Abstract:
We analyse BVI CCD data of five northern open star clusters in the Galaxy in order to
determine their mass functions. The clusters are Berkeley 81, Berkeley 99, NGC 6603, NGC
7044 and NGC 7510. They are distant ( ≥3 kpc) and compact (radius ≤2.8 arcmin) objects.
Except for NGC 7510 whose age is 10 Myr, all are intermediate-age and old star clusters with
ages between 0.5 and 3.2 Gyr. The observed cluster luminosity function (LF) is corrected for
both data incompleteness and field star contamination. Theoretical stellar evolutionary
isochrones are used to convert LFs into mass functions (MFs). The MF slope becomes flatter
if a correction for data incompleteness is not applied, while it becomes steeper if a correction
for field star contamination is ignored; however, both corrections increase with decreasing
stellar brightness.
In the mass range ~1–14 M☉, the MF slope of NGC 7510 is 1.1 ± 0.2. As the cluster is not
dynamically evolved, its present-day MF can be considered as the initial MF. It is not too
different from the Salpeter value (x = 1:35). In a narrow mass range ~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 and 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 agree with the Salpeter value within the errors. However,
significantly different values of MF slopes, 2.5 ± 02 and 0.3 ± 0.2, are found for Berkeley
81 and NGC 7044 respectively. We therefore conclude that the MF does vary among this
cluster sample.
The effects of mass segregation are observed in all the four intermediate-age and old
clusters; this segregation is most probably due to dynamical evolution, as the ages of the
clusters are much longer than the corresponding dynamical relaxation times. There is no obvious dependence of the MF slope on either Galactocentric distance or age of
the well-studied Galactic open clusters. Except for some of the dynamically evolved older (age
≥ 50 Myr) clusters, the MF slopes of the clusters are not too different from the Salpeter value.