SCYON Abstract

Received on July 25 2002

On the mass function of star clusters

AuthorsPavel Kroupa (1), Christian Boily (2)
Affiliation(1) University of Kiel
(2) Observatoire de Strasbourg
Accepted byMonthly Notices of the Royal Astronomical Society


Clusters that form in total 103 < N < 105 stars (type II clusters) lose their gas within a dynamical time as a result of the photo-ionising flux from O stars. Sparser (type I) clusters get rid of their residual gas on a timescale longer or comparable to the nominal crossing time and thus evolve approximately adiabatically. This is also true for massive embedded clusters (type III) for which the velocity dispersion is larger than the sound speed of the ionised gas. On expelling their residual gas, type I and III clusters are therefore expected to lose a smaller fraction of their stellar component than type II clusters. We outline the effect this has on the transformation of the mass function of embedded clusters (ECMF), which is directly related to the mass function of star-forming molecular cloud cores, to the "initial" MF of bound gas-free star clusters (ICMF). The resulting ICMF has, for a featureless power-law ECMF, a turnover near 104.5 Msun and a peak near 103 Msun. The peak lies around the birth masses of the Hyades, Praesepe and Pleiades clusters. We also find that the entire Galactic population II stellar spheroid can be generated if star formation proceeded via embedded clusters distributed like a power-law MF with exponent 0.9 < ß < 2.6.