Nebular Spectra and Explosion Asymmetry of Type Ia Supernovae
  The spectral signatures of asymmetry in Type Ia Supernova (SN Ia) explosions
are investigated, using a sample of late-time nebular spectra. First, a
kinematical model is constructed for SN Ia 2003hv, which can account for the
main features in its optical, Near-Infrared (NIR), and Mid-Infrared (Mid-IR)
late-time spectra. It is found that an asymmetric off-center model can explain
the observed characteristics of SN 2003hv. This model includes a relatively
high density, Fe-rich region which displays a large velocity off-set, and a
relatively low density, extended 56Ni-rich region which is more spherically
distributed. The high density region consists of the inner stable Fe-Ni region
and outer 56Ni-rich region. Such a distribution may be the result of a
delayed-detonation explosion, in which the first deflagration produces the
global asymmetry in the innermost ejecta, while the subsequent detonation can
lead to the bulk spherical symmetry. This configuration, if viewed from the
direction of the off-set, can consistently explain the blueshift in some of the
emission lines and virtually no observed shift in other lines in SN 2003hv. For
this model, we then explore the effects of different viewing angles and the
implications for SNe Ia in general. The model predicts that a variation of the
central wavelength, depending on the viewing angle, should be seen in some
lines (e.g., [Ni II]7378), while the strongest lines (e.g., [Fe III] blend at
4700A) will not show this effect. By examining optical nebular spectra of 12
SNe Ia, we have found that such a variation indeed exists. We suggest that the
global asymmetry in the innermost ejecta, as likely imprint of the deflagration
flame propagation, is a generic feature of SNe Ia (abridged).
