REDISCUSSION OF EXTRAGALACTIC SUPERNOVA RATES DERIVED FROM EVANS 1980-1988 OBSERVATIONS

Supernova rates are derived from observations by Evans under the assumption that (1) the discovery probability of supernovae is independent of parent galaxy inclination and (2) supernovae of all types have luminosity functions of finite width at maximum light. For spirals of types Sab-Sd the total supernova rate is calculated to be approximately 1.3(H0/75)2 SNu. Modeling shows that symmetrical broadening of the luminosity functions of supernovae at maximum light does not change the supernova rates significantly from those computed from Evans's observations under the assumption that each type of supernova has a single value of M(V)(max). However, systematic shifts in the mean luminosities of supernovae (or changes in the amount of interstellar extinction) result in dramatic changes in the calculated supernova rates. The rate of core-collapse supernovae within 4 kpc of the Sun is calculated to be approximately 2.3(H0/75)2 per millennium. This rate is in excellent agreement with the fact that four such objects are known to have occurred within 4 kpc during the last 2000 years.