Photometric and kinematic studies of open star clusters III. NGC 4103, NGC5281, and NGC 4755

We present CCD photometry and proper motion studies of the three open star clusters NGC4103, NGC 5281, and. NGC 4755 (kappa Cru). By fitting isochrone s to the colour magnitude diagrams, we found that all three objects are you ng, open star clusters with ages of at most t = 45 Myr. They are located at distances from approx. 1600 pc to 2200 pc, derived from distance moduli (m -M)(0) ranging from 11 mag to 12 mag. We combined membership determinations based on propel. motions and statistical field star subtraction to derive the initial mass function (IMF) of the clusters. The shape of the IMFs coul d be represented by power laws with exponents of Gamma = -1.46 +/- 0.22 for NGC 4103, Gamma = -1.60 +/- 0.50 for NGC 5281, and Gamma = -1.68 +/- 0.14 for NGC4755, when - as a reference - Salpeter's (1955) value would be Gamma = -1.35. These results agree well with other IMF studies of open star clus ters.