Spectral properties of Type Ia supernovae up to z~0.3
  Aims: Spectroscopic observations of Type Ia supernovae obtained at the New
Technology Telescope (NTT) and the Nordic Optical Telescope (NOT), in
conjunction with the SDSS-II Supernova Survey, are analysed. We use spectral
indicators measured up to a month after the lightcurve peak luminosity to
characterise the supernova properties, and examine these for potential
correlations with host galaxy type, lightcurve shape, colour excess, and
redshift. Methods: Our analysis is based on 89 Type Ia supernovae at a redshift
interval z = 0.05 - 0.3, for which multiband SDSS photometry is available. A
lower-z spectroscopy reference sample was used for comparisons over cosmic
time. We present measurements of time series of pseudo equivalent widths and
line velocities of the main spectral features in Type Ia supernovae. Results:
Supernovae with shallower features are found predominantly among the
intrinsically brighter slow declining supernovae. We detect the strongest
correlation between lightcurve stretch and the Si ii 4000 absorption feature,
which also correlates with the estimated mass and star formation rate of the
host galaxy. We also report a tentative correlation between colour excess and
spectral properties. If confirmed, this would suggest that moderate reddening
of Type Ia supernovae is dominated by effects in the explosion or its immediate
environment, as opposed to extinction by interstellar dust.
