The spatial morphology and dynamical status of a young , still-forming stellar cluster provide valuable clues on the conditions during the star formation event and the processes that regulated it .
We analyze the Orion Nebula Cluster ( ONC ) , utilizing the latest censuses of its stellar content and membership estimates over a large wavelength range .
We determine the center of mass of the ONC , and study the radial dependence of angular substructure .
The core appears rounder and smoother than the outskirts , consistent with a higher degree of dynamical processing .
At larger distances the departure from circular symmetry is mostly driven by the elongation of the system , with very little additional substructure , indicating a somewhat evolved spatial morphology or an expanding halo .
We determine the mass density profile of the cluster , which is well fitted by a power law that is slightly steeper than a singular isothermal sphere .
Together with the ISM density , estimated from average stellar extinction , the mass content of the ONC is insufficient by a factor \sim 1.8 to reproduce the observed velocity dispersion from virialized motions , in agreement with previous assessments that the ONC is moderately supervirial .
This may indicate recent gas dispersal .
Based on the latest estimates for the age spread in the system and our density profiles , we find that , at the half-mass radius , 90 % of the stellar population formed within \sim 5 – 8 free-fall times ( t _ { ff } ) .
This implies a star formation efficiency per t _ { ff } of \epsilon _ { ff } \sim 0.04 – 0.07 , i.e. , relatively slow and inefficient star formation rates during star cluster formation .