2026A&A...707A..57H

Appearing as secondary higher-velocity absorption components, high-velocity features (HVFs) have been observed in several absorption lines in many Type Ia supernovae (SNe Ia). The frequency and ubiquity of these components in silicon and calcium features specifically indicates that the mechanism through which they form must be a common occurrence among the majority of SNe Ia. Here we present the modelling of the HVF evolution in a sample of six well-observed SNe Ia with the radiative-transfer code TARDIS. A base model is derived for each of the SNe to reproduce the photospheric-velocity components, followed by a grid of simulations with Gaussian enhancements to the density profile at high velocities. We trained a set of neural networks to emulate the impact of these density enhancements upon the simulated silicon line profile. These networks were subsequently used within a Markov chain Monte Carlo (MCMC) framework to infer the density enhancement parameters that most closely reproduce the HVF evolution. While we obtain good matches for the silicon profile, we find that a single density enhancement alone cannot simultaneously produce the observed silicon and calcium HVF evolution. Our findings indicate that neither the delayed-detonation mechanism nor the double-detonation mechanism can produce these HVFs, which suggests that something may be missing from the models.