THE RATE OF SUPERNOVAE .2. THE SELECTION EFFECTS AND THE FREQUENCIES PER UNIT BLUE LUMINOSITY

We present new estimates of the observed rates of SNe determined with the control time method applied to the files of observations of two lo ng term, photographic SN searches carried out at the Asiago and Stembe rg Observatories. Our calculations are applied to a galaxy sample extr acted from RC3, in which 65 SNe have been discovered. This relatively large number of SNe has been redistributed in the different morphologi cal classes of host galaxies giving the respective SN rates. The magni tude of two biases, the overexposure of the central part of galaxies a nd the inclination of the spiral parent galaxies, have been estimated. We show that due to overexposure an increasing fraction of SNe is los t in galaxies of increasing distances. Also, a reduced number of SNe i s discovered in inclined galaxies (i > 30-degrees): SNII and Ib are mo re affected than Ia, as well as SNe in Sbc-Sd galaxies with respect to other spirals. We strengthen previous findings that the SN rates is p roportional to the galaxy blue luminosity for all SN and Hubble types. Other sources of errors, besides those due to the statistics of the e vents, have been investigated. In particular those related to the adop ted SN parameters (Cappellaro et al. 1993) and correction factor for o verexposure and inclination. Moreover, we show that the frequencies of SNe per unit luminosity vary if different sources for the parameters of the sample galaxies are adopted, thus hampering the comparison of S N rates based on different galaxy samples. The overall rates per unit blue luminosity are similar to the previous determinations but signifi cant differences show up for individual types. In particular, the rate in ellipticals, 0.11 SNu (H = 75 km s-1 Mpc-1), is significantly lowe r than previously reported and better agrees with the predictions of g alaxy evolutionary models. Contrary to recent claims, in late spirals the rates of SNIa (0.39 SNu) and Ib (0.27 SNu) are similar. The most f requent SNe in spiral galaxies are SNII (1.48 SNu). Even the possible occurrence of faint SNe similar to SN 1987A (< 0.5 SNu in late spirals ) does not significantly alter the total rate of SNII. In the Galaxy, the expected number of SNe is 1.7 +/- 0.9 per century.