Early time optical spectroscopy of supernova SN 1998S

Abstract

We present medium-resolution optical spectra of the type II supernova SN 1998S obtained during the pre-maximum, maximum and early decline phases. The early decline spectra show complex Balmer line profiles: a narrow P-Cygni emission feature superimposed on a broad emission. The narrow emission has a redshift similar to that of the host galaxy, indicating its origin from a pre-supernova circumstellar material. The Hα luminosity of the broad component implies a shell mass of ~ 0.1 Msun for the supernova ejecta, while the luminosity of the narrow component implies a mass loss rate of 10-4 M<sun yr-1 for the precursor. The photospheric temperature of the supernova shell decreased from a value of 18 400+/- 2900 K on day 17 to a value of 7500+/- 1200 K on day 44. The observations indicate SN 1998S belongs to type IIn supernovae where the supernova ejecta dynamically interact with a pre-supernova circumstellar material.