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tokens: the rise time of the light curve can be also used to estimate the radius of the progenitor of type IIP supernova through the relation between the progenitor radius and the rise time of the light curve, i.e. log R(R ⊙ ) = (1.225 ± 0.178) log t rise (d) + (1.692 ± 0.490) (Morozova et al. 2016 ). For SN 2017eaw, the rise time is estimated to be t rise = 6.81 ± 0.85 d in the g band, which gives an estimate of the radius as 515 ± 180 R ⊙ . The above two estimates of the progenitor radius agree within 1σ error. Figure 6. Constraining the progenitor radius of SN 2017eaw based on the cooling of the photospheric temperature in the early phase after the explosion. The red dots are the temperatures inferred from the blackbody fit to the observed UV and optical photometry; while the red curve represents the best fit of the radius. The green and black curves show the upper and lower limits of the estimates of the radius, respectively. The blue stars show the temperature evolution of SN 1987A, and the blue dashed line shows the corresponding best-fitting radius using the theoretical model (Rabinak Waxman 2011 ). We also try to fit the observed SED of the progenitor star from the prediscovery HST and Spitzer images with synthesized spectra calculated using the stellar evolution code marcs . 4 However, the best-fitting model spectrum has an effective temperature of only about 2500 K [A similar value of 2600 K was also suggested by Kilpatrick Foley ( 2018 )] with A V = 1.83 ± 0.59 mag by including the Spitzer data (Fig. 7 ), much lower than that inferred for the HST data alone. Given the luminosity, the lower temperature would result in an effective radius of 1330 R ⊙ for the progenitor, which is inconsistent with the estimate either from the rise time or temperature evolution of the shock cooling in the early phase. Despite an overall reasonable fit, the measurements in two Spitzer bands may suffer large contaminations of the nearby red sources due to large aperture photometry and/or the presence of circumstellar dust. In the latter case, the light of the progenitor star is scattered/absorbed by the surrounding dust and re-emitted at longer wavelengths [see also the analysis by Kilpatrick Foley ( 2018 )]. We also derive the F606W-band magnitude in 2004 by assuming that the dimming behaviour in F814W-band is totally due to the CSM dust. Taking into account this speculated F606W-band magnitude and the observed F814W-band magnitude obtained in 2004, the SED of the progenitor inferred in 2004 matches with the theoretical stellar spectra model characterized by an effective temperature of 3550 ± 150 K and a radius of 636 ± 155 R ⊙ (see Fig. 7 ). As an alternative, the dimming behaviour in F814W band could be also due to the intrinsic variation of an RSG star, but this explanation is inconsistent with either the observation taken at one year to a few days before the explosion or the theoretical prediction for its final stage evolution (see discussions in