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

3. H₀ FROM CEPHEID CALIBRATIONS

Cepheid-based calibrations of the absolute magnitudes of SNe Ia began with the discovery and measurement of Cepheid variables in IC 4182, the parent galaxy of SN 1937C, by the HST SN Ia Consortium (Sandage et al 1992, Saha et al 1994). The Cepheid distance modulus of IC 4182, µ = 28.36 ± 0.09, proved to be in agreement with the modulus that had been obtained much earlier by Sandage & Tammann (1982) on the basis of the brightest red stars in IC 4182, µ = 28.21 ± 0.2. As of this writing, the Consortium has determined Cepheid distances to four other galaxies in which five SNe Ia have appeared and hopes to be able to determine the distances to at least three more galaxies, including NGC 3627, the parent of SN 1989B; NGC 4527, the parent of SN 1991T; and NGC 1316 Fornax A, the rather early-type (Sa peculiar) parent of SNe 1980N and 1981D. The HST H₀ Key Project team (Freedman et al 1997) is expected to contribute a distance to NGC 4414, the parent of SN 1974G.

Figure 8. An empirical family of SN Ia light and color curves parameterized by luminosity. Data for some training-set members are shown as open circles (SN 1991T), open squares (1994ae), triangles (1980N), plus signs (SN 1992A), crosses (1986G), and open diamonds (1991bg). The lines are the light and color curves constructed from the training-set data, parameterized in terms of relative absolute magnitude (). From Riess et al (1996a).

A critical assessment of the reliability of the Cepheid distances is beyond the scope of this article. Thorough discussions of the discovery and photometry of the Cepheids, the observed period-luminosity relations, and the extinction and derived distances are given in the papers of Saha et al (1994, 1995, 1996a, b, 1997). The Consortium uses the V and I band Cepheid period-luminosity (P - L) relations of Madore & Freedman (1991), which are assumed to be independent of metallicity. Mild revision of the zero-points of the P - L relations on the basis of parallaxes of Galactic Cepheids measured by the Hipparcos satellite, in the sense of making Cepheid distances longer and Cepheid-based estimates of H₀ lower, may prove to be required (Feast & Catchpole 1997, Sandage & Tammann 1998, Madore & Freedman 1998) at the level of 4%. The metallicity dependence of Cepheid distances (Gould 1994, Kochanek 1997, Saha et al 1997, Sasselov et al 1997, Kennicutt et al 1998) is an important issue, but the P - L relations in M_bol appear to be independent of [Fe/H] at the level of 0.05 mag for even a change of 1.7 dex in the metallicity (Stothers 1988, Choisi et al 1992, Sandage 1996, Saio & Gautschy 1998).