THE SINGLE-DEGENERATE SCENARIO FOR TYPE IA SUPERNOVAE IN COSMIC PERSPECTIVE

The occurrence and properties of Type Ia supernovae (SN Ia's) in singl e-degenerate binary systems (white dwarf [WD] + nondegenerate companio n) is examined for galaxies of different types and as a function of re dshift. The rates and characteristics (peak luminosities, expansion ve locities of the ejecta) expected from the explosion of mass-accreting WDs in symbiotic systems and ''helium star cataclysmics'' are found to be different from those arising in another class of candidate systems : cataclysmic-like (contact) systems (CLSs), where a CO WD accretes hy drogen on a thermal timescale from a Roche lobe-fiIling main-sequence or subgiant companion. We derive the evolution of the SN Ia rate and p roperties resulting from the thermonuclear explosion of sub-Chandrasek har mass WDs in such systems when they detonate a helium layer accumul ated from steady burning of hydrogen at the surface. A fraction of CLS s are believed to form a subset of the observed luminous supersoft X-r ay sources (SSSs). Sub-Chandrasekhar explosions from CLSs are disfavor ed in all types of galaxies at redshifts z greater than or similar to 1. On the other hand, CLSs in which the WD succeeds to grow to the Cha ndrasekhar mass are more likely found in spiral galaxies and absent fr om early-type galaxies. SN Ia statistics could (if the uncertainties s till involved are reduced) help to discriminate among proposed SN Ia s cenarios. The range of variation of the characteristics of SN Ia's in the CLS scenario should be narrower than in symbiotics. The predicted correlation between peak luminosity and velocity of the ejecta in SN I a's coming from these systems is weak. For CLSs, the distinction betwe en the characteristics of SN Ia's respectively arising from sub-Chandr asekhar and from Chandrasekhar-mass explosions should be sharp, since all sub-Chandrasekhar explosions would be produced by low-mass WDs.