On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster

We suggest that the Orion A cloud is gravitationally collapsing on largescales, and is producing the Orion Nebula Cluster due to the focusing effectsof gravity acting within a finite cloud geometry. In support of thissuggestion, we show how an elliptical rotating sheet of gas with a modestdensity gradient along the major axis can collapse to produce a structurequalitatively resembling Orion A, with a fan-shaped structure at one end,ridges or filaments along the fan, and a narrow curved filament at the otherend reminiscent of the famous integral-shaped filament. The model produces alocal concentration of mass within the narrow filament which in principle couldform a dense cluster of stars like that of the Orion Nebula. We suggest thatglobal gravitational contraction might be a more common feature of molecularclouds than previously recognized, and that the formation of star clusters is adynamic process resulting from the focusing effects of gravity acting upon thegeometry of finite clouds.Comment: 23 pages, 6 figures, to appear in the Astrophysical Journa