1999ApJ...522..802S

Repeated imaging observations have been made of NGC 3627 with the Hubble Space Telescope in 1997-1998 over an interval of 58 days. Images were obtained on 12 epochs in the F555W band and on five epochs in the F814W band. The galaxy hosted the prototypical, ``Branch normal'', type Ia supernova SN 1989B. A total of 83 variables have been found, of which 68 are definite Cepheid variables with periods ranging from 75 to 3.85 days. The dereddened distance modulus is determined to be (m-M)₀=30.22±0.12 (internal uncertainty) using a subset of the Cepheid data whose reddening and error parameters are secure.

The photometric data of Wells and coworkers combined with the Cepheid data for NGC 3627 give M_B(max)=-19.36±0.18 and M_V(max)=-19.34±0.16 for SN 1989B. The previous six calibrations in this program combined with two additional calibrations determined by others gives the mean absolute magnitudes at maximum of <M_B≥-19.48±0.07 and <M_V≥-19.48±0.07 for ``Branch normal'' SNe Ia at this interim stage in the calibration program.

Using the argument by Wells et al. that SN 1989B here is virtually identical in decay rate and colors at maximum with SN 1980N in NGC 1316 in the Fornax cluster, and that such identity means nearly identical absolute magnitude, it follows that the difference in the distance modulus of NGC 3627 and NGC 1316 is 1.62±0.03 mag. Thus the NGC 3627 modulus implies that (m-M)₀=31.84 for NGC 1316.

The second-parameter correlations of M(max) of blue SNe Ia with decay rate, color at maximum, and Hubble type are reinvestigated. The dependence of <M(max)> on decay rate is nonlinear, showing a minimum for decay rates between 1.0<Δm₁₅<1.6. Magnitudes corrected for decay rate show no dependence on Hubble type, but a dependence on color remains. Correcting both the fiducial sample of 34 SNe Ia with decay-rate data and the current eight calibrating SNe Ia for the correlation with decay rate as well as color gives

H₀=60±2(internal) km.s^–1.Mpc^–1 ,

in both B and V. The same value to within 4% is obtained if only the SNe Ia in spirals (without second-parameter corrections) are considered.

The correlation of SNe Ia color at maximum with M(max) cannot be due to internal absorption, because the slope coefficients in B, V, and I with the change in magnitude are far from or even opposite to the canonical reddening values. The color effect must be intrinsic to the supernova physics. ``Absorption'' corrections of distant blue SNe Ia will lead to incorrect values of H₀.

The Cepheid distances used in this series are insensitive to metallicity differences. The zero point of the P-L relation is based on an assumed LMC modulus of (m-M)₀=18.50. Because this may have to be increased by 0.06-0.08 mag, all distances in this paper will follow, and H₀ will decrease by 3%-4%.