Exploring the origins of high-velocity features in SNe Ia with the spectral synthesis code TARDIS

¹ School of Physics, Trinity College Dublin, College Green Dublin 2, Ireland
² Heidelberg Institute for Theoretical Studies Schloss-Wolfsbrunnenweg 35 69118 Heidelberg, Germany
³ European Southern Observatory, Alonso de Córdova 3107 Casilla 19 Santiago, Chile
⁴ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off.104 Providencia Santiago, Chile
⁵ Graduate Institute of Astronomy, National Central University 300 Jhongda Road 32001 Jhongli, Taiwan
⁶ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC) 08860 Castelldefels (Barcelona), Spain
⁸ CENTRA, Instituto Superior Técnico, Universidade de Lisboa Av. Rovisco Pais 1 1049-001 Lisboa, Portugal
⁹ Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland
¹⁰ Instituto de Alta Investigación, Universidad de Tarapacá Casilla 7D Arica, Chile

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Received: 4 April 2025
Accepted: 12 December 2025

Abstract

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.