High-Velocity Features in Type Ia Supernova Spectra
  We use a sample of 58 low-redshift (z <= 0.03) Type Ia supernovae (SNe Ia)
having well-sampled light curves and spectra near maximum light to examine the
behaviour of high-velocity features (HVFs) in SN Ia spectra. We take advantage
of the fact that Si II 6355 is free of HVFs at maximum light in all SNe Ia,
allowing us to quantify the strength of HVFs by comparing the structure of
these two lines. We find that the average HVF strength increases with
decreasing light-curve decline rate, and rapidly declining SNe Ia (dm_15(B) >=
1.4 mag) show no HVFs in their maximum-light spectra. Comparison of HVF
strength to the light-curve colour of the SNe Ia in our sample shows no
evidence of correlation. We find a correlation of HVF strength with the
velocity of Si II 6355 at maximum light (v_Si), such that SNe Ia with lower
v_Si have stronger HVFs, while those SNe Ia firmly in the "high-velocity"
(i.e., v_Si >= 12,000 km/s) subclass exhibit no HVFs in their maximum-light
spectra. While v_Si and dm_15(B) show no correlation in the full sample of SNe
Ia, we find a significant correlation between these quantities in the subset of
SNe Ia having weak HVFs. In general, we find that slowly declining (low
dm_15(B)) SNe Ia, which are more luminous and more energetic than average SNe
Ia, tend to produce either high photospheric ejecta velocities (i.e., high
v_Si) or strong HVFs at maximum light, but not both. Finally, we examine the
evolution of HVF strength for a sample of SNe Ia having extensive pre-maximum
spectroscopic coverage and find significant diversity of the pre-maximum HVF
behaviour.
