1 Introduction

The supernova 1998S in NGC 3877 was discovered by Zhou Wan at Beijing Astronomical Observatory (BAO) on 1998 March 3 UT (Li & Li 1998). It is located at R.A. = 11 $^{{\rm h}}$46 $^{{\rm m}}$06 $^{{\rm s}}$, $\delta$ = +47$^{\circ}$29'0 (equinox 2000.0), which is 16'' west and 46'' south of the nucleus of NGC 3877. CCD images taken on Feb. 23.7 showed no star at the position of SN 1998S. As the object is embedded in the galaxy, the BAO magnitude was obtained from a subtracted image.

Spectra obtained by Filippenko & Moran (1998a) showed that the supernova was a type-II supernova due to the presence of broad H$\alpha$ emission superposed on a featureless continuum. Further observation (Filippenko & Moran 1998b) indicated that the object was not a typical supernova, and suggested that the supernova belonged to the type IIn based on the presence of the narrow H$\alpha$ emission component. More recently, Leonard et al. (1999) presented optical spectropolarimetry and spectra of the supernova and found that the SN exhibits a high degree of linear polarization, implying significant asphericity for its continuum-scattering environment.

Type II supernovae are defined by the presence of hydrogen. They can be divided photometrically into II-P and II-L (Barbon et al. 1979). The II-P light curves show an essentially constant-light phase from approximately 30 to 80 days post-maximum, while the II-L light curves decay quite linearly, at a rate of about 0.05 mag day^-1 in the B band from the maximum to about 100 days past maximum. Schlegel (1996) suggested that the spectroscopic differences between these two subtypes were that the early spectra of SNe II-L lacked a strong P Cygni at H$\alpha$ line. In addition to the two well-known photometric subclass of SNII, Schlegel (1990) defined a new subclass of type IIn, distinguished by relatively narrow emission lines with little or no P Cygni absorption component and slowly declining light curves. The typical spectrum of a type IIn supernova is a narrow component superposed on a broad component. The centroid of the base is blueshifted with respect to the systemic velocity (Filippenko 1989). The evolution of the IIn spectrum is slow, with the overall appearance remaining approximately constant for 50-100 days.

In this paper, we present the spectral and photometric evolution of SN 1998S through the first four months after discovery based on BAO observations. The pre-maximum spectrum was dominated by the Wolf-Rayet star lines, while the post maximum spectrum was dominated by double P Cygni profile at H$\alpha$ to H $\varepsilon$. Possible interpretations of these features are discussed.