The evolution of the initial mass function: from globular toyoung clusters^*

Abstract

The main sequence mass functions (MF) of a large sample of Galacticclusters (young and old) can be well reproduced with a tapered powerlaw distribution function with an exponential truncation of the formdN/dM ∝ m^-α[1 - exp(-m/m_p)^β]. Theaverage value of the power-law index α is very close to Salpeter(~2.3), whereas the peak mass m_p is in the range0.1-0.6 M_⊙ and does not seem to vary in a systematic way withthe present cluster parameters such as metal abundance and centralconcentration. A remarkable correlation with age, however, is seen inthat older disc clusters have higher m_p, although this trend doesnot extend to globular clusters, whose value of m_p is lower thanthat of old open clusters. This trend most likely results from theonset of mass segregation following early dynamical interactions in theloose cluster cores. Differences between globular and younger clustersmay depend on the initial environment of star formation, which in turnaffects their total mass. Mass functions of field populations such asthe solar neighbourhood and bulge are consistent with the hypothesisthat they were built up over time by contributions from many functionsof this type with different peak masses.