On spectral line profiles in Type Ia supernova spectra

We present a detailed analysis of spectral line profiles in Type Ia supernova (SN Ia) spectra. We focus on the feature at similar to 3500-4000 angstrom, which is commonly thought to be caused by blueshifted absorption of Ca H&K. Unlike some other spectral features in SN Ia spectra, this feature often has two overlapping (blue and red) components. It is accepted that the red component comes from photospheric calcium. However, it has been proposed that the blue component is caused by either high-velocity calcium (from either abundance or density enhancements above the photosphere of the supernova, SN) or Si II lambda 3858. By looking at multiple data sets and model spectra, focusing on spectra near maximum brightness, we conclude that the blue component of the Ca H&K feature is caused by Si II lambda 3858 for most SNe Ia, although high-velocity calcium is likely important for some SNe. The strength of the Si II lambda 3858 feature varies strongly with the light-curve shape of an SN. As a result, the velocity measured from a single-Gaussian fit to the full line profile correlates with light-curve shape. The velocity of the Ca H&K component of the profile does not correlate with light-curve shape, contrary to previous claims. We detail the pitfalls of assuming that the blue component of the Ca H&K feature is caused by calcium, with implications for our understanding of SN Ia progenitors, explosions and cosmology.