ar X iv : a st ro - p h / 06 09 67 9 v 1 2 5 Se p 20 06 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 large scales, and is producing the Orion Nebula Cluster due to the focusing effects of gravity acting within a finite cloud geometry. In support of this suggestion, we show how an elliptical rotating sheet of gas with a modest density gradient along the major axis can collapse to produce a structure qualitatively resembling Orion A, with a fan-shaped structure at one end, ridges or filaments along the fan, and a narrow curved filament at the other end reminiscent of the famous integralshaped filament. The model produces a local concentration of mass within the narrow filament which in principle could form a dense cluster of stars like that of the Orion Nebula. We suggest that global gravitational contraction might be a more common feature of molecular clouds than previously recognized, and that the formation of star clusters is a dynamic process resulting from the focusing effects of gravity acting upon the geometry of finite clouds. Subject headings: ISM: clouds, ISM: structure, stars: formation, open clusters and associations: individual (Orion Nebula Cluster)