The rate of supernovae (SNe) is a key parameter regulating the chemical evolution of galaxies, the kinematics of the interstellar medium, the production of cosmic rays, in addition to being a fundamental constraints for stellar evolution theories.
In recent years there has been a renewed interest in the search for SNe and in fact the number of discoveries in the last decade almost equaled those of the previous century. Despite these efforts, published estimates of the rate of SN still bring large uncertainties.
In order to obtain a direct measurement of the rate of SNe in a given galaxy sample one has to divide the number of SN discoveries by the period of surveillance. The latter can be estimated either by making some "reasonable" assumptions (e.g. Tammann et al. 1994 and reference therein) or by computing the control time through the detailed analysis of the log of a given SN search (Zwicky 1938, 1942). Since only the SNe discovered in the particular search enter the computation, in general the main problem with this approach is the small statistics. This can be improved by joining into a single database the data of different SN searches but, so far, this has been done only for the Asiago and Crimea SN searches (Cappellaro et al. 1993a, 1993b hereafter PI and PII).
In this paper we present new estimates of the rate of SNe based on the joint efforts of five SN searches, namely the Asiago and Crimea searches, the search by Evans (van den Bergh et al. 1987, Evans et al. 1989), the OCA search (Pollas 1994) and the Calán/Tololo search (Hamuy et al. 1993). In this way we collected the largest SN sample ever used for SN rate calculation.
In the following, after introducing the individual searches, we describe the recipe used in the computation with emphasis on the updating and improvements with respect to previous works. We make an effort to explicitly mention all the critical assumptions and parameters involved in the calculation and to discuss in detail the biases of SN searches. Finally, the computed SN rates are reported and compared with previous estimates.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998