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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. 

ABSTRACT 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. 

Abstract We present deep, nebular-phase spectropolarimetry of the Type II-P/L SN 2013ej, obtained 167 days after explosion with the European Southern Observatory’s Very Large Telescope. The polarized flux spectrum appears as a nearly perfect (92% correlation), redshifted (by ∼4000 km s⁻¹) replica of the total flux spectrum. Such a striking correspondence has never been observed before in nebular-phase supernova spectropolarimetry, although data capable of revealing it have heretofore been only rarely obtained. Through comparison with 2D polarized radiative transfer simulations of stellar explosions, we demonstrate that localized ionization produced by the decay of a high-velocity, spatially confined clump of radioactive⁵⁶Ni—synthesized by and launched as part of the explosion with final radial velocity exceeding 4500 km s⁻¹—can reproduce the observations through enhanced electron scattering. Additional data taken earlier in the nebular phase (day 134) yield a similarly strong correlation (84%) and redshift, whereas photospheric-phase epochs that sample days 8 through 97 do not. This suggests that the primary polarization signatures of the high-velocity scattering source only come to dominate once the thick, initially opaque hydrogen envelope has turned sufficiently transparent. This detection in an otherwise fairly typical core-collapse supernova adds to the growing body of evidence supporting strong asymmetries across nature’s most common types of stellar explosions, and establishes the power of polarized flux—and the specific information encoded by it in line photons at nebular epochs—as a vital tool in such investigations going forward. 

The linear polarization of the optical continuum of type II supernovae (SNe), together with its temporal evolution is a promising source of information about the large-scale geometry of their ejecta. To help access this information, we undertook 2D polarized radiative transfer calculations to map the possible landscape of type II SN continuum polarization (P_cont) from 20 to 300 days after explosion. Our simulations were based on crafted 2D axisymmetric ejecta constructed from 1D nonlocal thermodynamic equilibrium time-dependent radiative transfer calculations for the explosion of a red supergiant star. Following the approach used in our previous work on SN 2012aw, we considered a variety of bipolar explosions in which spherical symmetry is broken by material within ~30° of the poles that has a higher kinetic energy (up to a factor of two) and higher⁵⁶Ni abundance (up to a factor of about five, allowing for⁵⁶Ni at high velocity). Our set of eight 2D ejecta configurations produced considerable diversity inP_cont(λ~ 7000 Å), although its maximum of 1–4% systematically occurs around the transition to the nebular phase. Before and after this transition,P_contmay be null, constant, rising, or decreasing, which is caused by the complex geometry of the depth-dependent density and ionization and also by optical depth effects. Our modest angle-dependent explosion energy can yield aP_contof 0.5–1% at early times. Residual optical-depth effects can yield an angle-dependent SN brightness and constant polarization at nebular times. The observed values ofP_conttend to be lower than obtained here. This suggests that more complicated geometries with competing large-scale structures cancel the polarization. Extreme asymmetries seem to be excluded. 

Type Ia Supernovae (SNe Ia) arise from carbon oxygen white dwarfs, but the true nature of their progenitor systems and explosion mechanisms remains the subject of considerable debate. The various progenitor models and methods of ignition result in different ejecta morphologies and/or distributions of material. By observing the polarization of SNe spectra we can gather insight into the geometry of these explosions. A key diagnostic that appears to be correlated with other SN Ia properties is the change in polarization observed across the Si II 6355 Å feature near maximum light. To investigate this, we are undertaking a systematic analysis of this feature in a uniformly obtained sample of SNe Ia observed at multiple epochs as part of the Supernova Spectropolarimetry (SNSPOL) Project, which gathered data, from 2010-2018, using the CCD Imaging/Spectropolarimeter (SPOL) on the 61" Kuiper, 6.5 m MMT, and 90" Bok telescopes. Here we present a preliminary analysis of the Si II feature in a particularly well-observed object from our sample, SN 2018gv, and present 10 epochs of data spanning from 10 days before, to 22 days after, peak light. We compare our near-maximum SNSPOL data with complementary data presented by Yang et al. [1]. This work was supported by NSF grants AST-1210311 and AST-2010001, and NASA grant NNX15AU81G. References: [1] Yang, Yi et al. 2020, ApJ, 902. 

Type IIb supernovae (SNe IIb) are core-collapse events whose optical spectra show strong hydrogen features that disappear over time, implying that their progenitors were nearly, but not completely, stripped of their hydrogen envelopes prior to core collapse. Thus, compared to hydrogen-rich SNe II, SNe IIb can provide a closer examination of the underlying structure of the progenitor system, particularly during early photospheric phases (less than +70 days relative to max. light). I will present early-time multi-epoch optical spectropolarimetry of several SNe IIb, obtained using the SPOL instrument at the University of Arizona. Using polarization diagnostics provides a way to track structural changes in the depleted hydrogen envelopes of these SNe as deeper layers of helium and other elements emerge and evolve. I find significant temporal polarization increases in the absorption wings of their H and He lines. Some of these line features make "loops" in Stokes Q-U diagrams, suggesting non-axisymmetic structure in the ejecta, perhaps arising from a transient absorbing clump. Furthermore, the majority of these SNe show polarimetric evidence for aspherical explosions along a preferred, or dominant, axis. I discuss the implications these findings have on the 3D geometry of the explosions by comparing the observed polarization to published synthetic spectropolarimetry that models axial symmetry and clump structures in stripped-envelope, core-collapse SNe. This comparative study naturally facilitates a broader discussion around the unresolved question as to what extent this SNe subclass shows common polarization characteristics. 

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. 

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. 

ABSTRACT We present photometry, spectra, and spectropolarimetry of supernova (SN) 2014ab, obtained through ∼200 d after peak brightness. SN 2014ab was a luminous Type IIn SN (MV < −19.14 mag) discovered after peak brightness near the nucleus of its host galaxy, VV 306c. Pre-discovery upper limits constrain the time of explosion to within 200 d prior to discovery. While SN 2014ab declined by ∼1 mag over the course of our observations, the observed spectrum remained remarkably unchanged. Spectra exhibit an asymmetric emission-line profile with a consistently stronger blueshifted component, suggesting the presence of dust or a lack of symmetry between the far side and near side of the SN. The Pa β emission line shows a profile very similar to that of H α, implying that this stronger blueshifted component is caused either through obscuration by large dust grains, occultation by optically thick material, or a lack of symmetry between the far side and near side of the interaction region. Despite these asymmetric line profiles, our spectropolarimetric data show that SN 2014ab has little detected polarization after accounting for the interstellar polarization. We are likely seeing emission from a photosphere that has only small deviation from circular symmetry in the plane normal to our line of sight, but with either large-grain dust or significant asymmetry in the density of circumstellar material or SN ejecta along our line of sight. We suggest that SN 2014ab and SN 2010jl (as well as other SNe IIn) may be events with similar geometry viewed from different directions.