For this PhD thesis, 13 open star clusters were studied. The main goals were
the determination of the parameters age, distance, and reddening of the
clusters and their initial mass function (IMF) from the individual
main-sequence turn-off point down to stellar masses of approx. 1 MO.
To separate field and cluster stars we used a combination of membership
probabilities derived from stellar proper motions and statistical field star
subtraction. The IMF was determined with a maximum likelihood technique.
The mass functions of all objects could be well represented by power laws with
IMF indices ranging from Gamma=-0.69 (NGC 2541) to Gamma = -2.27
(Blanco 1), when Salpeter's (1955) value would be Gamma = -1.35. The
differences in the indices are higher than statistically expected, indicating
real IMF variations.
With a survey of earlier IMF studies a weak dependence of the IMF index on
the cluster age was detected. The IMF appears not to depend on other
parameters like the location of the object within the Galaxy.