The optical light curves of SN 1980N and SN 1981D in NGC 1316 (Fornax A)

Abstract

We present optical photometry of the two supernovae, 1980N and 1981D, which appeared in the peculiar D-type galaxy NGC 1316 (Fornax A). These data are combined with published observations to produce definitive optical light curves. We find that the maximum-light magnitudes of both supernovae were the same to within +/- 0.1 mag, in agreement with infrared light curve observations. The shapes of the UBV light curves of the best observed of the two supernovae, 1980N, closely resembled those of the type Ia prototype SN 1981B. We also show that an optical spectrum of SN 1980N taken 30 days after B maximum was virtually identical to a spectrum of SN 1981B obtained at the same point in its evolution. These findings lend support to claims that the majority of type Ia supernovae form a highly homogeneous class of objects. Nevertheless, the B - V colors at B maximum of the NGC 1316 supernovae were 0.3-0.5 mag redder than previous estimates of the intrinsic B - V color type of Ia supernovae at this phase. Although dust extinction within NGC 1316 could explain this difference, there is little evidence to support such a large reddening. By comparing the photometric data of SN 1980N with the light curves of SN 1984A in NGC 4419, and by assuming that the absolute magnitudes of the majority of type Ia supernovae are indeed very similar, NGC 1316 would appear to be at essentially the same distance as the core of the Virgo Cluster.