The SNLS-VLT Type Ia Spectrum Evolution with Redshift: a Demographic Effect?

We build composite spectra at z <= 0.5 and z > 0.5 from the 3 year SuperNova Legacy Survey (SNLS) VLT spectral sample and study their differences around maximum light. We use 93 spectra near maximum of confirmed SNe Ia observed at the VLT between 2003 and 2006 as part of the 3 first years of operation of the SNLS. We find differences in the absorption depth of some intermediate mass elements (Ca II, Si ii). Average stretches of observed distributions are 0.975 +/- 0.016 and 0.983 +/- 0.014 for the z <= 0.5 and z > 0.5 sample respectively. We use the A+B model of Scannapieco & Bildsten (2005) to interpret these results in terms of a possible demographic evolution.