Type IA supernovae in elliptical and spiral galaxies - Possible differences in photometric homogeneity

It is shown that β, the initial postmaximum rate of SN brightness decline (in the B band) defined by Pskovskii, may have a smaller dispersion among SNe Ia in elliptical galaxies than in all other types of galaxies. Contamination of the sample by SNe Ib is unlikely to be the primary cause of this difference. Although the number of objects is very small, it is also possible that the velocity of SN Ia ejecta in elliptical galaxies is lower than in spiral galaxies. If correct, these observations provide the first direct evidence for physical differences among SNe Ia in different environments; reddening variations due to gas and dust are unlikely to produce most of the observed dispersion in β among spirals. One obvious possibility is that the SNe Ia in spiral galaxies come from intermediate-mass stars and that differences in the metallicities, accretion rates, or other properties account for the observations. A more extreme, improbable explanation is that not all SNe Ia in spiral galaxies result from carbon deflagrations of carbon-oxygen white dwarfs.