Evidence for a correlation between the Si II λ width and type Ia supernova color

We study the pseudo-equivalent width of the Si II λ4000 feature of Type Ia supernovae (SNe Ia) in the redshift range 0.0024 ≤ z ≤ 0.634. We find that this spectral indicator correlates with the light curve color excess (SALT2c) as well as previously defined spectroscopic subclasses (Branch types) and the evolution of the Si II λ6150 velocity, i.e., the so-called velocity gradient. Based on our study of 55 objects from different surveys, we find indications that the Si II λ4000 spectral indicator could provide important information to improve cosmological distance measurements with SNe Ia. © 2011. The American Astronomical Society. All rights reserved.