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

   https://doi.org/10.1051/0004-6361/202140409  

   Dessart, Luc; John Hillier, D.; Leonard, Douglas C. (July 2021, Astronomy & Astrophysics)

   null (Ed.)

   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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2. Multiepoch VLT–FORS spectropolarimetric observations of supernova 2012aw reveal an asymmetric explosion

   https://doi.org/10.1051/0004-6361/202140281  

   Dessart, Luc; Leonard, Douglas C.; John Hillier, D.; Pignata, Giuliano (July 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present VLT–FORS spectropolarimetric observations of the type II supernova (SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to 120 d after explosion. We corrected for interstellar polarization by postulating that the SN polarization is naught near the rest wavelength of the strongest lines – this is later confirmed by our modeling. SN 2012aw exhibits intrinsic polarization, with strong variations across lines, and with a magnitude that grows in the 7000 Å line-free region from 0.1% at 16 d up to 1.2% at 120 d. This behavior is qualitatively similar to observations gathered for other type II SNe. A suitable rotation of Stokes vectors places the bulk of the polarization in q , suggesting the ejecta of SN 2012aw is predominantly axisymmetric. Using an upgraded version of our 2D polarized radiative transfer code, we modeled the wavelength- and time-dependent polarization of SN 2012aw. The key observables may be explained by the presence of a confined region of enhanced 56 Ni at ~4000 km s −1 , which boosts the electron density in a cone having an opening angle of ~50 deg and an observer’s inclination of ~70 deg to the axis of symmetry. With this fixed asymmetry in time, the observed evolution of the SN 2012aw polarization arises from the evolution of the ejecta optical depth, ionization, and the relative importance of multiple versus single scattering. However, the polarization signatures exhibit numerous degeneracies. Cancellation effects at early times imply that low polarization may even occur for ejecta with a large asymmetry. An axisymmetric ejecta with a latitudinal-dependent explosion energy can also yield similar polarization signatures as asymmetry in the 56 Ni distribution. In spite of these uncertainties, SN 2012aw provides additional evidence for the generic asymmetry of type II SN ejecta, of which VLT–FORS spectropolarimetric observations are a decisive and exquisite probe. 

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3. Early Time Spectropolarimetry of the Aspherical Type II Supernova SN 2023ixf

   https://doi.org/10.3847/2041-8213/acf1a3  

   Vasylyev, Sergiy S.; Yang 杨, Yi 轶.; Filippenko, Alexei V.; Patra, Kishore C.; Brink, Thomas G.; Wang, Lifan; Chornock, Ryan; Margutti, Raffaella; Gates, Elinor L.; Burgasser, Adam J.; et al (October 2023, The Astrophysical Journal Letters)

   Abstract We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from ∼2 to 15 days after the explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory, representing the earliest such observations ever captured for an SN. We observe a high continuum polarizationp_cont≈ 1% on days +1.4 and +2.5 before dropping to 0.5% on day +3.5, persisting at that level up to day +14.5. Remarkably, this change coincides temporally with the disappearance of highly ionized “flash” features. The decrease of the continuum polarization is accompanied by a ∼70° rotation of the polarization position angle (PA) as seen across the continuum. The early evolution of the polarization may indicate different geometric configurations of the electron-scattering atmosphere as seen before and after the disappearance of the emission lines associated with highly ionized species (e.g., Heii, Civ, and Niii), which are likely produced by elevated mass loss shortly prior to the SN explosion. We interpret the rapid change of polarization and PA from days +2.5 to +4.5 as the time when the SN ejecta emerge from the dense asymmetric circumstellar material (CSM). The temporal evolution of the continuum polarization and the PA is consistent with an aspherical SN explosion that exhibits a distinct geometry compared to the CSM. The rapid follow-up spectropolarimetry of SN 2023ixf during the shock ionization phase reveals an exceptionally asymmetric mass-loss process leading up to the explosion. 

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4. The Type Ibn Supernova 2019kbj: Indications for Diversity in Type Ibn Supernova Progenitors

   https://doi.org/10.3847/1538-4357/acb432  

   Ben-Ami, Tom; Arcavi, Iair; Newsome, Megan; Farah, Joseph; Pellegrino, Craig; Terreran, Giacomo; Burke, Jamison; Hosseinzadeh, Griffin; McCully, Curtis; Hiramatsu, Daichi; et al (March 2023, The Astrophysical Journal)

   Abstract Type Ibn supernovae (SNe) are a rare class of stellar explosions whose progenitor systems are not yet well determined. We present and analyze observations of the Type Ibn SN 2019kbj, and model its light curve in order to constrain its progenitor and explosion parameters. SN 2019kbj shows roughly constant temperature during the first month after peak, indicating a power source (likely circumstellar material interaction) that keeps the continuum emission hot at ∼15,000 K. Indeed, we find that the radioactive decay of⁵⁶Ni is disfavored as the sole power source of the bolometric light curve. A radioactive decay + circumstellar material (CSM) interaction model, on the other hand, does reproduce the bolometric emission well. The fits prefer a uniform-density CSM shell rather than CSM due to a steady mass-loss wind, similar to what is seen in other Type Ibn SNe. The uniform-density CSM shell model requires ∼0.1M_⊙of⁵⁶Ni and ∼1M_⊙total ejecta mass to reproduce the light curve. SN 2019kbj differs in this manner from another Type Ibn SN with derived physical parameters, SN 2019uo, for which an order of magnitude lower⁵⁶Ni mass and larger ejecta mass were derived. This points toward a possible diversity in SN Ibn progenitor systems and explosions. 

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5. Spectropolarimetry of the Type IIP supernova 2021yja: an unusually high continuum polarization during the photospheric phase

   https://doi.org/10.1093/mnras/stad3352  

   Vasylyev, Sergiy S; Yang, Yi; Patra, Kishore C; Filippenko, Alexei V; Baade, Dietrich; Brink, Thomas G; Hoeflich, Peter; Maund, Justyn R; Patat, Ferdinando; Wang, Lifan; et al (November 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from ∼25 to 95 d after the explosion. An unusually high continuum linear polarization of $$p \approx 0.9~{{\ \rm per\ cent}}$$ is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behaviour has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the Hα and Ca ii near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter (CSM), although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density CSM. 

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