Evidence for a correlation between the Si II 4000 width and Type Ia
  supernova color
  We study the pseudo equivalent width of the Si II 4000 feature of Type Ia
supernovae in the redshift range 0.0024 < z < 0.634. We find that this spectral
indicator correlates with the lightcurve color excess (SALT2 c) as well as
previously defined spectroscopic subclasses (Branch types) and the evolution of
the Si II 6150 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
4000 spectral indicator could provide important information to improve
cosmological distance measurements with Type Ia supernovae.
