MASS FUNCTIONS OF 5 DISTANT NORTHERN OPEN STAR-CLUSTERS

We analyse BVI CCD data of five northern open star clusters in the Gal axy in order to determine their mass functions. The clusters are Berke ley 81, Berkeley 99, NGC 6603, NGC 7044 and NGC 7510. They are distant (greater than or equal to 3 kpc) and compact (radius less than or equ al to 2.8 arcmin) objects. Except for NGC 7510 whose age is 10 Myr, al l are intermediate-age and old star clusters with ages between 0.5 and 3.2 Gyr. The observed cluster luminosity function (LF) is corrected f or both data incompleteness and field star contamination. Theoretical stellar evolutionary isochrones are used to convert LFs into mass func tions (MFs). The MF slope becomes flatter if a correction for data inc ompleteness is not applied, while it becomes steeper if a correction f or field star contamination is ignored; however, both corrections incr ease with decreasing stellar brightness. In the mass range similar to 1-14 M., the MF slope of NGC 7510 is 1.1 +/- 0.2. As the cluster is no t dynamically evolved, its present-day MF can be considered as the ini tial ME It is not too different from the Salpeter value (x = 1.35). In a narrow mass range similar to 0.6 - 2 M., the values of the MF slope in the four intermediate-age and old clusters range from 0.3 to 2.5 a nd differ significantly from each other. For Berkeley 99 and NGC 6603, the MF slopes are 1.4 +/- 0.6 and 1.1 +/- 0.4 respectively. They agre e with the Salpeter value within the errors. However, significantly di fferent values of MF slopes, 2.5 +/- 0.2 and 0.3 +/- 0.2, are found fo r Berkeley 81 and NGC 7044 respectively. We therefore conclude that th e MF does vary among this cluster sample. The effects of mass segregat ion are observed in all the four intermediate-age and old clusters; th is segregation is most probably due to dynamical evolution, as the age s of the clusters are much longer than the corresponding dynamical rel axation times. There is no obvious dependence of the MF slope on eithe r Galactocentric distance or age of the well-studied Galactic open clu sters. Except for some of the dynamically evolved older (age greater t han or equal to 50 Myr) clusters, the MF slopes of the clusters are no t too different from the Salpeter value.