The Interaction of Supernova 2018evt with a Substantial Amount of Circumstellar Matter -- An SN1997cy-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 H 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.

The Fall and Rise of Cosmic Dust in SN 2018evt

“…The slowly declining luminosity in the optical and NIR (Figures 1 and Extended Data 4) and the broad, long-lived Hα line (as shown in [27] and Figures Extended Data 7 and Extended Data 10) dominating the late-time spectra of SN 2018evt support the suggestion of a continuous interaction between the expanding SN ejecta and an extended ambient CSM. In such a context, a cold dense shell (CDS) develops during the ejecta-CSM interaction, with the CDS being located at a region in between the shocked CSM and the shocked ejecta [33].…”

“…After day +310, the dust formed in the CDS gradually blocks the receding side of the Hα line again, resulting in a blueward shift of the line profiles (panel (c) of Figure 3). The presence of a highly asymmetric ejecta-CSM interaction zone is also supported by detailed spectropolarimetry of SN 2018evt which shows a wavelength-independent degree of polarization with non-evolving position angles that is characteristic of electron scattering/dust from a highly axisymmetric configuration [21,42,27]. Both models A and B are spherically symmetric.…”

“…The Hα emission is powered by the interaction between the ejecta and the CSM [22,25]. The Hα spectral profile evolution can be understood by an axisymmetric geometry of the emitting region which is still compatible with the evolution of the MIR fluxes and the spectroscopic polarimetry of SN 2018evt [27]. The SN ejecta running into a highly asymmetric disk-like CSM leads to a high-density torus inclined at an angle towards the observer.…”

“…The CSM masses derived from optical and optical/MIR luminosities in shock interaction regions are ∼ 0.2 − 4.5 M ⊙ (see Supplementary Information on the progenitor's mass loss for details; [27]), corresponding to mass loss rates of Ṁ = 1 × 10 −3 − 9 × 10 −2 M ⊙ yr −1 . They are larger than the dust mass in the same region assumed in model B by 10 5 times, suggesting a very low dust to gas ratio in the CSM as a consequence of the destruction of dust particles by the energetic particles from the SN [43].…”

“…SN 2018evt (ASASSN-18ro [26]) is a SN 2002ic-like Type Ia found in the spiral galaxy MCG-01-35-011 at redshift z = 0.02523 [27]. SN 2018evt shares some common optical spectral features as typical Type Ia SN 1991T-like SNe as shown in Figure Extended Data 1 (a).…”