A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics

Abstract

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 reÑect the geometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass King cluster models. The model Ðts 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 0.16È0.21 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 1È2 as would be M _ , expected for general mass 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 suggest that the mass segregation reÑects a preference for higher mass stars to form in dense, central cluster regions.

Tightening the belt: Constraining the mass and evolution in SDC335

“…Hillenbrand (1997) finds evidence for mass segregation in the more evolved ONC with the most massive stars preferentially located within projected radii of <0.3 pc. In a later paper Hillenbrand & Hartmann (1998) find that this distribution of stars is unlikely to be the result of dynamical mass segregation since its timescale is larger than the age of the ONC stars. The authors instead favour a primordial origin of the observed mass segregation.…”

Radio Measurements of the Stellar Proper Motions in the Core of the Orion Nebula Cluster

“…Some authors (e.g., Parenago 1954;Strand 1958;Fallon et al 1977) have claimed evidence of expansion or contraction, but it has also been claimed that these results are due to observational errors (e.g., Vasilevskis 1962Vasilevskis , 1971Allen et al 1974). Furthermore, given the velocity dispersions, one would expect the ONC core to be virialized and not contracting nor expanding motions are expected (e.g., Hillenbrand & Hartmann 1998).…”

“…In the core of the ONC (Hillenbrand & Hartmann 1998), there are two sub-regions of particular interest: the Orion Trapezium and the Orion BN/KL region, which is located behind the ONC. Together they cover an area of a few square arcminutes.…”

Sequential star formation in IRAS 06084-0611 (GGD 12-15)

“…The observation that the most massive stars are concentrated towards the center of the cluster is observed in many more young clusters (Hillenbrand & Hartmann 1998;Gouliermis et al 2004). Mass segregation to the center of the cluster could be primordial or dynamical: the most massive stars can be concentrated at or near the center or migrate into the center of the cluster due to two-body interactions.…”