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The spatial morphology and dynamical status of a young, still-forming stellarcluster provide valuable clues on the conditions during the star formationevent and the processes that regulated it. We analyze the Orion Nebula Cluster(ONC), utilizing the latest censuses of its stellar content and membershipestimates over a large wavelength range. We determine the center of mass of theONC, and study the radial dependence of angular substructure. The core appearsrounder and smoother than the outskirts, consistent with a higher degree ofdynamical processing. At larger distances the departure from circular symmetryis mostly driven by the elongation of the system, with very little additionalsubstructure, indicating a somewhat evolved spatial morphology or an expandinghalo. We determine the mass density profile of the cluster, which is wellfitted by a power law that is slightly steeper than a singular isothermalsphere. Together with the ISM density, estimated from average stellarextinction, the mass content of the ONC is insufficient by a factor $\sim 1.8$to reproduce the observed velocity dispersion from virialized motions, inagreement with previous assessments that the ONC is moderately supervirial.This may indicate recent gas dispersal. Based on the latest estimates for theage spread in the system and our density profiles, we find that, at thehalf-mass radius, 90% of the stellar population formed within $\sim 5$-$8$free-fall times ($t_{\rm ff}$). This implies a star formation efficiency per$t_{\rm ff}$ of $\epsilon_{\rm ff}\sim 0.04$-$0.07$, i.e., relatively slow andinefficient star formation rates during star cluster formation.

The Structure, Dynamics and Star Formation Rate of the Orion Nebula Cluster