Two populations of progenitors for type Ia supernovae

Type Ia supernovae (SNe) play a key role in the determination of the cosmological parameters because their absolute luminosities can directly be derived from measurements of their light curves. Despite several decades of observations and theoretical studies, the nature of SN Ia progenitors is still unknown, and this fact brings ambiguity into the interpretation of recent cosmological results. One of the most promising approaches to clarify the nature of SN Ia progenitors consists in studying the delay time distribution (DTD) between the formation of the progenitor star and its explosion as a SN. We use recent observations of the evolution of the SN Ia rate with redshift (Dahlen et al. 2004), the dependence of the SN Ia rate on the colours of the parent galaxies (Mannucci et al. 2005), and the enhancement of the SN Ia rate in radio-loud early-type galaxies (Della Valle et al. 2005) to derive, for the first time on robust empirical grounds, the DTD. We find it to be bimodal, in that about 50% of type Ia SNe (dubbed “prompt” SN Ia) explode soon after their stellar birth, in a time of the order of 10 years, while the remaining 50% (“tardy” SN Ia) have a much broader distribution, well described by an exponential function with a decay time of about 3 Gyr. It appears that prompt SN Ia originate from stellar progenitors more massive than 5.5M⊙, which have an explosion efficiency about 3 times higher than that of less massive progenitors. The bimodality of the DTD, coupled with the well established systematics in the properties of SN Ia, strongly support the existence of two classes of progenitors. Several theoretical models, based on both singleand double-degenerate schemes, can reproduce the evolution of SN Ia rate with redshift and the dependence on the colors of the hosts, while none can account for the enhancement of the SN Ia rates in radio-loud Ellipticals. Prompt SN Ia are expected to occur up to same high redshifts as core-collapse (CC) SNe (e.g., z∼ 5 − 10) in a ratio SN CC/Ia∼10. Since prompt SN Ia explosions at high z are able to provide a substantial input of Fe into their environments, in the early Universe the O/Fe ratio may be expected to be approximately constant at a level moderately higher than measured in the local Universe. Since the two classes of SNe dominate at different redshifts, the confident use of the SN Ia as “standardizable” candles in the local and in the distant universe requires a good understanding of the properties of the two classes.