A PRELIMINARY-STUDY OF THE ORION NEBULA CLUSTER STRUCTURE AND DYNAMICS

We use optical and near-infrared star counts to explore the structure and dynamics of the Orion Nebula Cluster !ONC). This very young (<1 My r) cluster is not circularly symmetric in projection but is elongated north-south in a manner similar to the molecular gas distribution in t he region, suggesting that the stellar system may still reflect the ge ometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass Kin g cluster models. The model fits suggest that the inner Trapezium regi on should be regarded as the core of the ONC, not as a distinct entity as sometimes advocated. We estimate that the core radius of the clust er is 0.16-0.21 pc and that the central stellar density approaches 2 x 10(4) stars pc(-3). Adopting the stellar velocity dispersion from pub lished proper-motion studies, virial equilibrium would require a total mass within about 2 pc of the Trapezium of similar to 4500 M., slight ly more than twice the mass of the known stellar population and compar able to the estimated mass in molecular gas projected onto the same re gion of the sky. If greater than or similar to 20% of the remaining mo lecular gas is converted into stars, thus adding to the binding mass, given that the present stellar population alone has a total energy clo se to zero, the ONC is likely to produce a gravitationally bound clust er. The ONC also exhibits mass segregation, with the most massive (Tra pezium) stars clearly concentrated toward the center of the cluster an d some evidence for the degree of central concentration to decrease wi th decreasing mass down to 1-2 M., as would be expected for general ma ss segregation. Given the extreme youth of the stars compared with the estimated range of collisional relaxation times, the mass segregation is unlikely to be the result of cluster relaxation. Instead, we sugge st that the mass segregation reflects a preference for higher mass sta rs to form in dense. central cluster regions.