Infrared and optical spectroscopy of Type Ia supernovae

The spectroscopic behaviour of Type Ia supernovae before and just after maximum light is of great interest since this covers the time when the photosphere recedes through most of the ejecta. While a number of optical spectra exist for this period, there are only a few early-time infrared spectra. Those few show a flux 'deficit' which spans 1.1-1.5 mu m and which deepens dramatically just after maximum light. Recent IR spectroscopy of SNe 1994D and 1995D shows that the deficit is already present a week before maximum light and is still present as late as 100 days post-maximum light. While a satisfactory explanation for the early-time behaviour has yet to be found, it seems likely that the deficit at later times is due to the smaller number of lines in the infrared region. A P-Cygni-like feature at 1.05/1.08 mu m was seen at early times in SNe 1991T and 1994D. We discuss its origin in terms of a detached shell of scattering material, possibly helium or magnesium.