Three-dimensional models for high-velocity features in Type Ia supernovae

Spectral synthesis in three-dimensional space for the earliest spectra of Type Ia supernovae (SNe Ia) is presented. In particular, the high-velocity absorption features that are commonly seen at the earliest epochs (similar to 10 days before maximum light) are investigated by means of a three-dimensional Monte Carlo spectral synthesis code. The increasing number of early spectra available allows statistical study of the geometry of the ejecta. The observed diversity in strength of the high-velocity features (HVFs) can be explained in terms of a "covering factor,'' which represents the fraction of the photosphere that is concealed by high-velocity material. Various geometric models involving high-velocity material with a clumpy structure or a thick torus can naturally account for the observed statistics of HVFs. HVFs may be formed by a combination of density and abundance enhancements. Such enhancements may be produced in the explosion itself or may be the result of interaction with circumstellar material or an accretion disk. Models with one or two blobs, as well as a thin torus or disklike enhancement, are unlikely as a standard situation.