Volume 651, July 2021
|Number of page(s)||19|
|Published online||01 July 2021|
Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae
Institut d’Astrophysique de Paris, CNRS-Sorbonne Université,
98bis boulevard Arago,
2 Department of Physics and Astronomy & Pittsburgh Particle Physics, Astrophysics, and Cosmology Center (PITT PACC), University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA
3 Department of Astronomy, San Diego State University, San Diego, CA 92182-1221, USA
Accepted: 1 May 2021
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 56Ni “blob” that breaks spherical symmetry. Our ansatz, supported by nonlocal thermodynamic equilibrium radiative transfer calculations, is that the primary role of such a 56Ni 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 Ne, fac, its velocity location Vblob, and its spatial extent. For plausible Ne, fac values of a few tens, a high-velocity blob can deliver a continuum polarization Pcont of 0.5–1.0% at 200 d. Our simulations reproduce the analytic scalings for Pcont, 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 Vblob, the polarized spectrum appears as a replica of the full spectrum, scaled down by a factor of 100–1000 (i.e., 1∕Pcont) and redshifted by an amount Vblob (1 − cosαlos), where αlos is the line-of-sight angle. As Vblob 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 (∝ sin2αlos), the polarization preferentially reveals asymmetries in the plane perpendicular to the line-of-sight (αlos = 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.
Key words: radiative transfer / polarization / supernovae: general
© L. Dessart et al. 2021
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