Fast and Furious—High-velocity Features of Ca and Si in the Spectra of Type Ia Supernovae*

High-velocity absorption lines of Ca II and Si II (high-velocity features (HVFs)) are common in the early spectra of Type Ia supernovae (SNe Ia), but their physical origin is still poorly understood. We used 181 optical spectra of 56 SNe Ia that show HVFs in at least the Ca II near-infrared (NIR) triplet to better understand the nature of HVFs. To model the profiles of HVFs we applied Gaussian fitting as well as spectrum synthesis modeling with the SYNOW code. We found that Gaussian fitting usually underestimates the velocity of HVFs compared to SYNOW, and derived a linear formula for correction. In accord with many previous studies, the observed properties of HVFs (velocity, strength, and duration) were found to correlate with the general observables of SNe Ia (decline rate and spectral subtype). Brighter, slow-declining supernovae usually exhibit stronger, higher-velocity HVFs, while fainter objects typically show weak, low-velocity HVFs. SNe Ia in early-type galaxies showed weaker and slower HVFs compared to events in late-type galaxies in our sample. Comparing these observational findings with theoretical predictions, we conclude that the overall physical picture of HVFs is quite complex, and could be explained by multiple models. A double-detonation explosion model is promising as it is inherently capable of explaining the creation of high-velocity shells that may produce HVFs, but an asymmetric delayed detonation model or interaction with circumstellar material cannot be ruled out.