Line Polarization of Si II $λ$6355 Å in Type Ia Supernovae: A New Statistical Approach to Probe the Explosion Physics and Diversity

TL;DR

This work uses pre-maximum spectropolarimetry of $24$ SNe Ia and state-of-the-art 3D non-LTE radiative-transfer simulations to diagnose explosion geometry via Si II $\lambda$6355 polarization. It tests two main asphericity classes: Class I (off-center, lopsided abundances from delayed-detonation or WD collisions) and Class II (global density asphericities from rotation or mergers), linking polarization to ejecta velocity and brightness decline $\Delta m_{15}$. The results show normal-bright and transitional SNe Ia are largely explained by Class I geometries, Subluminous SNe Ia by Class II, and that viewing angle strongly shapes the observed $P_{\text{SiII}}$, with potential small contributions from Class III clumpiness. The findings imply a viewing-angle dependent luminosity component in local SNe Ia, with important implications for precision cosmology at high redshift and a framework for disentangling explosion physics from observational perspective.

Abstract

Spectropolarimetry provides a unique probe of ejecta asphericities, offering direct insights into the underlying explosion physics of Type Ia supernovae (SNe Ia). We analyze the statistical properties of pre-maximum spectropolarimetric data for 24 SNe Ia observed with VLT/FORS, focusing on the Si II $λ$6355 Åline. Previous studies have revealed a correlation between the peak Si II polarization degree and the expansion velocity. Here, we combine these observations with multi-dimensional non-LTE radiative transfer simulations. We consider two asphericity classes: (i) lopsided abundance distributions produced by off-center delayed-detonation transitions in near-$M_{Ch}$ white dwarfs or, for example, WD collisions (Class I), and (ii) global, axisymmetric density asphericities such as those arising from explosions of rapidly rotating WDs or mergers (Class II). Our model grid spans normal to subluminous SNe Ia and successfully reproduces the observed Si II velocity-polarization trend, with higher velocities associated with stronger asphericities. Consistent with observations, transitional SNe Ia and the faint end of the normal SNe Ia population show the highest Si II polarization and are best explained by Class I scenarios. In contrast, subluminous SNe Ia are dominated by Class II asphericities, characterized by lower Si II polarization but significant continuum polarization. The observed distribution of Si II polarization depends on both the observer's viewing angle $θ$ and the intrinsic asphericity. Statistical analysis of these spectropolarimetric snapshots enables the separation of Class I and Class II contributions and highlights the intrinsic diversity among SNe Ia. Our results imply viewing-angle-dependent luminosities in our local sample, which may have implications when using high-redshift SNe Ia as evidence for the need of non-standard cosmology.

Figures (5)

• Figure 1: Maximum linear polarization of the Si ii$\lambda$6355 Å line between -11 and +1 days relative to peak brightness, as a function of the interpolated Si ii$\lambda$6355 Å velocity at -5 days relative to B-band peak brightness. The black solid line shows the linear least-squares fit to the data, while the dashed lines indicate the 1$\sigma$ uncertainty. The figure is reproduced from Cikota2019MNRAS.490..578C.
• Figure 2: Left: Illustration of two model classes of asphericities: (i) lopsided large-scale configurations from off-center detonations (Class I) and (ii) global density asphericities (Class II), adapted from 2001ApJ...556..302H. Right: Schematic illustration of polarization formation in an off-center DDT, viewed from different angles (adapted from 2006NewAR..50..470H). The measured net polarization at a viewing angle of +90 degrees is zero, owing to the nearly spherical distribution of absorbing material (e.g., Silicon). At +30 degrees, the aspherical distribution produces strong line polarization and pronounced loops in the Stokes $Q$-$U$ plane, whereas at -90 degrees, small clumps produce weaker polarization and smaller loops. Any combination with the mechanisms illustrated in the left figures will produce loops in time series in the $Q$-$U$ plane (except in the case of a purely axisymmetric geometry), similar to what has been observed (e.g. 1997ApJ...476L..27WCikota2019MNRAS.490..578C2020ApJ...902...46Y).
• Figure 3: Comparison of the peak polarization in Si ii as a function of the expansion velocity 5 days before peak brightness in the $B$ band (left panel), and the expansion velocity as a function of the luminosity decline rate (right panel), for a series of models from Hoeflich_2017, compared to observations from Cikota2019MNRAS.490..578C. Model 10 assumes an elliptical oblate density structure with an axis ratio of A/B = 0.82 and reproduces subluminous SNe Ia. Models HeD10 and HeD6 represent sub-$\rm M_{Ch}$ SNe Ia, which include detonating He layers. Model 16 represents transitional SNe Ia, and models 23 and 25 represent normal-bright SNe Ia. Note that the polarization results for a viewing angle of +90 degrees are 0%, and are not plotted. Such low polarization occurs only within a very narrow cone around the north pole ($\Delta \theta \approx 5^o$) and is expected in only about 1/50 SNe Ia. In the models, off-center delayed-detonation explosions viewed from angles near $\theta \rightarrow +90^\circ$ produce high expansion velocities for a given $\Delta$m$_{15}$ and low polarization ($\rm P_{\text{Si\,ii}}$$\approx 0$).
• Figure 4: Observed (blue) vs. intrinsic (red) polarization assuming maximum entropy for our SN sample assuming $\sim 0.1\%$ polarization (see text). The x-axis lists individual SNe Ia, sorted by their $\rm P_{\text{Si\,ii}}$ polarization degree. Note that the differences between the observed and intrinsic polarization degrees are small, comparable to the intrinsic errors and the assumed polarization due to small-scale clumps.
• Figure 5: Viewing angle dependence of the $\rm P_{\text{Si\,ii}}$ polarization. Left: Normalized Si ii polarization (peak $\rm P_{\text{Si\,ii}}$ = 1) produced by abundance inhomogeneities in an off-center DDT model (Class I) vs. ellipsoidal density structure (Class II) as a function of viewing angle $\theta$. In addition, we give the likelihood $F\propto \cos^2(\theta)$ that a SN Ia is observed from a direction $\theta$. As examples we show the normal-bright Model 23 with $\rm M_{DDT}\,=\,0.3\,M_{WD}$2023MNRAS.520..560H and the HeD10 model with an axis ratio of 0.68 (see Table \ref{['table:simulationsresults']}). Note that all models of the same class have very similar $\rm P_{\text{Si\,ii}}$$(\theta)$. Right: Distributions of the observed intrinsic maximum polarization (black dots, with observed uncertainties in N(SN) given in light and dark gray), the normal-bright models 23 with $\rm M_{DDT}\,=\,0.3\ \&\ 0.6\,M_{WD}$ (Class I, blue and red hexagons, respectively), and the normal-bright HeD10 with axis ratios of 0.82 and 0.68 (Class II, light and dark purple dots). The distribution of the off-center delayed-detonation models has been normalized to the total number of observations, with a fraction of $\approx 15 \%$ based on the high-polarization observations.