A HUBBLE-SPACE-TELESCOPE ULTRAVIOLET-SPECTRUM OF SN 1993J

We obtained a Hubble Space Telescope ultraviolet spectrum of the Type II supernova SN 1993J in M81 on 1993 April 15. The similar to 1650-2900 Angstrom region is smoother than observed for SN 1987A and SN 1992A and lacks strong P Cygni absorptions caused by iron peak element lines. Synthetic spectra calculated using a parameterized LTE procedure and a simple model atmosphere do not fit the UV region. Radio observations suggest that SN 1993J is embedded in a thick circumstellar envelope. The UV spectra of other supernovae that are believed to have thick circumstellar envelopes also have similar to 1650-2900 Angstrom regions lacking in strong P Cygni absorptions. Interaction of supernova ejecta and circumstellar matter may cause the smooth UV spectrum. If so, UV observations of supernovae will provide insight into the circumstellar environment of the supernova progenitors.