Fig. 4: Modelling to the MIR flux excesses of SN 2018evt.

The single-shell CSM dust model (solid green line) with an inner radius of 2.6 × 10¹⁷ cm can fit well the declining flux excess in the MIR at day ≲+310 and infer a flatter power-law index of the dust density s = 1.15. In the case of the steady-wind mass loss s = 2.0 for the double-shell model (solid black line), dust grains within the inner shell at 2.2 × 10¹⁶ cm were continuously destroyed by the expanding forward shock between days ~+200 and +310, causing a monotonically decreased flux excess in the MIR (dashed grey line). The presence of an outer CSM dust shell with an inner radius of 6.0 × 10¹⁷ cm is necessary to account for the time evolution of the flux excess before day +310 (dotted grey line). The prominent rise of the MIR flux excess of SN 2018evt after day +310, which cannot be explained by the thermal emission of any pre-existing dust content, demands a substantial amount of new dust to form promptly in the postshock regions (dotted red line). Panels (a) and (b) present the flux excesses of SN 2018evt at ~3.5 μm (Spitzer CH1 and NEOWISE W1) and ~4.6 μm (Spitzer CH2 and NEOWISE W2), respectively. The error bars shown represent 1σ uncertainties of monochromatic luminosity excesses.