Nucleosynthesis in Slow Deflagration and Constraints on the Progenitor of Type IA Supernovae

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Among the major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae (SNe Ia) are the companion star of the accreting white dwarf (or the accretion rate that determines the carbon ignition density) and the flame speed after ignition. To constrain the accretion rate from nucleosynthesis point of view, we calculate explosive nucleosynthesis in a carbon deflagration wave that propagates at a speed v_def as slow as 1.5 - 3 % of the sound speed v_s as suggested from recent multi-dimensional simulations. In the deflagration wave, electron capture enhances neutron excess, which depends on both v_def and the central density of the white dwarf rho_9 = rho_c /10(9) g cm(-3) . We adopt two cases of rho_9 = 1.3 and 2.4 at the thermonuclear runaway. Because of slow propagation, a significant amount of neutron-rich species such as (54) Cr, (50) Ti, (58) Fe, (62) Ni, etc. are synthesized in the central region. For v_def = 0.015 v_s and rho_9 = 2.4, for example, the ratios of (54) Cr/(56) Fe and (50) Ti/(56) Fe exceed the solar ratios by a factor of ~ 20 and 15, respectively, assuming 0.6 M_sun (56) Ni production in the outer layers of SNe Ia. For rho_9 = 1.3, the excess of the (54) Cr/(56) Fe ratio over the solar is a factor of ~ 4, which seems to be marginally consistent with the solar abundances if about a half of the solar ({56) Fe originate from SNe Ia. Therefore, the Chandrasekhar mass models should have rho_9 < 1.3, which can be realized by such a rapid accretion as dot M > 5 x 10(-7) M_sun yr(-1) . Such rapidly accreting white dwarfs might correspond to the super-soft X-ray sources.

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