The supernova 1998S in NGC 3877: Another supernova with Wolf-Rayet star features in pre-maximum spectrum

Abstract

We present the BVR photometry and the spectra of SN 1998S taken about four months after discovery, beginning a week and 6 days before maximum brightness respectively. The light curves show a steep and linear decline, which are consistent with those of SN 1979C and are typical of a SN II-L. An absolute magnitude at maximum MB0 ≤ -18.7 is derived, which is more than 2 mag brighter than the regular SNII and indicates this SN is a member of the rare class of Bright SNII. The B - V color evolution at early epoch is also typical of linear SN II with a maximum (B - V) value of +1.35, similar to the typical Type II-L SN 1980K. The spectral evolution is typical for SN II-L, very similar to that of the SN 1979C. They, however, showed some differences. The spectrum before maximum showed high-ionization N III and He II emission lines superposed on a strong blue continuum. Around the maximum light the high-ionization emission lines disappeared, leaving weak H I, He I emission lines, which are similar to those of SN 1983K. The extraordinary feature on the post-maximum spectra is the narrow emission P Cygni profile superposed on a much broader P Cygni structure in all five Balmer lines, Hα to Hε. Such feature is reminiscent of the post-maximum spectra of SN 1984E. Rapid spectral and photometric evolution, together with the presence of obvious P Cygni profile, suggests that SN 1998S is more consistent with the nature of a type II-L supernova than that of a IIn supernova, although narrow lines are present on the premaximum spectrum.