Rates and Delay Times of Type Ia Supernovae
  We analyze the evolution of binary stars to calculate synthetic rates and
delay times of the most promising Type Ia Supernovae progenitors. We present
and discuss evolutionary scenarios in which a white dwarf reaches the
Chandrasekhar-mass and potentially explodes in a Type Ia supernova. We
consider: Double Degenerate (DDS), Single Degenerate (SDS), and AM Canum
Venaticorum scenarios. The results are presented for two different star
formation histories; burst (elliptical-like galaxies) and continuous
(spiral-like galaxies). It is found that delay times for the DDS in our
standard model (with common envelope efficiency alpha = 1) follow a power-law
distribution. For the SDS we note a wide range of delay times, while AM CVn
progenitors produce a short burst of SNe Ia at early times. We point out that
only the rates for two merging carbon-oxygen white dwarfs, the only systems
found in the DDS, are consistent with the observed rates for typical Milky
Way-like spirals. We also note that DDS progenitors are the dominant population
in elliptical galaxies. The fact that the delay time distribution for the DDS
follows a power-law implies more Type Ia supernovae (per unit mass) in young
rather than in aged populations. Our results do not exclude other scenarios,
but strongly indicate that the DDS is the dominant channel generating SNe Ia in
spiral galaxies, at least in the framework of our adopted evolutionary models.
Since it is believed that white dwarf mergers cannot produce a thermonuclear
explosion given the current understanding of accreting white dwarfs, either the
evolutionary calculations along with accretion physics are incorrect, or the
explosion calculations are inaccurate and need to be revisited (Abridged).
