The research of YY has been supported through a Benoziyo Prize Postdoctoral Fellowship and the Bengier-Winslow-Robertson Fellowship. MB acknowledges support from the Swedish Research Council (Reg. no. 2020-03330). TWC is grateful for funding from the Alexander von Humboldt Foundation and the EU Funding under Marie Skłodowska-Curie grant H2020-MSCA-IF-2018-842471. A. V. Filippenko’s group at U.C. Berkeley acknowledges generous support from the TABASGO Foundation, the Christopher R. Redlich fund, the Miller Institute for Basic Research in Science (in which A.V.F. was a Miller Senior Fellow), Sunil Nagaraj, Landon Noll, Gary and Cynthia Bengier, Clark and Sharon Winslow, Sanford Robertson, and many additional donors. LG acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and by the European Social Fund (ESF) ‘Investing in your future’ under the 2019 Ramón y Cajal program RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project. AG-Y’s research is supported by the EU via ERC grant 725161, the ISF GW excellence center, an IMOS space infrastructure grant and BSF/Transformative and GIF grants, as well as The Benoziyo Endowment Fund for the Advancement of Science, the Deloro Institute for Advanced Research in Space and Optics, The Veronika A. Rabl Physics Discretionary Fund, Minerva, Yeda-Sela, and the Schwartz/Reisman Collaborative Science Program; AG-Y is the incumbent of the The Arlyn Imberman Professorial Chair. D. Hiramatsu was supported by NSF grants AST-1313484 and AST-1911225, as well as by NASA grant 80NSSC19kf1639. PH acknowledges support from the NSF project ‘Signatures of Type Ia Supernovae, New Physics, and Cosmology,’ grant AST-1715133. EYH and MS acknowledge support provided by NSF grant AST-161347 and the Florida Space Research Program. The research of JM is supported through a Royal Society University Research Fellowship. CM and GH were supported by NSF grant AST-1313484. CP was supported by NSF grant AST-1911225. Time-domain research by DJS is supported by NSF grants AST-1821987, 1813466, and 1908972, and by the Heising-Simons Foundation under grant #2020-1864. The supernova research by LW is supported by NSF award AST-1817099 and HST-GO-14139.001-A. LW is sponsored in part by the Chinese Academy of Sciences (CAS), through a grant to the CAS South America Center for Astronomy (CASSACA) in Santiago, Chile. XW is supported by the National Natural Science Foundation of China (NSFC grants 11178003 and 11325313). JCW is supported by NSF grant AST-1813825. The LCO team is supported by NSF grants AST-1911225 and AST-1911151.

A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN 2018evt obtained by the ESO Very Large Telescope from 172 to 219 d after the estimated time of peak luminosity to study the geometry of the CSM. The non-zero continuum polarization decreases over time, suggesting that the mass-loss of the progenitor star is aspherical. The prominent H α emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN 2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN 2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN 2018evt has been significantly enriched at a rate of ∼0.1 M⊙ yr−1 over a period of >100 yr.

Y. Yang et al.

Peer reviewed