3 Comparison with other supernovae

In order to show how unusual SN 1998S is we compare SN 1998S to several SNe which share some characteristics.

3.1 SN 1983K

The first time Wolf-Rayet star lines were identified in a supernova was in SN 1983K before maximum light (Niemela et al. 1985). The Wolf-Rayet lines in the SN disappeared close to the maximum light. Niemela et al. (1985) suggested that the surface layers of the progenitor were significantly overabundant in nitrogen. The progenitor was assumed to be a red supergiant (Phillips et al. 1990).

To our knowledge, SN 1998S is the second one with those lines. The spectra of SN 1998S before maximum light are very similar to that of SN 1983K, but with a much stronger H$\alpha$ intensity. Although only blue spectra of SN 1983K are available in the literature, the spectral evolution of the two SNe after maximum light is different. The most striking difference between the two SNe, however, is the evolution of light curves although their absolute magnitudes are comparable. After maximum light, the magnitudes of SN 1983K decayed very slowly and the supernova itself was defined as Type II-P by Phillips et al. (1990), while that of SN 1998S dropped dramatically (Fig. 1). Consequently, though the spectra before the maximum light are superficially similar to those seen in SN 1983K, there appears to be little relationship between the two supernova events.

3.2 SN 1984E

The first time CSM lines were identified in a supernova was in SN 1984E close to maximum light. These lines were very luminous, but only short-lived. The line widths imply a velocity of 250-350 ${\rm km~s}^{-1}$, which is faster than typical red supergiant winds. Schlegel (1990) defined the sub-class Type IIn to emphasize the presence of the narrow emission components. Recently there have been a substantial number of other SNe IIn discovered, including SNe 1988Z, 1994Y, and 1994ak. Filippenko (1997) suggested that SNe 1983K and 1984E might also be variants of SNe IIn. Dopita et al. (1984) suggested that the CSM was the product of a superwind shortly before collapse, but Gaskell & Keel (1988) found that the material had been ejected from the progenitor in a relatively discrete event less than 30 years before the explosion. The CSM emission in SN 1984E had disappeared by one month after maximum (Henry & Branch 1987), and the supernova itself decayed rapidly, with a light curve of a typical type II-L.

Light curves of SN 1998S are similar to those of SN 1984E, but the maximum magnitude of SN 1998S is over 2 mag brighter than that of SN 1984E. Although no spectrum before maximum light in SN 1984E was reported, its post-maximum spectrum looks very similar to that of SN 1998S, except for the very strong narrow lines. The narrow lines in SN 1998S also disappeared one month later.

3.3 SN 1979C

SN 1979C in NGC 4321 is one of the prototypes of Linear SNII. The photometric evolution of SN 1998S is consistent with that of SN 1979C (Fig. 1), and the maximum absolute magnitude is comparable. Although no premaximum spectrum is available, a glance at the spectral evolution can find the similarity of SN 1998S with SN 1979C (Branch et al. 1981). Similar weak, narrow H I absorption lines were observed just after maximum in the Type II supernova 1979C. A comparison of the April 24 and May 25 spectra with the spectra of SN 1979C at nearly same epochs after maximum light shows good qualitative agreement (Fig. 4). However, there are also some differences between the two SNe. For example, the emission line intensity of SN 1998S within one month after maximum is a little weaker than that seen in SN 1979C, which may imply the difference of the CSM between them. P Cygni absorption is not very apparent in the spectra of SN 1979C, while in SN 1998S, it is obviously present in the early spectra. It is interesting to note that although the width of the emission line profile in SN 1998S is roughly similar to that observed in SN 1979C at a corresponding phase, the profiles are much different (Fig. 4).

Figure 4: The spectra of SN 1998S obtained on April 24 and May 25 are compared to the spectra of SN 1979C at nearly the same epochs. Wavelengths are in the rest frames of the parent galaxies