The outermost ejecta of Type Ia supernovae

The properties of the highest velocity ejecta of normal Type Ia supernovae (SNe Ia) are studied via models of very early optical spectra of six SNe. At epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si ii k6355 line velocity (HVG) have a larger photospheric velocity than SNe with a slowly evolving Si ii k6355 line velocity (LVG). Since the two groups have comparable luminosities, the temperature at the photosphere is higher in LVG SNe. This explains the different overall spectral appearance of HVG and LVG SNe. However, the variation of the Ca ii and Si ii absorptions at the highest velocities (vk20;000 km s 1) suggests that additional factors, such as asphericity or different abundances in the progenitor white dwarf, affect the outermost layers. The C ii k6578 line is marginally detected in three LVG SNe, suggesting that LVGs undergo less intense burning. The carbon mass fraction is small, only less than 0.01 near the photosphere, so that he mass of unburned C is only P0.01M . Radioactive 56Ni and stable Fe are detected in both LVG and HVG SNe. Different Fe-group abundances in the outer layers may be one of the reasons for spectral diversity among SNe Ia at the earliest times. The diversity among SNe Ia at the earliest phases could also indicate an intrinsic dispersion in the LC width-luminosity relation.M. T. is supported by the JSPS (Japan Society for the Promotion of Science) Research Fellowship for Young Scientists. G. P. acknowledges support by the Proyecto FONDECYT 3070034. This research was supported in part by the Grant-in- Aid for Scientific Research (18104003, 18540231) and the 21st Century COE Program (QUEST) from the JSPS and MEXT of Japan