Star Formation in Space and Time: The Orion Nebula Cluster

We examine the pattern of star birth in the Orion Nebula Cluster (ONC), withthe goal of discerning the cluster's formation mechanism. Outside of theTrapezium, the distribution of stellar masses is remarkably uniform, and is notaccurately described by the field-star initial mass function. The deconvolved,three-dimensional density of cluster members peaks at the Trapezium stars,which are truly anomalous in mass. Using theoretical pre-main-sequence tracks,we confirm the earlier finding that star formation has accelerated over thepast 10 Myr. We further show that the rate of acceleration has been the samefor all masses. Thus, there is no correlation between stellar age and mass,contrary to previous claims. Finally, the acceleration has been spatiallyuniform throughout the cluster. Our reconstruction of the parent molecularcloud spawning the cluster shows that it had a mass of 6700 solar masses priorto its destruction by the Trapezium. If the cloud was supported againstself-gravity by mildly dissipative turbulence, then it contracted in aquasi-static, but accelerating manner. We demonstrate this contractiontheoretically through a simple energy argument. The mean turbulent speedincreased to its recent value, which is reflected in the present-day stellarvelocity dispersion. The current ONC will be gravitationally unbound once clouddestruction is complete, and is destined to become a dispersing OB association.We hypothesize that similarly crowded groups seen at the centers of distant OBassociations are also unbound, and do not give rise to the Galactic populationof open clusters. Finally, accelerating star formation implies that most clumpswithin giant molecular complexes should have relatively low formation activity.Sensitive infrared surveys could confirm this hypothesis.Comment: 25 pages, 10 figures, to appear in Ap