The Hubble Diagram for Supernovae of Type Ia. II. The Effect on the Hubble Constant of a Correlation between Absolute Magnitude and Light Decay Rate
New Hubble diagrams in B and V are derived for supernovae of type I based on light curves from the archive literature plus 13 new light curves with superior modern photometry observed in the Cerro Tololo/University of Chile program (Hamuy et al, 1995). The sample is restricted to SNe Ia whose light curves are defined by photometry beginning 5 days or less after maximum light and with (B - V)max < 0.5 mag. Supernovae of known type Ib or Ic are also excluded. The resulting Hubble diagrams, extending to redshifts of 30,00 km s^- 1^, have dispersions in absolute magnitude of 0.34 mag in B and 0.33 mag in V, confirming that spectroscopically "normal" (Branch et al. 1993) SNe Ia are among the best standard candles known. A solution for the slope of the Hubble diagram gives n(B) = 0.977 +/- 0.025 and n(V) = 1.020 +/- 0.024 for the exponent in ν~D^n^, proving linearity of the expansion field to a high level. The residuals in magnitude from the ridge line of the Hubble diagram are compared with the light decay rate during the first 15 days to test the correlation between the two suggested by Pskovskii and by Phillips. The strongest possible correlation using the extant data has a slope 3 times smaller than that derived by Phillips, and 2 times smaller than suggested by Hamuy et al., leading to a decrease of less than 10% in the distance scale based on the present (1995) SNe Ia calibration by means of three supernovae whose distances are known from Cepheids in their parent galaxies. Applying the maximum possible correction to M(max) for a Psko'vskii- Phillips effect would give Hubble constants of H_0_(B)<= 54 +/- 4 km s^-1^ Mpc^-1^, and H_0_(V) <= 59 +/- 4 km s^-1^ Mpc^-1^, where the errors are internal. It is argued that the absence of measurable bias effects in the Hubble diagrams shows that the three local (nearest) SNe Ia presently calibrated via Cepheid distances cannot all be overluminous relative to the average of more distant SNe. If they are underluminous, which must be the case by the statistics of the Malmquist effect if the large dispersion in M(max) for SNe Ia claimed by Hamuy et al. applies to the calibrators, then the value of H_0_ = 52 km s^-1^ Mpc^-1^ given by Saha et al. is an upper limit to the Hubble constant.