The interaction of supernova 2018evt with a substantial amount of circumstellar matter – An SN 1997cy-like event

Abstract

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 days after the estimated time of peak luminosity to study the geometry of the CSM. The nonzero continuum polarization decreases over time, suggesting that the mass loss of the progenitor star is aspherical. The prominent Hα 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.

Newly formed dust within the circumstellar environment of SN Ia-CSM 2018evt

“…The inset of Extended Data Fig. 1b shows the early phase B − V colour curve, which is also in general agreement with that of SN 1991T after correcting the host reddening of E ( B − V ) < 0.32 mag, which has been estimated from the equivalent width (EW) of the Na i D lines 20 . The presence of the Hα line makes it a Ia-CSM SN similar to SN 2002ic.…”

“…2 , Extended Data Table 2 and ref. 20 ), Paβ and Brγ profiles (Extended Data Fig. 3 and Extended Data Table 2 ).…”

“…4 ) and the broad, long-lived Hα line (as shown in ref. 20 and Extended Data Figs. 2 and 6 ) dominating the late-time spectra of SN 2018evt both indicate that a substantial amount of late-time emission would arise from kinetic energy from the ejecta–CSM interaction converted to radiation 27 .…”

“…19 )) is a Ia-CSM SN found in the spiral galaxy MCG-01-35-011 at redshift z = 0.02523 (ref. 20 ). SN 2018evt shares some common optical spectral features with typical Type Ia SN 1991T-like supernovae, as shown in Extended Data Fig.…”

Monte Carlo Radiation Transport for Astrophysical Transients Powered by Circumstellar Interaction

“…The late-time multiwavelength observations of this SNe show evidence of CSI (Chandra et al 2022). In rare cases, luminous SNe are observed to have SN Ialike features before exhibiting SN IIn trends, which could be attributed to a white dwarf exploding into dense CSM (e.g., SN 2018evt; Yang et al 2023).…”