We give an update of current research on the use of nebular spectra of SNe Ia as distance indicators. Results of the application of the method to a group of SNe Ia are reported. We describe the status of the research including theoretical and observational requirements of the method. Our results point toward a shorter distance scale than methods based on the “standard candle” hypothesis for Type Ia SNe.
The use of SNe Ia as “standard candles” to determine the extragalactic distance scale has been recurrently debated. The correlation found by Pskovskii (1977, 1984) and by Branch (1981) between the postmaximum decline rate of the light curve and the magnitude at maximum cast doubts concerning this method. The validity of the correlation was questioned by Boisseau & Wheeler (1991), who found that such an effect might reflect contamination from the light of the underlying galaxy. But new evidence on differences in the light curve decline rate (Phillips 1993; Suntzeff, this volume) opens again the question of the correlation of magnitude at maximum and slope of the light curve soon after maximum. The value of the absolute magnitude of SNe Ia as a class loses much of its meaning if the considerable spread in magnitudes found in recent work is confirmed.
Uncertainties in the absolute magnitudes of SNe Ia are amplified by extinction. The discrepant “observationally-inferred” values obtained for SN 1986G (Phillips et al. 1992a; Delia Valle & Panagia 1992; Phillips 1993) show that when reddening is high the usual prescriptions to obtain this quantity from the color curves and from the equivalent width of the Na I D interstellar line towards the supernova give different results.