▪ Abstract Because calibrated light curves of type Ia supernovae have become a major tool to determine the local expansion rate of the universe and also its geometrical structure, considerable atte...

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Type IA Supernova Explosion Models

AM Canum Venaticorum (AM CVn) binaries consist of a degenerate helium donor and a helium, C/O, or O/Ne white dwarf accretor, with accretion rates of M-dot=10{sup -13}-10{sup -5}M{sub odot}yr{sup -1}. For accretion rates 0.8 M {sub sun} will undergo dynamical burning. Triple-{alpha} reactions during the hydrostatic evolution set a lower limit to the {sup 12}C mass fraction of 0.001-0.05 when dynamical burning occurs, but core dredge-up may yield {sup 12}C, {sup 16}O, and/or {sup 20}Ne mass fractions of {approx}0.1. Accreted {sup 14}N will likely remain {sup 14}N during the accretion and convective phases, but regardless of {supmore » 14}N's fate, the neutron-to-proton ratio at the beginning of convection is fixed until the onset of dynamical burning. During explosive burning, the {sup 14}N will undergo {sup 14}N({alpha}, {gamma}){sup 18}F({alpha}, p){sup 21}Ne, liberating a proton for the subsequent {sup 12}C(p, {gamma}){sup 13}N({alpha}, p){sup 16}O reaction, which bypasses the relatively slow {alpha}-capture onto {sup 12}C. Future hydrodynamic simulations must include these isotopes, as the additional reactions will reduce the Zel'dovich-von Neumann-Doering length, making the propagation of the detonation wave more likely.« less

https://iopscience.iop.org/article/10.1088/0004-637X/699/2/1365/pdf

Unstable Helium Shell Burning on Accreting White Dwarfs

AM Canum Venaticorum (AM CVn) binaries consist of a degenerate helium donor and a helium, C/O, or O/Ne WD accretor, with accretion rates of Mdot = 1e-13 - 1e-5 Msol/yr. For accretion rates 0.8 Msol will undergo dynamical burning. Triple-alpha reactions during the hydrostatic evolution set a lower limit to the 12C mass fraction of 0.001 - 0.05 when dynamical burning occurs, but core dredge-up may yield 12C, 16O, and/or 20Ne mass fractions of ~ 0.1. Accreted 14N will likely remain 14N during the accretion and convective phases, but regardless of 14N's fate, the neutron-to-proton ratio at the beginning of convection is fixed until the onset of dynamical burning. During explosive burning, the 14N will undergo 14N(a,g)18F(a,p)21Ne, liberating a proton for the subsequent 12C(p,g)13N(a,p)16O reaction, which bypasses the relatively slow alpha-capture onto 12C. Future hydrodynamic simulations must include these isotopes, as the additional reactions will reduce the Zel'dovich-von Neumann-Doring (ZND) length, making the propagation of the detonation wave more likely.

This work presents the results of a numerical study of two-dimensional cellular thermonuclear detonations in degenerate C+O matter at densities 106-3×107 g cm-3. We found three levels of detonation cells of very different sizes which correspond to the C-, O-, and Si-reaction length scales. The Si cells are comparable to or even larger than the white dwarf size at densities below 107 g cm-3. In the presence of the cellular structures, the averaged half-reaction length compared to the one-dimensional case increases by the factor of 1.3 for the Si-reaction, and by a factor of 1.6 for the O- and C-reactions. The formation of unreacted pockets and overreacted regions behind the cellular detonation front creates concentration fluctuations in the reaction zone. The composition fluctuations and the reaction length increase may affect the spectra of Type Ia supernovae predicted by delayed-detonation models.

Multilevel structure of cellular detonations in type Ia supernovae

We describe a mechanism by which a failed deflagration of a Chandrasekhar-mass carbon-oxygen white dwarf can turn into a successful thermonuclear supernova explosion, without invoking an ad hoc high-density deflagration-detonation transition. Following a pulsating phase, an accretion shock develops above a core of ~1 M☉ composed of carbon and oxygen, inducing a converging detonation. A three-dimensional simulation of the explosion produced a kinetic energy of 1.05 × 1051 ergs and 0.70 M☉ of 56Ni, ejecting scarcely 0.01 M☉ of C-O moving at low velocities. The mechanism works under quite general conditions and is flexible enough to account for the diversity of normal Type Ia supernovae. In given conditions the detonation might not occur, which would reflect in peculiar signatures in the gamma-ray and UV wavelengths.

Beyond the Bubble Catastrophe of Type Ia Supernovae: Pulsating Reverse Detonation Models

Results of computations of equilibrium parameters of steady detonation, deflagration and spontaneous flame propagation both in degenerate12C+16O mixtures and helium4He are presented in a wide density range from 106 to 5×109 g cm−3. The limiting regimes of supersonic and subsonic burning are discussed. The maximum velocity of deflagration is obtained as a function of initial density of matter.

A. M. Khokhlov

Papers

Hugoniot adiabates for the waves of combustion in degenerate matter of supernovae