THE FINE-STRUCTURE OF THE PLEIADES LUMINOSITY FUNCTION AND PRE-MAIN-SEQUENCE EVOLUTION

The luminosity function (LF) of the Pleiades cluster stars was constru cted for the study of the LF fine structure related to pre-MS stellar evolution. Theoretical luminosity functions based on present-day pre-M S and MS stellar models were constructed and compared with observation s. We tested both power- and log-normal laws describing the cluster st ar IMF. Both single star formation burst- and age spread-models were e xamined. The agreement between the observed Pleiades CMD with the new HIPPARCOS distance and the theoretical ZAMS for a normal metallicity i s excellent when the model positions in the HRD are corrected to the h elium abundance Y=0.34. The corresponding age of the cluster is log t = 7.95. Three features (dips) were found in the observed cluster LF in a magnitude range M-V = 5(m) - 12(m). Two of them (at M-V = 7.5(m) an d 9.5(m)) are assumed to be field LF features: Wielen and Kroupa dips. Theoretical models fail to reproduce them. We attributed the third (b rightest) detail (the dip at M-V = 5.5(m)) to the pre-MS evolution of Pleiades stars. The observed Pleiades LF corresponds in its brighter p art to the standard Population I IMF. The log-normal IMF fits the obse rvations much better than a simple power-law IME The brightest LF feat ure could be reproduced in the theoretical LF if a substantial age spr ead of order of several tens of Myrs is supposed to exist among the Pl eiades stars.