ON THE TEMPORAL EVOLUTION OF THE STELLAR MASS FUNCTION IN GALACTIC CLUSTERS

We show that we can obtain a good fit to the present-day stellar mass functions (MFs) of a large sample of young and old Galactic clusters in the range 0.1-10 M-circle dot with a tapered power-law distribution function with an exponential truncation of the form dN/dm alpha m(alpha) [1 -e(-(m/mc)beta)]. The average value of the power-law index alpha is similar to -2, that of beta is similar to 2.5, whereas the characteristic mass m(c) is in the range 0.1-0.8 M-circle dot and does not seem to vary in any systematic way with the present cluster parameters such as metal abundance, total cluster mass, or central concentration. However, mc shows a remarkable correlation with the dynamical age of the cluster, namely, m(c)/M-circle dot similar or equal to 0.15 + 0.5 x tau(3/4)(dyn), where tau(dyn) is the dynamical age taken as the ratio of cluster age and dissolution time. The small scatter seen around this correlation is consistent with the uncertainties in the estimated value of tdyn. We attribute the observed trend to the onset of mass segregation via two-body relaxation in a tidal environment, causing the preferential loss of low-mass stars from the cluster and hence a drift of the characteristic mass mc toward higher values. If dynamical evolution is indeed at the origin of the observed trend, it would seem plausible that high-concentration globular clusters, now with median m(c) similar or equal to 0.33 M-circle dot, were born with a stellar MF very similar to that measured today in the youngest Galactic clusters and with a value of m(c) similar or equal to 0.15 M-circle dot. This hypothesis is consistent with the absence of a turnover in the MF of the Galactic bulge down to the observational limit at similar to 0.2 M-circle dot and, if correct, it would carry the implication that the characteristic mass is not set by the thermal Jeans mass of the cloud.