We present a theoretical delay-time distribution (DTD) of Type la supernovae on the basis of our new evolutionary models of single-degenerate (SD) progenitor systems. Our model DTD has almost a featureless power law shape (⊙-n with n ≈1) for the delay time from t ∼ 0.1 to 10 Gyr. This is in good agreement with the recent direct measurement of DTDs. The observed featureless property of the DTD has been suggested to be favorable for the double-degenerate (DD) scenario but not for the SD scenario. If the mass range of the companion star to the white dwarf (WD) were too narrow in the SD model, its DTD would be too limited around the companion's main-sequence lifetime to be consistent with the observed DTD. However, this is not the case in our SD model that consists of the two channels of WD+RG (red giant) and WD+MS (main-sequence star). In these channels, the companion stars have a mass range of ∼0.9-3 M⊙ (WD+RG) and ∼2-6 M⊙ (WD+MS). The combined mass range is wide enough to yield the featureless DTD. We emphasize that the SD scenario should include two important processes: the optically thick winds from the mass-accreting WD and the mass-stripping from the companion star by the WD wind.
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