Early time optical spectroscopy of supernova SN1998S

We present medium-resolution optical spectra of the type II supernova SN 19 98S obtained during the pre-maximum, maximum and early decline phases. The early decline spectra show complex Balmer line profiles: a narrow P-Cygni e mission 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 Ha luminosity of the broad compon ent implies a shell mass of similar to0.1 M-circle dot for the supernova ej ecta, while the luminosity of the narrow component implies a mass loss rate of 10(-4) M-circle dot yr(-1) for the precursor. The photospheric temperat ure of the supernova shell decreased from a value of 18 400 +/- 2900 K on d ay 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 dynamicall y interact with a pre-supernova circumstellar material.