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1. The 3D shape of Type IIb SN 2011hs

   https://doi.org/https://ui.adsabs.harvard.edu/link_gateway/2019MNRAS.485..102S/doi:10.1093/mnras/stz265  

   H. F. Stevance1?, J.R. Maund1y (July 2019, Monthly notices of the Astronomical Society of London)

   We observed seven epochs of spectropolarimetry in optical wavelengths for the Type IIb SN 2011hs, ranging from 􀀀3 to +40 days with respect to V -band maximum. A high degree of interstellar polarization was detected (up to 3 percent), with a peak lying blueward of 4500A. Similar behaviours have been seen in some Type Ia SNe, but had never been observed in a Type IIb. We nd that it is most likely the result of a relative enhancement of small silicate grains in the vicinity of the SN. Signi cant intrinsic continuum polarization was recovered at 􀀀3 and +2 days (p = 0.55  0.12 percent and p = 0.75  0.11 percent, respectively). We discuss the change of the polarization angle across spectral lines and in the continuum as diagnostics for the 3D structure of the ejecta. We see a gradual rotation by about 􀀀50 in the continuum polarization angle between 􀀀2 and +18 days after V -band maximum. A similar rotation in He i 5876, H and the Ca ii infrared triplet seems to indicate a strong in uence of the global geometry on the line polarization features. The dierences in the evolution of their respective loops on the Stokes q 􀀀 u plane suggest that line speci c geometries are also being probed. Possible interpretations are discussed and placed in the context of literature. We nd that the spectropolarimetry of SN 2011hs is most similar to that of SN 2011dh, albeit with notable dierences. 

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2. Spectropolarimetry of the Type Ia SN 2019ein rules out significant global asphericity of the ejecta

   https://doi.org/10.1093/mnras/stab3136  

   Patra, Kishore C; Yang; Brink, Thomas G; Höflich, Peter; Wang, Lifan; Filippenko, Alexei V; Kasen, Daniel; Baade, Dietrich; Foley, Ryan J; Maund, Justyn R; et al (December 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Detailed spectropolarimetric studies may hold the key to probing the explosion mechanisms and the progenitor scenarios of Type Ia supernovae (SNe Ia). We present multi-epoch spectropolarimetry and imaging polarimetry of SN 2019ein, an SN Ia showing high expansion velocities at early phases. The spectropolarimetry sequence spans from ∼−11 to +10 d relative to peak brightness in the B band. We find that the level of the continuum polarization of SN 2019ein, after subtracting estimated interstellar polarization, is in the range 0.0–0.3 per cent, typical for SNe Ia. The polarization position angle remains roughly constant before and after the SN light-curve peak, implying that the inner regions share the same axisymmetry as the outer layers. We observe high polarization (∼1 per cent) across both the Si ii λ6355 and Ca ii near-infrared triplet features. These two lines also display complex polarization modulations. The spectropolarimetric properties of SN 2019ein rule out a significant departure from spherical symmetry of the ejecta for up to a month after the explosion. These observations disfavour merger-induced and double-detonation models for SN 2019ein. The imaging polarimetry shows weak evidence for a modest increase in polarization after ∼20 d since the B-band maximum. If this rise is real and is observed in other SNe Ia at similar phases, we may have seen, for the first time, an aspherical interior similar to what has been previously observed for SNe IIP. Future polarization observations of SNe Ia extending to post-peak epochs will help to examine the inner structure of the explosion. 

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3. The core normal Type Ia supernova 2019np – an overall spherical explosion with an aspherical surface layer and an aspherical 56Ni core

   https://doi.org/10.1093/mnras/stad172  

   Hoeflich, Peter; Yang, Yi; Baade, Dietrich; Cikota, Aleksandar; Maund, Justyn R.; Mishra, Divya; Patat, Ferdinando; Patra, Kishore C.; Wang, Lifan; Wheeler, J. Craig; et al (January 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14.5 to +14.5 d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0.21 ± 0.09 per cent at the first epoch. Between days −11.5 and  +0.5, pcont remained ∼0 and by day +14.5 was again significant at 0.19 ± 0.10 per cent. Not considering the first epoch, the dominant axis of $${\rm Si\, {\small II}}$$ λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day  +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont ≈ 0.2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3.5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the $${\rm Ca\, {\small II}}$$ near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to $${\rm Si\, {\small II}}$$ λ6355 and $${\rm Ca\, {\small II}}$$ NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN 2019np to other SNe and develop future objectives and strategies for SN Ia spectropolarimetry. 

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4. 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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5. Multi-epoch spectropolarimetry for a sample of Type IIn Supernovae: persistent asymmetry in dusty circumstellar material

   https://doi.org/10.1093/mnras/stae380  

   Bilinski, Christopher; Smith, Nathan; Williams, G. Grant; Smith, Paul S.; Leonard, Douglas C.; Hoffman, Jennifer L.; Andrews, Jennifer E.; Milne, Peter (February 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present multi-epoch spectropolarimetry and spectra for a sample of 14 Type IIn supernovae (SNe IIn). We find that after correcting for likely interstellar polarization, SNe IIn commonly show intrinsic continuum polarization of 1–3 per cent at the time of peak optical luminosity, although a few show weaker or negligible polarization. While some SNe IIn have even stronger polarization at early times, their polarization tends to drop smoothly over several hundred days after peak. We find a tendency for the intrinsic polarization to be stronger at bluer wavelengths, especially at early times. While polarization from an electron scattering region is expected to be grey, scattering of SN light by dusty circumstellar material (CSM) may induce such a wavelength-dependent polarization. For most SNe IIn, changes in polarization degree and wavelength dependence are not accompanied by changes in the position angle, requiring that asymmetric pre-SN mass loss had a persistent geometry. While 2–3 per cent polarization is typical, about 30 per cent of SNe IIn have very low or undetected polarization. Under the simplifying assumption that all SN IIn progenitors have axisymmetric CSM (i.e. disc/torus/bipolar), then the distribution of polarization values we observe is consistent with similarly asymmetric CSM seen from a distribution of random viewing angles. This asymmetry has very important implications for understanding the origin of pre-SN mass loss in SNe IIn, suggesting that it was shaped by binary interaction. 

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