A DETAILED STUDY OF THE OPEN CLUSTER PISMIS-20

The very young open cluster Pis 20 has been investigated using CCD UBV R photometry. New blue faint members were found down to V = 19 mag. Wi th them we obtain a better definition of the lower cluster main sequen ce. Using the fitting method, we derive a cluster distance of d = 3600 pc (V-0 - M(V) = 12.80). The mean color excess is (E(B-V)) = 1.24 mag . From the observed characteristics and spatial location of the bluest cluster stars we conclude that the star WR 67 has a good chance to be a cluster member. In that case, the mass of its progenitor should be greater than 50 M. according to evolutionary models. We obtain for Pis 20 an age of 5 (+/-1) 10(6) y from isochrone fitting. We discuss the cluster's mass spectrum whose slope, within the observational uncertai nties, appears to be similar to the one in Salpeter's law. We notice t hat high WR/OB and WC/WN number ratios are present not only within the cluster itself but also in the association where Pis 20 is situated. Although the cluster distance derived here places Pis 20 nearer than t he supernova remnant MSH 15-52, we cannot rule out the possibility tha t they are spatially related. The presently observed cluster mass is a round 600 M..