Abstract

The Calan/Tololo supernova survey has discovered ~30 type Ia supernovae at redshifts out to z -0.1. Using BV(I)_KC_ data for these objects and nearby SNe Ia, we have shown that there exists a significant dispersion in the intrinsic luminosities of these objects. We have devised a robust X^2^ minimization technique simultaneously fitting the BVI light curves to parametrize the SN event as a function of [t_B_,m_i_,{DELTA}m_15_(B)], where t_B_ is the time of B maximum, m is the peak BVI magnitude corrected for luminosity variations, and {DELTA}m_15_(B) is a single parameter describing the whole light curve morphology. When properly corrected for {DELTA}_m_15_(B), SNe Ia prove to be high precision distance indicators, yielding relative distances with errors ~7%-10%. The corrected peak magnitudes are used to construct BVI Hubble diagrams, and with Cepheid distances recently measured with the Hubble Space Telescope to four nearby SNe Ia (1937C, 1972E, 1981B, and 1990H) we derive a value of the Hubble constant of H_0_ = 63.1+/-3.4 (internal) +/-2.9 (external) km s^-1^ Mpc^-1^. This value is ~10%-15% larger than the value obtained by assuming that SNe Ia are perfect standard candles. As we have shown in Paper V, there is now strong evidence that galaxies with younger stellar population (spirals and irregulars) appear to host the slowest declining, and therefore most luminous SNe Ia. Hence, the use of Pop I objects such as Cepheids to calibrate the zero point of the SNe Ia Hubble diagram can easily bias the results toward luminous SNe Ia, unless the absolute magnitude-decline relation is taken into account. Using the inertial reference frame of the distant SNe Ia, and the "corrected" apparent magnitudes of the best- observed SNe Ia in the Virgo and Fornax clusters, we have evaluated the recession velocities of these two clusters based on the Calan/Tololo Hubble diagrams. We find a cosmic recession velocity for Virgo of 1223+/-115 km s^-1^ and for Fornax of 1342+/-70 km s^-1^, and a relative distance modulus of {DELTA}mu_(Fornax-Virgo) = 0.19^m^+/-0.23^m^.