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; merger of two white dwarfs), Single Degenerate (SDS; white dwarf accreting from H-rich companion) and AM Canum Venaticorum (AM CVn; white dwarf accreting from He-rich companion) 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 αCE = 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. The DDS median delay time falls between ∼ 0.5−1 Gyr; the SDS between ∼ 2− 3 Gyr; and the AM CVn between ∼ 0.8− 0.6 Gyr depending on the assumed αCE. For a Milky Way-like galaxy we estimate the rates of SNe Ia arising from different progenitors as: ∼ 10−4 yr−1 for the SDS and AM CVn, and ∼ 10−3 yr−1 for the DDS. 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 powerlaw 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. Subject headings: binaries: close — supernovae: general