AN EARLY-TIME INFRARED AND OPTICAL STUDY OF THE TYPE IA SUPERNOVAE SN1994D AND 1991T

We present early-time infrared (IR) and optical spectroscopy, and opti cal photometry, of the Type Ia supernova 1994D. These observations pro vide the most complete optical-IR spectral coverage ever achieved for a Type Ia at this phase. Optical and IR spectra were obtained as early as 9 d before maximum light. The combined optical and IR spectra of S N 1994D reveal a flux 'deficit' in the R, I and J bands as early as ma ximum light, They also illustrate the dramatic deepening of the J-band deficit after maximum light. We also present a maximum light IR spect rum of the peculiar Type Ia SN 1991T. This also shows a deficit in the J band, but it does not show such a pronounced deficit in the R and I bands as SN 1994D. Both supernovae show a P Cygni-like feature with t he absorption at similar to 1.05 mu m and the emission at similar to 1 .08 mu m. In SN 1994D the absorption shows no wavelength shift during the period between -8.5 and -1.5 d before maximum light. After this th e feature rapidly weakened. We argue that, in this event, the P Cygni line feature formed in a discrete shell-like zone lying above the phot osphere. In SN 1991T a continuous, shallow density gradient scattering zone seems more appropriate. For both supernovae we explore possible identifications with He I 1.0830 mu m and Mg II 1.0926 mu m, but there are difficulties with either option. In SN 1994D it may be that the f eature is actually the result of a blend of the helium and magnesium l ines. However, for both supernovae it is not ruled out that the J-band feature is really the result of a transition in one or more unidentif ied species. Consideration of the velocities associated with the featu re indicates that, if its origin is helium, then in SN 1994D it is mor e likely that it formed in the alpha-rich freeze-out, but some mixing to higher velocities is also required. In contrast, in SN 1991T we fav our an origin in the form of a residual or accreted layer on the surfa ce of the progenitor white dwarf. If the feature is due to magnesium t hen the derived velocities for SN 1994D are in good agreement with the predictions of explosion model W7. In SN 1991T, identification with m agnesium presents problems in accounting for the inferred velocity str ucture.