Broad emission lines from the opaque electron-scattering environment of SN 1998S

I propose that broad narrow-topped emission lines with full width at zero intensity >20 000 km s−1 revealed by early-time spectra of SN 1998S originate from a dense circumstellar shell (CS) with an outer radius R≈1015 cm. The tremendous line width is the result of multiple scattering of the photons of the narrow line by the thermal electrons of the opaque CS gas. The Hα line on 1998 March 6 is reproduced by Monte Carlo simulations, provided that the Thomson optical depth of the CS shell is 3–4 at this epoch. The high density of the CS shell implies that a cool dense shell (CDS), which forms at the ejecta–wind interface, is opaque in the Paschen continuum for about 40–50 d after the explosion. This explains why strong lines from the supernova ejecta were not seen during an initial period of about 40 d after the discovery of this object. The Thomson optical depth recovered from the electron-scattering wings is consistent with observational constraints imposed by the bolometric luminosity, photospheric radius and expansion velocity of the CDS.