Accreting white dwarf models of Type I supernovae. III - Carbon deflagration supernovae

Abstract

The carbon deflagration models in accreting C + O white dwarfs are presented as a plausible model for Type I supernovae. The evolution of the white dwarf is calculated from the beginning of accretion. The relatively rapid accretion studied here (Mdot > 4 x 10-8 Msun yr-1) leads to the initiation of the carbon deflagration at the center. Subsequent propagation of the convective carbon deflagration wave and associated explosive nucleosynthesis are calculated for several cases of mixing length in the convection theory. The deflagration wave synthesizes 0.5-0.6 Msun 56Ni in the inner layer of the star; this amount is sufficient to power the light curve of Type I supernovae by the radioactive decays of 56Ni and 56Co. In the outer layers, substantial amount of intermediate mass elements, Ca, Ar, S, Si, Mg, and O are synthesized in the decaying deflagration wave; this is consistent with the spectra of Type I supernovae near maximum light. As a result of large nuclear energy release, the star is disrupted completely, leaving no compact star remnant behind. Thus the carbon deflagration model can account for many of the observed features of Type I supernovae. Nucleosynthesis in the deflagration models is discussed based on the reaction network calculation including 205 species: The abundance ratios of these species with respect to 56Fe normalized to the solar values are shown. These ratios are ∼1 for 40Ca and ∼0.5 for 36Ar, 32S, and 28Si. This suggests that Type I supernovae produce a significant fraction of these elements in the Galaxy besides iron peak elements, which may be complementary to the nucleosynthesis in massive star models for Type II supernovae. The production of neutronrich isotopes, γ-radioactivities, and 5- and r-process elements are discussed. Finally, the light curves and early time spectra based on the present models are compared with Type I supernova observations.