The spectrum of SN 1998S is unusual. The H I Balmer and weak He I lines observed around maximum light
almost certainly were produced in a preexisting circumstellar shell, implying that the progenitor of SN 1998S
had undergone significant mass loss. The most noticeable feature in the spectrum before maximum light is the
presence of high-excited lines such as He II
4686 and
5411, C III and C IV, and N III
4634, 4640, 4641. SN 1998S is one of the only two SNe with those lines at pre-
maximum light. The prominent nitrogen lines very likely imply that at least some of the gas in the
progenitor star was significantly nitrogen enriched. This spectrum abruptly disappeared at the approach of
maximum light, which is the point at which the shock wave most likely emerged at the photosphere. The
similarity of the high-excitation lines in two different SNII, 1983K and 1998S suggests that nitrogen
abundant and excited state in CSM of the progenitor before maximum light are not very different.
The rapid decline of light curves suggests that SN 1998S has relatively low-mass hydrogen envelope. The CSM is likely dense, but small in extent and mass, and was probably disrupted when collided with the expanding SN envelope. Rapid spectral evolution also suggests that the photon diffusion time in envelope is short, thereby implying a low-density envelope, with which a linear light curve will be expected.
The extraordinary feature of the post-maximum spectra is the narrow emission P Cygni
profile superposed on a much broader absorption or P Cygni structure in all five Balmer lines, H
to
H
.
This behaviour does not approximate that of the proposed SN IIn. The narrow emission lines are
typical of hot nebulae and arise in circumstellar material released from the progenitor as a stellar wind
and excited by the UV flash shortly after core collapse, while the weak broad component could be
produced by accelerated material at the shock caused by the collision of the supernova envelope with the
circumstellar material. When the CSM matter was eventually engulfed by the expanding SN ejecta at
later epochs, the spectra are dominated by the broadened lines produced by the accelerated material at the
shock front.
Panagia et al. (1980) suggested presence of a shell where the gas is highly ionized. It is likely that the shell consists of gas originally ejected by the progenitor in the form of a more or less continuous wind. The wind material may have subsequently been compressed and accelerated as a result of the SN explosion, possibly by the radiation pressure of an initial soft X-ray burst (Klein & Chevalier 1978). The narrow P Cygni feature at H-Balmer may also be explained by this accelerated, extended shell. Such a narrow P Cygni feature, superposed upon a broad, partially filled in P Cygni line, could give a line profile such as we observed.
The comparison of the observations of this supernova with those of other SN II suggests that most of the features of SN 1998S may not be consistent with that of the type IIn, and that the SN may be suitable to classify as a type II-L, although the narrow lines exist at the early spectrum. SN 1998S provides strong evidence for a physical continuity between the features of CSM and likely explosion mechanisms of type II-L SNe and type IIn SNe. We suggest that a SN IIn should exhibit slowly declining light curves and slow spectral evolution besides the presence of narrow lines, since narrow lines are easily present at the early spectrum due to the significant mass loss of the progenitor as pointed out by Stathakis & Sadler (1991). From this viewpoint, the SN 1984E may not be classified as a SN IIn although there are very narrow and strong lines at the early spectrum.
Acknowledgements
We are grateful to Prof. A.V. Filippenko for his permitting to use his data in Ann. Rev. Astron. Astrophys. We thank an anonymous referee for useful comments and suggestions. QZL acknowledges the financial support from Wang Kuan-cheng foundation of Chinese Academy of Sciences. This work is partially supported by the National Project for Fundamental Research by the Ministry of Science and Technology and by the National Climbing Programme on Fundamental Researches, China.
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