SIMBAD references

The very young open cluster Pis 20 has been investigated using CCD UBVR photometry. New blue faint members were found down to V=19mag. With them we obtain a better definition of the lower cluster main sequence. Using the fitting method, we derive a cluster distance of d=3600pc (V₀-M_V=12.80). The mean color excess is <E_B–V≥1.24mag. 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 50M_☉ according to evolutionary models. We obtain for Pis 20 an age of 5(±1)x10⁶yr from isochrone fitting. We discuss the cluster's mass spectrum whose slope, within the observational uncertainties, appears to be similar to the one in Salpeter's law. We notice that 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 the supernova remnant MSH 15-52, we cannot/rule out the possibility that they are spatially related. The presently observed cluster mass is around 600M_☉.