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The linear polarization of the optical continuum of type II supernovae (SNe), together with its temporal evolution is a promising source of information about the large-scale geometry of their ejecta. To help access this information, we undertook 2D polarized radiative transfer calculations to map the possible landscape of type II SN continuum polarization (Pcont) from 20 to 300 days after explosion. Our simulations were based on crafted 2D axisymmetric ejecta constructed from 1D nonlocal thermodynamic equilibrium time-dependent radiative transfer calculations for the explosion of a red supergiant star. Following the approach used in our previous work on SN 2012aw, we considered a variety of bipolar explosions in which spherical symmetry is broken by material within ~30° of the poles that has a higher kinetic energy (up to a factor of two) and higher56Ni abundance (up to a factor of about five, allowing for56Ni at high velocity). Our set of eight 2D ejecta configurations produced considerable diversity inPcont(λ~ 7000 Å), although its maximum of 1–4% systematically occurs around the transition to the nebular phase. Before and after this transition,Pcontmay be null, constant, rising, or decreasing, which is caused by the complex geometry of the depth-dependent density and ionization and also by optical depth effects. Our modest angle-dependent explosion energy can yield aPcontof 0.5–1% at early times. Residual optical-depth effects can yield an angle-dependent SN brightness and constant polarization at nebular times. The observed values ofPconttend to be lower than obtained here. This suggests that more complicated geometries with competing large-scale structures cancel the polarization. Extreme asymmetries seem to be excluded.
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Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae
Type II supernovae (SNe) often exhibit a linear polarization, arising from free-electron scattering, with complicated optical signatures, both in the continuum and in lines. Focusing on the early nebular phase, at a SN age of 200 d, we conduct a systematic study of the polarization signatures associated with a 56 Ni “blob” that breaks spherical symmetry. Our ansatz, supported by nonlocal thermodynamic equilibrium radiative transfer calculations, is that the primary role of such a 56 Ni blob is to boost the local density of free electrons, which is otherwise reduced following recombination in Type II SN ejecta. Using 2D polarized radiation transfer modeling, we explore the influence of such an electron-density enhancement, varying its magnitude N e, fac , its velocity location V blob , and its spatial extent. For plausible N e, fac values of a few tens, a high-velocity blob can deliver a continuum polarization P cont of 0.5–1.0% at 200 d. Our simulations reproduce the analytic scalings for P cont , and in particular the linear growth with the blob radial optical depth. The most constraining information is, however, carried by polarized line photons. For a high V blob , the polarized spectrum appears as a replica of the full spectrum, scaled down by a factor of 100–1000 (i.e., 1∕ P cont ) and redshifted by an amount V blob (1 − cos α los ), where α los is the line-of-sight angle. As V blob is reduced, the redshift decreases and the replication deteriorates. Lines whose formation region overlaps with the blob appear weaker and narrower in the polarized flux. Because of its dependence on inclination (∝ sin 2 α los ), the polarization preferentially reveals asymmetries in the plane perpendicular to the line-of-sight ( α los = 90 deg). This property also weakens the broadening of lines in the polarized flux. With the adequate choice of electron-density enhancement, some of these results may apply to asymmetric explosions in general or to the polarization signatures from newly formed dust in the outer ejecta.
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- Award ID(s):
- 2010001
- PAR ID:
- 10280595
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 651
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A10
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202140409
