The Reddening-Free Decline Rate Versus Luminosity Relationship for Type IA Supernovae

We develop a method for estimating the host galaxy dust extinction for type Ia supernovae based on an observational coincidence first noted by Lira, who found that the B-V evolution during the period from 30 to 90 days after V maximum is remarkably similar for all events, regardless of light-curve shape. This fact is used to calibrate the dependence of the B_max-V_max and V_max-I_max colors on the light-curve decline rate parameter Δm_15(B), which can, in turn, be used to separately estimate the host galaxy extinction. Using these methods to eliminate the effects of reddening, we reexamine the functional form of the decline rate versus luminosity relationship and provide an updated estimate of the Hubble constant of H_0=63.3+/-2.2(internal)+/-3.5(external) km s^-1 Mpc^-1.