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 ∼ 12 days after the B maximum denoted by ∆C12. By introducing such a novel color parameter, ∆C12, this empirical correlation allows us to uniformly standardize SNe Ia with decline rates ∆m15 in the range of 0.8 < ∆m15 < 2.0 and to reduce scatters in estimating their peak luminosities from ∼ 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 < ∼ 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. Subject headings: cosmological parameters – cosmology: observations – distance scale – supernova: general National Astronomical Observatories of China, Chinese Academy of Sciences, A20, Datun Road, Beijing 100012, China Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing, 100084, China Lawrence Berkeley National Laboratory 50-232, 1 Cyclotron Road, Berkeley, CA 94720, USA Department of Astronomy and Astrophysics, the University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA Astronomy Department, Beijing Normal University, 19 Xinjiekouwai Dajie, Beijing, 100875, China