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 $\\sim 12$ days after the $B$ maximum denoted by $\\Delta C_{12}$. By introducing such a novel color parameter, $\\Delta C_{12}$, this empirical correlation allows us to uniformly standardize SNe Ia with decline rates $\\Delta m_{15}$ in the range of $0.8<\\Delta m_{15}<2.0$ and to reduce scatters in estimating their peak luminosities from $\\sim 0.5$ 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$(B-V)_{host}\\lsim 0.06$ 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.

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