Key points are not available for this paper at this time.
We use optical and near-infrared star counts to explore the structure and dynamics of the Orion Nebula Cluster (ONC). This very young (\1 Myr) cluster is not circularly symmetric in projection but is elongated north-south in a manner similar to the molecular gas distribution in the region, suggesting that the stellar system may still reect the geometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass King cluster models. The model ts suggest that the inner Trapezium region 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 cluster is 0. 160. 21 pc and that the central stellar density approaches 2 ] 104 stars pc~3. Adopting the stellar velocity dispersion from published proper-motion studies, virial equilibrium would require a total mass within about 2 pc of the Trapezium of D4500 slightly more than twice the mass of the known stellar M _, population and comparable to the estimated mass in molecular gas projected onto the same region of the sky. If of the remaining molecular gas is converted into stars, thus adding to the binding Z20% mass, given that the present stellar population alone has a total energy close to zero, the ONC is likely to produce a gravitationally bound cluster. The ONC also exhibits mass segregation, with the most massive (Trapezium) stars clearly concentrated toward the center of the cluster and some evidence for the degree of central concentration to decrease with decreasing mass down to 12
Hillenbrand et al. (Sat,) studied this question.