Detailed study of open cluster PIS 20

Abstract

The very young open cluster Pis 20 has been analyzed by employing CCD UBVR photometry. Down to V = 19 mag, new blue and faint members were found. With them we obtain a better definition of the lower cluster main sequence. Using the fitting method we situate the cluster at a distance of d = 3600 pc (Vo-Mv = 12.80). The main color excess value is <E(B-V)> = 1.24 mag. In analyzing observed characteristics and spatial positions 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. From isochrone fitting the age of Pis 20 is 5.1 x 10⁶ y. We discuss the cluster mass spectrum whose slope, within the observational uncertainties, appears to be similar to a Salpeter 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. The cluster distance derived here prevents to assume any spatial connection between Pis 20 and the supernova remnant MSH 15-52. The total cluster mass is around 600 M⊙.