SN 1992bc and SN 1992bo: Evidence for intrinsic differences in type IA supernova luminosities

Abstract

Based on the two best-observed Type Ia supernovae found in the Calan/Tololo Supernova Search, in this Letter we present convincing evidence that Type la supernovae with a fast rate of decline in the B band are intrinsically subluminous with respect to those with a slower decline rate. The effect is greater in the B band than in the I band. Specifically, we show that the two Type la supernovae SN 1992bc and SN 1992bo differ by 0.80 +/- 0.2 mag in their peak B absolute magnitudes, and by only 0.41 +/- 0.2 mag in the I band. SN 1992bc occurred in a spiral galaxy, whereas SN 1992bo exploded very far from the nucleus of a lenticular galaxy. These findings suggest the possibility that these two Type Ia supernovae may have had progenitors with significant physical differences. Both objects are normal Type Ia supernovae from a photometric and spectroscopic point of view. Even though these objects confirm the dependence of the absolute B magnitude on the rate of decline, that relation could be less pronounced than suggested recently by Phillips (1993).