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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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Polarized lines illuminate the progenitors of core-collapse supernovae
Polarized emission and absorption lines in core-collapse supernovae (CCSNe) trace detailed structures in the ejecta and surrounding circumstellar medium (CSM) that contain clues to the end stages of their progenitors’ lives as massive stars. Extracting this geometrical information, however, requires careful computational modeling that takes into account the 3-D distribution of the ejecta and CSM and the potentially polarizing nature of line scattering. I present the results of such modeling for several CCSNe with multi-epoch polarization observations obtained by the Supernova Spectropolarimetry Project with the SPOL spectropolarimeter. In the cases of “interacting supernovae,” time-variable line polarization can constrain the shape and density of the surrounding CSM, yielding valuable information about the progenitor’s late-stage mass loss episodes. In stripped-envelope supernovae, it provides new, quantitative evidence for the asymmetric nature of the explosions and the possible role of binary companions in shaping the supernova ejecta.
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- Award ID(s):
- 2009996
- PAR ID:
- 10288551
- Date Published:
- Journal Name:
- Astropol 2020: New Era of Multiwavelength Polarimetry
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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