A Linear Relation between the Color Stretch s BV and the Rising Color Slope s0*(B−V) of Type Ia Supernovae

Using data from the Complete Nearby (redshift z host < 0.02) sample of Type Ia Supernovae (CNIa0.02), we find a linear relation between two parameters derived from the B − V color curves of Type Ia supernovae: the color stretch s BV and the rising color slope s 0 * ( B − V ) after the peak, and this relation applies to the full range of s BV . The s BV parameter is known to be tightly correlated with the peak luminosity, especially for fast decliners (dim Type Ia supernovae), and the luminosity correlation with s BV is markedly better than with the classic light-curve width parameters such as Δm 15(B). Thus, our new linear relation can be used to infer peak luminosity from s 0 * . Unlike s BV (or Δm 15(B)), the measurement of s 0 * ( B − V ) does not rely on a well-determined time of light-curve peak or color maximum, making it less demanding on the light-curve coverage than past approaches.