Hubble Space Telescope and Ground-based Observations of SN 1993J and SN 1998S: CNO Processing in the Progenitors

Ground-based and Hubble Space Telescope observations are presented for SN 1993J and SN 1998S. SN 1998S shows strong, relatively narrow circumstellar emission lines of N III-V and C III-IV, as well as broad lines from the ejecta. Both the broad ultraviolet and optical lines in SN 1998S indicate an expansion velocity of similar to 7000 km s super(-1). The broad emission components of Ly alpha and Mg II are strongly asymmetrical after day 72 past the explosion and differ in shape from H alpha . Different models based on dust extinction from dust in the ejecta or shock region, in combination with H alpha from a circumstellar torus, are discussed. It is concluded, however, that the double-peaked line profiles are more likely to arise as a result of optical depth effects in the narrow, cool, dense shell behind the reverse shock than in a torus-like region. The ultraviolet lines of SN 1993J are broad, with a boxlike shape, coming from the ejecta and a cool, dense shell. The shapes of the lines are well fitted by a shell with inner velocity similar to 7000 km s super(-1) and outer velocity similar to 10,000 km s super(-1). For both SN 1993J and SN 1998S a strong nitrogen enrichment is found, with N/C approximately 12.4 in SN 1993J and N/C approximately 6.0 in SN 1998S. From a compilation of all supernovae with determined CNO ratios, we discuss the implications of these observations for the structure of the progenitors of Type II supernovae.