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Abstract Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia Supernovae (SNe Ia), observed with FORS on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si ii λ6355 Å line (p$$_{\rm Si\, {\small II}}$$) as a function of time which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the Δm15–p$$_{\rm Si\, {\small II}}$$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the Δm15–p$$_{\rm Si\, {\small II}}$$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si ii λ6355 Å line before maximum brightness and the Si ii line velocity and suggest that this, along with the Δm15–p$$_{\rm Si\, {\small II}}$$ relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the Δm15–v$$_{\rm Si\, {\small II}}$$ plane and found a dichotomy in the polarization properties between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe display evolution of loops in the q–u plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. Finally, we compared our statistical sample of the Si ii polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models.
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This content will become publicly available on December 24, 2026
Line Polarization of Si ii λ 6355 Å in Type Ia Supernovae: A New Statistical Approach to Probe the Explosion Physics and Diversity
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 premaximum spectropolarimetric data for 24 SNe Ia observed with the FOcal Reducer and low dispersion Spectrograph on the Very Large Telescope, focusing on the Siiiλ6355 Å line. Previous studies have revealed a correlation between the peak Siiipolarization degree and the expansion velocity. Here, we combine these observations with multidimensional nonthermodynamical equilibrium radiative transfer simulations. We consider two asphericity classes: (i) lopsided abundance distributions produced by off-center delayed-detonation transitions in near-MChwhite dwarfs (WDs) 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 Siiivelocity–polarization trend, with higher velocities associated with stronger asphericities. Consistent with observations, transitional SNe Ia and the faint end of the normal SN Ia population show the highest Siiipolarization and are best explained by class I scenarios. In contrast, subluminous SNe Ia are dominated by class II asphericities, characterized by lower Siiipolarization but significant continuum polarization. The observed distribution of Siiipolarization depends on both the observer’s viewing angleθand 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 for nonstandard cosmology.
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- Award ID(s):
- 2306395
- PAR ID:
- 10688902
- Publisher / Repository:
- chicago press
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 996
- Issue:
- 1
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L10
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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