SPECTRA OF LOW-MASS HELIUM STAR MODELS AND THE TYPE-IC SUPERNOVA SN-1987M

Low-mass helium star models that reproduce the first 100 days of the light curve of the Type Ic SN 1987M are used in model atmosphere calculations. The resulting spectra are compared to observations from 60 to 140 days post-maximum. The small column depth to Co-56 decay photons leads to strong optical helium lines at 60 days after maximum which fade into the continuum for times t greater than or similar to 200 days while the He I 10830 angstrom line remains a strong P-Cygni feature at all times examined. The lack of observed optical helium lines suggests SN 1987M has a low helium abundance with a total helium mass fraction not much in excess of approximately 7%. This suggests a significantly different evolutionary path from the helium-rich Type Ib SN 1984L. The effects on the theoretical spectrum of mixing the ejecta and/or artificially converting the helium to oxygen are also explored. In spite of very large departure coefficients, the predicted optical helium lines at late times are not strong enough to be identified in the current Type Ib or Type Ic data base. This provides an explanation for the similarity between optical spectra of these supernova subtypes at times exceeding about 200 days even if Type Ib contain substantial amounts of helium and Type Ic do not. The spectroscopic implications for Type Ic progenitor evolution, including the possible presence of a small amount of hydrogen in the ejecta, is discussed.