SN 2020uem: A Possible Thermonuclear Explosion within A Dense Circumstellar Medium (II) The Properties of The CSM from Polarimetry and Light Curve Modeling

Abstract

Type IIn/Ia-CSM supernovae (SNe IIn/Ia-CSM) are classified by their characteristic spectra, which exhibit narrow hydrogen emission lines originating from a strong interaction with a circumstellar medium (CSM) together with broad lines of intermediate-mass elements. We performed intensive follow-up observations of SN IIn/Ia-CSM 2020uem, including photometry, spectroscopy, and polarimetry. In this paper, we focus on the results of polarimetry. We performed imaging polarimetry at $66$ days and spectropolarimetry at $103$ days after the discovery. SN 2020uem shows a high continuum polarization of $1.0-1.5\%$ without wavelength dependence. Besides, the polarization degree and position angle keep roughly constant. These results suggest that SN 2020uem is powered by a strong interaction with a confined and aspherical CSM. We performed a simple polarization modeling, based on which we suggest that SN 2020uem has an equatorial-disk/torus CSM. Besides, we performed semi-analytic light-curve modeling and estimated the CSM mass. We revealed that the mass-loss rate in the final few hundred years immediately before the explosion of SN 2020uem is in the range of $0.01-0.05{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, and that the total CSM mass is $0.5-4{\mathrm{M}}_{\odot }$. The CSM mass can be accommodated by not only a red supergiant (RSG) but a red giant (RG) or an asymptotic-giant-branch (AGB) star. As a possible progenitor scenario of SN 2020uem, we propose a white-dwarf binary system including an RG, RSG or AGB star, especially a merger scenario via common envelope evolution, i.e., the core-degenerate scenario or its variant.

arXiv comment16pages, 9 figures, 1 table. Accepted for publication in ApJ

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