baron (1992) or Fryer et al (1999). Our B12 simulation suggests that if the progenitor core of such AICs rotates quickly and significant magnetic field amplification occurs after PNS formation, the event rate of the AIC of white dwarfs is doomed to be very small, of the order of 10 À6 yr À1 , less than 4 orders of magnitude lower than the event rate of massive star explosions.…”

“…Depending on the temperature in the envelope, the mass accretion rate on the newly formed white dwarf, and the mass of each white dwarf component, either a Type Ia or accretion-induced collapse (AIC) are possible. Overall, the occurrence rate of the AIC of white dwarfs is difficult to determine reliably, although it seems unlikely that they occur more frequently than once per 20Y50 Type Ia event ( Yungelson & Livio 1998, 2000Madau et al 1998;Fryer et al 1999;Nelemans et al 2001aNelemans et al , 2001bBlanc et al 2004;Mannucci et al 2005;Belczynski et al 2005;Greggio 2005;Scannapieco & Bildsten 2005;Dessart et al 2006b, hereafter D06).…”

“…In any case, the overlying envelope is fast-rotating, and the resulting progenitor is strongly aspherical (Guerrero et al 2004;Yoon et al 2007). 3 The collapse of ONeMg cores has been investigated in the past both in the context of white dwarfs ( Baron et al 1987b;Woosley & Baron 1992;Fryer et al 1999;D06) and in the context of 8Y10 M stars (i.e., low-mass massive stars; Hillebrandt et al 1984;Mayle & Wilson 1988;Baron et al 1987a;Kitaura et al 2006;Burrows et al 2007b). In D06, we presented results of radiation hydrodynamics simulations of the AIC of white dwarfs that extended these past investigations simultaneously to include multidimensionality, multigroup flux-limited diffusion for multiflavor neutrino transport, and, perhaps most importantly, rapid rotation.…”

Pair‐Instability Supernovae, Gravity Waves, and Gamma‐Ray Transients

“…The Kepler model was mapped into a one-dimensional Lagrangian code (Herant et al 1994 ;Fryer et al 1999a), which included the e †ects of general relativity (assuming spherical symmetry and without back- Dashed and dot-dashed lines give, respectively, the ( j eq ). speciÐc angular momentum of the last stable orbit (LSO) around a Schwarzschild and a Kerr black hole (BH) of that mass.…”

Constraints on the Progenitor of Cassiopeia A

“…The one-dimensional code uses a coupled set of equations of state to model the wide range of densities in the collapse phase (see Herant et al 1994;Fryer et al 1999 for details), including a 14 element nuclear network ( Benz et al 1989) to follow the energy generation. This network terminates at 56 Ni and cannot follow neutron excess.…”