Very Early Photometry of SN 1998S: Physical Parameters and Date of
  Explosion
  Context. We present very early optical lightcurves beginning 10 days before
maximum of the Type IIn supernova 1998S, covering the first four months after
discovery. Aims. We examine the light evolution and try to compare the
lightcurves to two analytical models(Nakar & Sari(2010) and Rabinak &
Waxman(2011)) for a red supergiant star. Methods. The photometry was carried at
the 60-cm telescope of the Xinglong Station of China. Broadband filters
Johnsons B, V and Cousins R were used. Results. The magnitude rose for the
first few days and then dropped slowly afterwards. The two different models we
use can fit the early lightcurves very well. The explosion date derived from
the models is within the range of 1998 March 1.34 - 2.64(JD 2450873.84 - JD
2450875.14.) The radius of the progenitor is found to be ~ 300 Rsun and ~ 2000
Rsun for the model of Nakar & Sari(2010) and Rabinak & Waxman(2011)
respectively. The constraint on mass and energy is not strong. The ranges of
these two parameters are within that of a red supergiant.
