Calcium versus silicon ejecta velocities and decline rates in supernovae Ia: The role of high-velocity features

TL;DR

We address how photospheric-velocity features (PVFs) and high-velocity features (HVFs) in Si II λ6355 and Ca II IR3 inform SN Ia ejecta structure and explosion physics. We analyze phase-matched PVF/HVF measurements for 145 nearby SNe Ia within ±5 days of B-band maximum using Gaussian Mixture Models to separate NV/HV components in Si II and to assess Ca II PVF/HVF properties across subclasses. The results show a robust bimodality in the Si II PVF distribution (normal-velocity and high-velocity components) while Ca II PVF is predominantly unimodal, with Ca II PVF bimodality emerging only at low Δm15, indicating formation-depth differences; HVFs do not significantly bias PVF distributions. A significant negative correlation between HVF strength and Ca II PVF velocity, together with a Δm15-linked offset between Ca II and Si II PVFs that strengthens for faster-declining SNe, indicates that HVFs and circumstellar interaction modulate observed kinematics and must be accounted for when interpreting SN Ia explosions. Overall, subclass and HVF physics are crucial for interpreting ejecta kinematics and constraining progenitor and explosion scenarios, motivating broader temporal coverage and direct CSM diagnostics in future work.

Abstract

Photospheric and high-velocity features (PVFs and HVFs) of Si II $λ$6355 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 $\pm$5 days of B-band maximum light, including normal, 91T-, and 91bg-like events with measured light-curve decline rates ($Δm_{15}$) 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 $Δm_{15}$, 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 $Δm_{15}$ 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 $Δm_{15}$ 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.

Figures (7)

• Figure 1: Left: Distributions of PVF velocities for the Si ii$\lambda$6355 and Ca ii IR3 lines for all SN Ia subclasses (black histograms). The bimodal and unimodal Gaussian fits to the distributions are presented by solid and dashed thick lines, respectively. The NV and HV components of the bimodal fit are indicated by red and blue thin lines, respectively. The insets display the same distributions and corresponding fits, but restricted to normal SNe Ia (green histograms). Right: Distributions of $V^{\rm PVF}_{\rm Si}$ and $V^{\rm PVF}_{\rm Ca}$, with overlaid smoothed histograms, separated by SN Ia subclasses: normal (green), 91T- (blue), and 91bg-like (red) SNe Ia. Arrows indicate the means of the distributions, with their standard deviations shown as horizontal error bars. The inset presents the histograms of phases at which the velocity measurements were obtained for each SN Ia subclass.
• Figure 2: Histogram of redshifts for all SNe Ia in our sample, divided into Near (red solid) and Far (blue dashed) subsamples using the median redshift (indicated by the vertical dashed line) as the division point. The median redshift of 0.019 is marked with an arrow. The upper and bottom insets display the CDFs of PVF velocities for the Si ii and Ca ii lines, respectively, separated according to the Near and Far subsamples.
• Figure 3: Comparison of PVF velocities of the Si ii$\lambda$6355 and Ca ii IR3 lines for SN Ia subclasses. Mean $V^{\rm PVF}$ values for each subclass are indicated by larger symbols, with error bars representing their standard deviations. For reference, the diagonal line denotes the 1 to 1 relation.
• Figure 4: Upper panel: Histograms of the LC decline rate $(\Delta m_{15})$ for SNe Ia, separated by subclasses: normal (green), 91T-like (blue), and 91bg-like (red) events. The vertical dashed line at $\Delta m_{15}=1.3$ mag delineates the rough boundary for faster-declining SNe (see text for explanation). Bottom panel: The difference between Ca ii and Si ii PVF velocities $(V^{\rm PVF}_{\rm Ca} - V^{\rm PVF}_{\rm Si})$ for the same SNe plotted against $\Delta m_{15}$. Mean values for each subclass are indicated by larger symbols, with error bars reflecting the standard deviations. Best linear fits for all SNe Ia (solid black) and for normal events only (dashed green) are overlaid. For reference, the horizontal line denotes zero velocity difference. For all SNe, grey lines represent all $10^4$ linear fits to the MC realisations, illustrating the effect of measurement uncertainties on the correlation.
• Figure 5: Distribution of Ca ii PVF velocities for all SN Ia subclasses (black histogram), restricted to events with $\Delta m_{15} \leq 1.3$ mag. The statistically preferred bimodal Gaussian fit is shown by the solid line, and the unimodal Gaussian fit by the dashed line. Inset shows the same distribution and fits restricted to normal SNe Ia (green histogram).