Supernova Type Ia Luminosities, Their Dependence on Second Parameters, and the Value of H0

A sample of 35 Type Ia supernovae (SNe Ia) with good to excellent photometry in B and V, minimum internal absorption, and 1200 < v ≲ 30,000 km s-1 is compiled from the literature. As far as their spectra are known, they are all Branch-normal. For 29 of the SNe Ia, peak magnitudes in I are also known. The SNe Ia have uniform colors at maximum, i.e., = -0.012 mag (σ = 0.051) and = -0.276 mag (σ = 0.078). In the Hubble diagram, they define a Hubble line with a scatter of σM = 0.21-0.16 mag, decreasing with wavelength. The scatter is further reduced if the SNe Ia are corrected for differences in decline rate, Δm15, or color (B-V). A combined correction reduces the scatter to σ ≲ 0.13 mag. After the correction, no significant dependence remains on Hubble type or Galactocentric distance. The Hubble line suggests some curvature that can be differently interpreted. A consistent solution is obtained for a cosmological model with ΩM = 0.3, ΩΛ = 0.7, which is also indicated by much more distant SNe Ia. Absolute magnitudes are available for eight equally blue (Branch-normal) SNe Ia in spirals whose Cepheid distances are known. If their well-defined mean values of MB, MV, and MI are used to fit the Hubble line to the above sample of SNe Ia, one obtains H0 = 58.3 km s-1 Mpc-1, or, after adjusting all SNe Ia to the average values of Δm15 and (B-V), H0 = 60.9 km s-1 Mpc-1. Various systematic errors are discussed whose elimination tends to decrease H0. The value finally adopted at the 90% level, including random and systematic errors, is H0 = 58.5 ± 6.3 km s-1 Mpc-1. Several higher values of H0 from SNe Ia, as suggested in the literature, are found to depend on large corrections for variations of the light-curve parameter and/or on an unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.