TYPE-IA SUPERNOVAE AS STANDARD CANDLES

The distribution of absolute blue magnitudes among Type Ia supernovae (SNs Ia) is studied using supernovae whose apparent magnitudes at maximum light have been well determined and whose parent galaxies have had their relative distances determined by the Tully-Fisher or D(n) - sigma techniques. Most supernovae in the sample appear to have been normal SNs Ia that were essentially unextinguished by dust. These supernovae have a mean absolute blue magnitude M(B) = - 19.72 +/- 0.06 + 5 log (H-0/50), with an observational dispersion about the mean of only sigma(M(B)) = 0.36, which is comparable to the expected combined errors in distance, apparent magnitude, and extinction. The mean (B - V) color at maximum light is 0.03 +/- 0.04, with a dispersion sigma(B - V) = 0.20, which may be due primarily to observational error. The Cepheid-based distance to IC 4182 (Sandage et al. 1992) the parent galaxy of the normal and unextinguished Type Ia SN 1937C, leads to a Hubble constant of H-0 = 51 +/- 12 km s-1 Mpc-1. The sample also includes a few SNs Ia that appear to have been intrinsically nor-mal but reddened and dimmed by dust in their parent galaxies (e.g., SN 1989B), and several having peculiar spectra or light curves which tend to be intrinsically subluminous and red (e.g., SN 1991bg). Considering the strong observational selection against subluminous supernovae, and the fact that even when they are discovered they are betrayed by their spectroscopic or photometric peculiarities, their existence does not seriously compromise the use of SNs la as distance indicators.