Mass functions of five distant Northern open star clusters

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 ± 0.2 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.