Annu. Rev. Astron. Astrophys. 1998. 36: 17-55 Copyright © 1998 by Annual Reviews. All rights reserved

3.1. The Cepheid-Calibrated SNe Ia

Table 6 of Saha et al (1997) lists distance moduli, apparent magnitudes, and absolute magnitudes for seven Cepheid-calibrated SNe Ia: SN 1937C in IC 4182, SNe 1895B and 1972E in NGC 5253, SN 1981B in NGC 4536, SN 1960F in NGC 4496A, SN 1990N in NGC 4639, and SN 1989B in NGC 3627. The Cepheid distance of the last galaxy has not yet been determined directly; it is assigned the mean Cepheid distance of three fellow members of the Leo group of galaxies. There has been little disagreement about the apparent magnitudes of these calibrator SNe Ia except in the case of SN 1937C (Schaefer 1994, 1996a, Pierce & Jacoby 1995, Jacoby & Pierce 1996). Extinction of the calibrators by dust in our Galaxy is low. Estimates of the extinction of the SNe Ia by dust in the parent galaxies are not so easy to come by, but parent-galaxy extinction of at least four of the calibrators, SNe 1895B, 1937C, 1960F, and 1972E, is likely to have been quite low. Saha et al (1997) assumed that the total extinction of these four was the same as the (low) mean extinction of the corresponding Cepheids, in which case the apparent distance modulus of the Cepheids combined with the apparent magnitude of the SN Ia gives the extinction-free SN Ia absolute magnitude. For SNe 1981B, 1989B, and 1990N, Saha et al (1997) obtained absolute magnitudes using the extinction-corrected distance moduli of the Cepheids together with assumed SN extinctions of E (B - V) = 0.1, 0.37 (Wells et al 1994), and 0.00, respectively. The mean absolute magnitudes listed by Saha et al (1997) are M_B = - 19.52 ± 0.07 based on seven events and M_V = -19.48 ± 0.07 based on six events.

Spectroscopically, SNe 1960F, 1981B, 1989B, and 1990N were observed at maximum light and they were normal. The earliest spectra of SNe 1937C and 1972E were obtained long enough after maximum light to raise the issue of whether they could have been peculiar in the sense of SN 1991T; arguments that they were spectroscopically normal have been made by Branch et al (1993, 1994). The single spectrum of SN 1895B (Schaefer 1995a) is consistent with normal, but an SN 1991T-type peculiarity cannot be excluded.

The inclusion of SN 1895B as a calibrator has been questioned on the grounds that its apparent magnitude may be unreliable, but Saha et al (1995) argued that it would be arbitrary to exclude it because there is no reason to expect a systematic error in its apparent magnitude. Also, it must be remembered that the uncertainty in a calibrator's absolute magnitude depends on the uncertainties not only in the apparent magnitude but also in the extinction and in the distance modulus; thus the uncertainties in the absolute magnitudes of the calibrators listed by Saha et al (1997) turn out to be of comparable size. The possibility that SN 1895B was a peculiar event like SN 1991T may be a better argument for its exclusion. If it is excluded, and if the most recent (and fainter) estimates of the apparent magnitudes of SN 1937C (Jacoby & Pierce 1996) and SN 1990N (Lira et al 1998) are adopted, the mean absolute magnitudes of the seven calibrators only drop from the values of Saha et al (1997), M_B = - 19.52 and M_V = - 19.48, to M_B = M_V = - 19.44.

In principle, the calibrators could provide an independent test of the magnitude-decline relation found for the Hubble-flow SNe Ia, but considering the uncertainties in the absolute magnitudes and the decline rates of the calibrators, the shallow slopes found by Tammann & Sandage (1995), Hamuy et al (1996a) can be neither confirmed nor denied. It is worth noting, though, that invoking a strong metallicity dependence of the Cepheid zero point, in the sense of reducing the Cepheid distances to metal-deficient galaxies (IC 4182, NGC 5253) relative to the distances of more metal-rich galaxies, would lead to an inverse magnitude-decline relation that is not plausible. This is a reason to doubt that introducing a metallicity dependence will cause any substantial revision of the results discussed in the next section.