ON THE STELLAR POPULATION AND STAR-FORMING HISTORY OF THE ORION NEBULA CLUSTER

We report on the first phase of a study of the stellar population comp rising the Orion Nebula Cluster (ONC). Approximately 50% of the simila r to 3500 stars identified to date within similar to 2.5 pc of the nam esake Trapezium stars are optically visible, and in this paper we focu s on that sample with I<17.5 mag. The large number and number density (n(peak)>10(4) pc(-3)) of stars, the wide range in stellar mass (simil ar to 0.1-50 M.), and the extreme youth (<1-2 Myr) of the stellar popu lation, make the ONC the best site for investigating: (1) the detailed shape of a truly ''initial'' mass spectrum; (2) the apparent age spre ad in a region thought to have undergone triggered star formation; (3) the time sequence of star formation as a function of stellar mass; an d (4) trends of all of the above with cluster radius. Nearly 60% of th e similar to 1600 optical stars have sufficient data (spectroscopy and photometry) for placement on a theoretical HR diagram; this subsample is unbiased with respect to apparent brightness or cluster radius, co mplete down to similar to 1 M., and representative of the total optica l sample below similar to 1 M. for the age and extinction ranges chara cteristic of the cluster. Comparison of the derived KR diagram with tr aditional pre-main-sequence evolutionary calculations shows a trend of increasing stellar age with increasing stellar mass. To avoid the imp lication of earlier characteristic formation times for higher-mass sta rs than for lower-mass stars, refinement of early evolutionary theory in a manner similar to the birthline hypothesis of Palla & Stahler [Ap J, 418, 414 (1993)], is required. Subject to uncertainties in the trac ks and isochrones, we can still investigate stellar mass and age distr ibutions in the ONC. We find the ONC as a whole to be characterized by a mass spectrum which is not grossly inconsistent with ''standard'' s tellar mass spectra. In particular, although there are structural diff erences between the detailed ONC mass spectrum and various models cons tructed from solar neighborhood data, the observed mass spectrum appea rs to a peak at similar to 0.2 M. and to fall off rapidly towards lowe r masses; several substellar objects are present. The abundance of low -mass stars relative to high-mass stars suggests that there: is no bi- modal star formation mode; somewhat ironically, the ONC probably conta ins fractionally more low-mass stars than the solar neighborhood since the population not yet located on the HR diagram is dominated by sub- solar-mass stars. Nonetheless, the ONC mass spectrum is biased towards higher-mass stars within the innermost cluster radii (r(projected)<0. 3 pc). We find the ONC as a whole to be characterized by a mean age of <1 Myr and an age spread which is probably less than 2 Myr, but also by a bias towards younger stars at smaller projected cluster radii. Al though the most massive stars and the youngest stars are found prefere ntially towards the center of the ONC it does not follow that the most massive stars are the youngest stars. A lower limit to the total clus ter mass in stars is M(stars)approximate to 900 M. (probably a factor of <2 underestimate). A lower limit to the recent low-mass star format ion rate is approximate to 10(-4) M. yr(-1). Ah observational data in this study as well as stellar parameters derived from them are availab le in electronic format. (C) 1997 American Astronomical Society.