The fine structure of the Pleiades luminosity function and pre-main sequence evolution

Abstract

The luminosity function (LF) of the Pleiades cluster stars was constructed for the study of the LF fine structure related to pre-MS stellar evolution. Theoretical luminosity functions based on present-day pre-MS and MS stellar models were constructed and compared with observations. We tested both power- and log-normal laws describing the cluster star IMF. Both single star formation burst- and age spread-models were examined. The agreement between the observed Pleiades CMD with the new HIPPARCOS distance and the theoretical ZAMS for a normal metallicity is excellent when the model positions in the HRD are corrected to the helium abundance Y=3D0.34. The corresponding age of the cluster is log t =3D 7.95. Three features (dips) were found in the observed cluster LF in a magnitude range M_V =3D 5() m - 12() m. Two of them (at M_V =3D 7.5() m and 9.5() m) are assumed to be field LF features: Wielen and Kroupa dips. Theoretical models fail to reproduce them. We attributed the third (brightest) detail (the dip at M_V =3D 5.5() m) to the pre-MS evolution of Pleiades stars. The observed Pleiades LF corresponds in its brighter part to the standard Population I IMF. The log-normal IMF fits the observations much better than a simple power-law IMF. The brightest LF feature could be reproduced in the theoretical LF if a substantial age spread of order of several tens of Myrs is supposed to exist among the Pleiades stars.