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