Supernova Type Ia Luminosities, Their Dependence on Second Parameters, and the Value ofH0

A sample of 35 SNe Ia with good to excellent photometry in B and V, minimuminternal absorption, and 1200 < v < \approx 30000 km/s is compiled from theliterature. As far as their spectra are known they are all Branch-normal. For29 of the SNe Ia also peak magnitudes in I are known. The SNe Ia have uniformcolors at maximum, i.e. =-0.012 mag (sigma=0.051) and =-0.276 mag(sigma=0.078). In the Hubble diagram they define a Hubble line with a scatterof $\sigma_M$=0.21-0.16 mag, decreasing with wavelength. The scatter is furtherreduced if the SNe Ia are corrected for differences in decline rate Delta_m_15or color (B-V). A combined correction reduces the scatter to sigma<=0.13 mag.After the correction no significant dependence remains on Hubble type orgalactocentric distance. The Hubble line suggests some curvature which can bedifferently interpreted. A consistent solution is obtained for a cosmologicalmodel with Omega_M=0.3, Omega_Lambda=0.7, which is indicated also by much moredistant SNe Ia. Absolute magnitudes are available for eight equally blue(Branch-normal) SNe Ia in spirals, whose Cepheid distances are known. If theirwell defined mean values of M_B, M_V, and M_I are used to fit the Hubble lineto the above sample of SNe Ia one obtains H_0=58.3 km/s/Mpc, or, afteradjusting all SNe Ia to the average values of Delta_m_15 and (B-V), H_0=60.9km/s/Mpc. Various systematic errors are discussed whose elimination tends todecrease H_0. The finally adopted value at the 90-percent level, includingrandom and systematic errors, is H_0=58.5 +/- 6.3 km/s/Mpc. Several highervalues of H_0 from SNe Ia, as suggested in the literature, are found to dependon large corrections for variations of the light curve parameter and/or on anunwarranted reduction of the Cepheid distances of the calibrating SNe Ia.Comment: 42 pages, including 9 figures; submitted to Ap