The Cepheid Calibration of Type Ia Supernovae as Standard Candles

Abstract

The experiment to establish the peak brightness of nearby supernovae of type Ia (SNe Ia) as standard candles is summarized. We show that while the entire set of SNe Ia can differ in peak brightness by over 1.5 mag, it is possible to define a subset based on color at maximum light, which has well behaved properties. By using second parameter corrections on this subset (aka the Parodi sample), the scatter in peak brightness in V is reduced to 0.13 mag rms, which makes these excellent standard candles. The results for the absolute calibration of the peak brightness of the Parodi sample via Cepheid distances to nearby SNe Ia host galaxies are presented. Procedural details that we have used to minimize the errors (on a case by case basis) in determining the peak brightness of these SNe Ia are described. We highlight the sources of the difference between our derived value of the Hubble constant H₀ = 60 kms⁻¹ Mpc⁻¹ and the 20% higher value obtained by the HST ‘key project’ group, and demonstrate the differences lie not in photometry, but in the details of the methodology, in how the problem of reddening in the host galaxy is handled and in external issues such as the calibration of the Cepheids themselves.