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1. Evidence for multiple origins of fast declining Type II supernovae from spectropolarimetry of SN 2013ej and SN 2017ahn

   https://doi.org/10.1093/mnras/stab1582  

   Nagao, T ; Patat, F ; Taubenberger, S ; Baade, D ; Faran, T ; Cikota, A ; Sand, D J ; Bulla, M ; Kuncarayakti, H ; Maund, J R ; et al ( June 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The origin of the diverse light-curve shapes of Type II supernovae (SNe), and whether they come from similar or distinct progenitors, has been actively discussed for decades. Here, we report spectropolarimetry of two fast declining Type II (Type IIL) SNe: SN 2013ej and SN 2017ahn. SN 2013ej exhibited high continuum polarization from very soon after the explosion to the radioactive tail phase with time-variable polarization angles. The origin of this polarimetric behaviour can be interpreted as the combination of two different aspherical structures, namely an aspherical interaction of the SN ejecta with circumstellar matter (CSM) and an inherently aspherical explosion. Aspherical explosions are a common feature of slowly declining Type II (Type IIP) SNe. By contrast, SN 2017ahn showed low polarization not only in the photospheric phase but also in the radioactive tail phase. This low polarization in the tail phase, which has never before been observed in other Type IIP/L SNe, suggests that the explosion of SN 2017ahn was nearly spherical. These observations imply that Type IIL SNe have, at least, two different origins: they result from stars that have different explosion properties and/or different mass-loss processes. This fact might indicate that 13ej-like Type IIL SNe originate from a similar progenitor to those of Type IIP SNe accompanied by an aspherical CSM interaction, while 17ahn-like Type IIL SNe come from a more massive progenitor with less hydrogen in its envelope. 

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2. 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)

   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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3. Record-breaking polarization from the interacting superluminous supernova 2017hcc

   https://doi.org/10.1093/mnras/stad3579  

   Mauerhan, Jon C. ; Smith, Nathan ; Williams, G. Grant ; Smith, Paul S. ; Filippenko, Alexei V. ; Bilinski, Christopher ; Zheng, WeiKang ; Brink, Thomas G. ; Hoffman, Jennifer L. ; Leonard, Douglas C. ; et al ( November 2023 , Monthly Notices of the Royal Astronomical Society)

   We present multi-epoch spectropolarimetry of Type IIn supernova SN2017hcc, 16–391 d after explosion. Continuum polarization up to 6 per cent is observed during the first epoch, making SN 2017hcc the most intrinsically polarized SN ever reported at visible wavelengths. During the first 29 d, when the polarization is strongest, the continuum polarization exhibits wavelength dependence that rises toward the blue, then becomes wavelength independent by day 45. The polarization drops rapidly during the first month, even as the flux is still climbing to peak brightness. None the less, unusually high polarization is maintained until day 68, at which point the polarization declines to levels comparable to those of previous well-studied SNe IIn. Only minor changes in position angle (PA) are measured throughout the evolution. The blue slope of the polarized continuum and polarized line emission during the first month suggests that an aspherical distribution of dust grains in pre-shock circumstellar material (CSM) is echoing the SN IIn spectrum and strongly influencing the polarization, while the subsequent decline during the wavelength-independent phase appears consistent with electron scattering near the SN/CSM interface. The persistence of the PA between these two phases suggests that the pre-existing CSM responsible for the dust scattering at early times is part of the same geometric structure as the electron-scattering region that dominates the polarization at later times. SN 2017hcc appears to be yet another, but more extreme, case of aspherical yet well-ordered CSM in Type IIn SNe, possibly resulting from pre-SN mass-loss shaped by a binary progenitor system.

    

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4. The aspherical explosion of the Type IIP SN 2017gmr†

   https://doi.org/10.1093/mnrasl/slz119  

   Nagao, T ; Cikota, A ; Patat, F ; Taubenberger, S ; Bulla, M ; Faran, T ; Sand, D J ; Valenti, S ; Andrews, J E ; Reichart, D E ( October 2019 , Monthly Notices of the Royal Astronomical Society: Letters)

   ABSTRACT Type IIP supernovae (SNe IIP), which represent the most common class of core-collapse (CC) SNe, show a rapid increase in continuum polarization just after entering the tail phase. This feature can be explained by a highly asymmetric helium core, which is exposed when the hydrogen envelope becomes transparent. Here we report the case of an SN IIP (SN 2017gmr) that shows an unusually early rise of the polarization, ≳30 d before the start of the tail phase. This implies that SN 2017gmr is an SN IIP that has very extended asphericity. The asymmetries are not confined to the helium core, but reach out to a significant part of the outer hydrogen envelope, hence clearly indicating a marked intrinsic diversity in the aspherical structure of CC explosions. These observations provide new constraints on the explosion mechanism, where viable models must be able to produce such extended deviations from spherical symmetry, and account for the observed geometrical diversity. 

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5. Early-time Ultraviolet and Optical Hubble Space Telescope Spectroscopy of the Type II Supernova 2022wsp

   https://doi.org/10.3847/2041-8213/ad0e6b  

   S_Vasylyev, Sergiy ; Vogl, Christian ; Yang_杨, Yi 轶 ; Filippenko, Alexei V ; Brink, Thomas G ; Brown, Peter J ; Matheson, Thomas ; Modjaz, Maryam ; Gal-Yam, Avishay ; Mazzali, Paolo A ; et al ( December 2023 , The Astrophysical Journal Letters)

   We report early-time ultraviolet (UV) and optical spectroscopy of the young, nearby Type II supernova (SN) 2022wsp obtained by the Hubble Space Telescope (HST)/STIS at about 10 and 20 days after the explosion. The SN 2022wsp UV spectra are compared to those of other well-observed Type II/IIP SNe, including the recently studied Type IIP SN 2021yja. Both SNe exhibit rapid cooling and similar evolution during early phases, indicating a common behavior among SNe II. Radiative-transfer modeling of the spectra of SN 2022wsp with theTARDIScode indicates a steep radial density profile in the outer layer of the ejecta, a solar metallicity, and a relatively high total extinction ofE(B−V) = 0.35 mag. The early-time evolution of the photospheric velocity and temperature derived from the modeling agree with the behavior observed from other previously studied cases. The strong suppression of hydrogen Balmer lines in the spectra suggests interaction with a preexisting circumstellar environment could be occurring at early times. In the SN 2022wsp spectra, the absorption component of the MgiiP Cygni profile displays a double-trough feature on day +10 that disappears by day +20. The shape is well reproduced by the model without fine-tuning the parameters, suggesting that the secondary blueward dip is a metal transition that originates in the SN ejecta.

    

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