The origin of the diversity of Type Ia supernovae and the environmental effects

Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as the morphology, metallici ty, and age of the host galaxies. The influence of these environmental prop erties on the resulting SNe Ia is studied in this Letter. First, it is show n that the carbon mass fraction X(C) in the C + O white dwarf SN Ia progeni tors tends to be smaller for a lower metallicity environment and an older b inary system. It is then suggested that the variation of X(C) causes the di versity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia . Further studies of the propagation of the turbulent flame are necessary t o confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low-metalli city environment, the number of bright SNe Ia is reduced, so that the varia tion in brightness among the SNe Ia is also smaller. Thus, our model can ex plain why the mean SN Ia brightness and its dispersion depend on the morpho logy of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z greater than or similar to 1), both the mean brightness of SNe Ia and its variation sho uld be smaller in spiral galaxies than in elliptical galaxies. These variat ions are within the range observed in nearby SNe Ia. Insofar as the variati on in X(C) is the most important cause for the diversity among SNe Ia, the light-curve shape method that is currently used to determine the absolute m agnitude of SNe Ia can also be applied to high-redshift SNe Ia.