Prompt Ia Supernovae Are Significantly Delayed

Type Ia supernovae (SNIa) provided the distance measurements that first uncovered the accelerating expansion of the universe (Perlmuter et al. 1999; Riess et al. 1998), but their origin remains unknown. While there is a consensus that they arise from the explosion of a carbon-oxygen white dwarf that has been accreting from a binary companion star (Hoyle & Fowler 1960), no fully satisfactory theoretical model of this accretion has yet been developed. One key constraint on these models is the time delay between the formation of a population of stars and the onset of SNIa, which sets tight limits on the masses and nature of SNIa progenitors. Here we use a new observational technique to measure this delay time and demonstrate that it lies between 200-500 million years. This implies that either the majority of Ia companion stars have main-sequence masses less than three times that of the Sun, or that most SNIa arise from double-white dwarf binaries. Our constraint also provides a key missing piece of information for interpreting the formation history of the Milky Way, whose oldest stars do not appear to have been contaminated with ejecta from SNIa (McWilliams 1992). Subject headings: stars: evolution – supernovae: general 1