A. A. Hakobyan, M. H. Gevorgyan, A. G. Karapetyan, G. A. Mamon, D. Kunth, V. Zh. Adibekyan, L. V. Barkhudaryan
Abstract
Photospheric and high-velocity features (PVFs and HVFs) of Si II λ6355 and Ca II IR3 lines in supernova Ia (SN Ia) spectra provide insights into ejecta structure, energetics, and circumstellar interaction, yet their interplay remains poorly understood. We analyse a representative sample of 145 nearby SNe Ia observed within ±5 d of B-band maximum light, including normal, 91T-, and 91bg-like events with measured light-curve decline rates (Δm15) and Si II and Ca II line properties from the literature. We model PVF and HVF velocity distributions using Gaussian Mixture Models, compare Si II and Ca II PVF velocity distributions, assess Ca II HVF properties, and test correlations between Si II PVF velocities and Δm15, with emphasis on HVF effects. For the first time, we show that the Ca II PVF velocity distribution, measured for the same events at the same phases as Si II, is predominantly unimodal, in contrast to the well-known bimodal Si II PVF distribution that supports the high-velocity/normal-velocity division. This contrast likely reflects a subclass-dependent formation depth of the Ca II line, as supported by a positive correlation (>3.3σ) between Δm15 and the velocity offset between Ca II and Si II PVFs, particularly in faster-declining SNe Ia. Importantly, HVFs do not significantly bias PVF velocity distributions. A significant negative correlation (>3.3σ) between Si II PVF velocity and Δm15 is found only for HVF-weak SNe Ia, consistent with more energetic explosions yielding faster ejecta, while this trend vanishes in HVF-strong events, likely due to circumstellar interaction. These results underscore the critical role of HVFs and SN Ia subclass in interpreting ejecta kinematics in both models and observations.
Keywords
methods: data analysis / supernovae: individual: Type Ia
Monthly Notices of the Royal Astronomical Society
Volume 546, Issue staf2281
2026 February
