Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae

¹ Institut d’Astrophysique de Paris, CNRS-Sorbonne Université, 98bis boulevard Arago, 75014 Paris, France
e-mail: dessart@iap.fr
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

Received: 24 January 2021
Accepted: 1 May 2021

Abstract

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 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 − cosα_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 (∝ sin²α_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.