Constraining the Asymmetry of Type Ia Supernovae using Late-time Spectra
Physical and Biological Sciences
Astrophysics
We use a relational database of Type Ia Supernovae (SNe Ia) spectra, kaepora, to probe the explosion physics of a sample of 55 SNe Ia. In particular, we examine their late-time (∼ 100 days or more after explosion) spectra, when the SNe are transparent, making bulk kinematic measurements feasible. We derive Doppler shifts, and thus line-of-sight velocities, from nickel and iron emission features in these spectra. We confirm a broad distribution of late-time velocities for the sample, which is an imprint of the explosion kinematics. We confirm the prediction from an asymmetric explosion model that events with redshifted nebular velocities are correlated with higher early-time velocities and vice versa. We infer this as an indication of an asymmetric explosion seen from different viewing angles. We investigate additional spectral differences between subsamples where the SN material is redshifted or blueshifted at late-times. Using kaepora, we create composite, or average, spectra acrosss all epochs. If the explosion has incomplete burning, some of the pristine carbon from the WD will remain in the ejecta, possibly visible as an absorption feature. About 1/3 of SNe Ia have detected carbon absorption features in their pre-maximum light spectra, which has been interpreted as incomplete burning in those particular SNe. In the pre-maximum light composite spectra, we find that the events with blueshifted late-time velocities show C II λ6580 absorption, while those with redshifted late-time velocities do not. We interpret this as all SNe Ia having the same overall amount of carbon burning,
but it is distributed asymmetrically where the side opposite the central offset has more complete burning. Through these observations, we have a unified model to explain the offset nebular lines, different maximum-light ejecta velocities, and presence of carbon as all being manifestations of an asymmetric explosion viewed from different lines of sight.
