Spectroscopy of twelve type Ia supernovae at intermediate redshift

We present spectra of twelve type Ia supernovae obtained in 1999 at the William Herschel Telescope and the Nordic Optical Telescope during a search for type Ia supernovae (SN Ia) at intermediate redshift. The spectra range from z=0.178 to z=0.493, including five high signal-to-noise ratio SN Ia spectra in the still largely unexplored range 0.15≤ z ≤ 0.3. Most of the spectra were obtained before or around restframe B-band maximum light. None of them shows the peculiar spectral features found in low-redshift over- or under-luminous SN Ia. Expansion velocities of characteristic spectral absorption features such as Si ii at 6355 Å, S ii at 5640 Å and Ca ii at 3945 Å are found to be consistent with those of their low-z SN Ia counterparts.