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 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.
Highlights
One possible source of polarization in core-collapse supernovae (SNe) is the presence of 56Ni at high velocity in the ejecta but confined within a restricted solid angle (Chugai 1992, 2006; Dessart et al 2021
In 1D, this boost corresponds to a shell, with no associated polarization
If the 56Ni enhancement is limited to a confined blob, a sizable polarized flux may result from the asymmetry introduced in the distribution of scatterers, but with little influence on the total flux
Summary
One possible source of polarization in core-collapse supernovae (SNe) is the presence of 56Ni at high velocity in the ejecta but confined within a restricted solid angle (Chugai 1992, 2006; Dessart et al 2021). We tend to use interchangeably the 56Ni blob and its associated electron-density enhancement because in our approach the 56Ni is microscopically mixed with other species at any ejecta location In nature, this microscopic mixing does not take place during the dynamical phase of the explosion, so that a 56Ni blob would be essentially pure 56Ni and be surrounded by material with a distinct composition (a high-velocity 56Ni blob would be surrounded by material rich in H and He in a Type II SN – i.e., a “cocoon,” as discussed in the previous section). A high-velocity 56Ni-rich blob should retain its original composition (the one it had at the time of explosion) but be surrounded by the H-rich material present in the outer ejecta This effect is not captured in the CMFGEN simulation above since we performed a macroscopic and a microscopic 2 In the work of Gabler et al (2021), radiative losses are neglected and the decay heating is treated as local. One explanation for this is that these lines form in very different regions of the ejecta, with O I 6300 Å forming deep in the ejecta (and being well reflected by the distant blob) whereas Hα forms over a large volume overlapping in part with the blob location – the blob is not exterior to the emitting region for a significant fraction of Hα photons
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