On the temporal evolution of the stellar mass function of Galactic clusters

Abstract

Abstract. We show that we can obtain a good fit to the present-day stellar-mass functions of a large sample of young and old Galactic clusters with a tapered Salpeter power-law distribution function with an exponential truncation of the form dN/dm ∝ m. The average value of the power-law index α is ∼ −2.2, very close to the Salpeter value of −2.3, while the characteristic mass, mc , is in the range 0.1-0.6 M 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, the characteristic mass shows a remarkable correlation with the dynamical age of the cluster, namely m c /M 0.15 + 0.5 ×td yn , where t d yn is the dynamical time, taken as the ratio of cluster age and dissolution time. The small scatter around this correlation is likely due to uncertainties on the estimated value of t d yn . We attribute the observed trend to the onset of mass segregation through two-body relaxation in a tidal environment, causing preferential loss of low-mass stars from the cluster and hence a drift of the characteristic mass towards higher values. If dynamical evolution is indeed at the origin of the observed trend, it seems plausible that globular clusters, now with m c 0.35 M , were born with a stellar mass function very similar to that measured today in the youngest Galactic clusters and with a value of mc around 0.15 M . This is consistent with the absence of a turn-over in the mass function of the Galactic bulge down to the observational limit at ∼ 0.2 M and argues for the universality of the initial mass function of Population I and II stars.