Astron. Astrophys. Suppl. Ser. 144, 219-225

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

Q.-Z. Liu^1,2,3 - J.-Y. Hu⁴ - H.-R. Hang¹ - Y.-L. Qiu^2,4 - Z.-X. Zhu¹ - Q.-Y. Qiao^2,4

Send offprint request: Q.-Z. Liu, e-mail: qzliu@astro.uva.nl

1 - Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
2 - National Astronomical Observatories, Chinese Academy of Sciences, China
3 - Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4 - Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China

Received January 3; accepted February 17, 2000

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 $M_{B}^{0}\leq -18.7$ 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, H$\alpha$ to H$\epsilon$. 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.

Key words: supernovae: general -- supernovae: individual (SN 1998S)