A novel color parameter as a luminosity calibrator for type Ia supernovae

Type Ia supernovae (SNe Ia) provide us with a unique tool for measuring extragalactic distances and determining cosmological parameters. As a result, the precise and effective calibration for peak luminosities of SNe Ia becomes extremely crucial and thus is critically scrutinized for cosmological explorations. In this Letter, we reveal clear evidence for a tight linear correlation between peak luminosities of SNe Ia and their B - V colors similar to 12 days after the B maximum denoted by DeltaC(12). By introducing such a novel color parameter, DeltaC(12), this empirical correlation allows us to uniformly standardize SNe Ia with decline rates Deltam(15) in the range of 0.8 < Delta m(15) < 2.0 and to reduce scatters in estimating their peak luminosities from similar to0.5 mag to the levels of 0.18 and 0.12 mag in the V and I bands, respectively. For a sample of SNe Ia with insignificant reddenings of host galaxies [e.g., E(B - V)(host) less than or similar to 0.06 mag], the scatter drops further to only 0.07 mag (or 3% - 4% in distance), which is comparable to observational accuracies and is better than other calibrations for SNe Ia. This would impact observational and theoretical studies of SNe Ia and cosmological scales and parameters.