The most luminous stars forming in molecular clouds: A hint to understand the initial stellar mass function

The maximum luminosity of YSOs forming in molecular clouds is investigated as a function of the parent cloud mass, on the basis of a rich cloud sample searched for in the literature. In total, we gathered 499 molecular clouds among the published data, out of which 243 clouds are found to be associated with candidates for YSOs selected from the IRAS point source catalog. A diagram of the maximum stellar luminosity in each cloud and the parent cloud mass shows that the young stars in the clouds associated with H II regions are apparently more luminous than those in clouds away from H II regions over the entire cloud mass range investigated (1 < M-CL/M. < 10(6)). In addition, we found that there are well-defined upper and lower limits in the maximum stellar luminosity distribution with the lower limit having a steeper dependence on the cloud mass (L-MAX proportional to M-CL(1.5)) than the upper one (L-MAX proportional to M-CL(0.8)). All these features could be naturally accounted for if we assume that the luminosity function of YSOs is controlled by the cloud mass and the external pressure imposed on the cloud surface. We introduce a simple model for the stellar luminosity function as a function of these quantities.