On the Relative Frequencies of the Kinds of Type I Supernovae

Examination of the optical spectra of 58 Type I supernovae has revealed that 31 had spectra of the 'ordinary' or 'classical' variety (Type Ia). No further members of the subtype exemplified by SN 1983n in M83 and SN 19841 in NGC 991 (Type Ib) have been identified. Five Type I supernovae (Type Ipec) showed individual peculiarities, and the subtypes of the remaining 18 cannot be established from the spectra available. The observed fractions of Type I supernovae of subtypes, Ia, Ib, and Ipec are 0.78 + or - 0.14, 0.10 + or - 0.05, and 0.12 + or - 0.06, respectively, and the mean absolute magnitudes are -18.2 + or - 0.2, -16.6 + or - 0.3, and -16.7 + or - 0.1. The true type Ib and Type Ipec fractions must be significantly higher than the observed fractions because both Ib and Ipec are fainter than Ia and therefore less likely to be discovered. The possibility that Type Ib and Type Ipec each may be as frequent as Type Ia, at least in spiral galaxies, can not be excluded.