Narrow lines from SN 1993J

Summary

Abstract

We report narrow emission lines observed during the first 10 days of supernova 1993J. The earliest spectra showed resolved, P-Cygni-like Hα emission which declined on a timescale of about 2 days. Fast-declining, unresolved He II and coronal iron lines were also detected. A higherresolution spectrum taken on day 8 after the explosion showed that the Hα line had narrowed to about 90 km s⁻¹ FWHM and had lost its P-Cygni profile. The narrow line emission followed ionisation, by the EUV flash, of dense circumstellar material close to the supernova. A likely explanation for the rapid disappearance of the these lines was that the circumstellar gas was overrun by the expanding supernova shock. However, it may be necessary to also invoke the decline of trapped UV radiation to account for the rapid fading of the Hα line.

Introduction

The detection of narrow emission lines from supernova 1993J gives us a rare opportunity to study the circumstellar medium produced in the preexplosion phase. This can give insights into the evolution and mass loss history of the progenitor star, and test theories of the interaction of a supernova with its surroundings.

Observations

Optical spectra of SN 1993J were obtained with the IDS and ISIS spectrographs on, respectively, the INT and WHT on La Palma. For the first two weeks, spectroscopy was carried out nightly, and less frequently thereafter. The resolution and wavelength coverage varied according to the scheduled observing programme being carried out at the time.