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Flows and Structures in Star Forming Regions (1Dec93) (from NOAO Highlights!, NOAO Newsletter No. 36, 1 December 1993) Much of recent work in the field of star formation has concentrated on examining the mechanisms of interaction between newly formed stars and the molecular cloud from which they formed. Structures are indeed present in the molecular cloud once stars form, but the state of the cloud, whether homogeneous, highly structured, or somewhere in between, prior to the start of star formation is uncertain. Once star formation begins, mechanisms such as shocks, infall, winds and mass outflow, and the radiation of the star itself interact with the molecular cloud, both producing structure and changing the thermal state of the medium. Regions of bi-polar flow and lenticular structures such as disks can be identified near newly formed stars of solar mass or larger. Changes in the thermal state of material near newly forming stars also modify or eliminate the cooling mechanism needed for star formation itself. In a cool molecular cloud, far infrared emission from molecules, neutral atoms, and possibly grains can remove the heat generated by the star formation process. When the molecules and grains are destroyed and the atoms ionized, these initial cooling mechanisms are reduced or eliminated, but others must arise to allow the process of star formation to continue. The interaction between newly formed stars and the surrounding medium can have a profound effect on the process of star formation itself. How is the process of star formation regulated? Does the interaction trigger or inhibit formation of stars in different mass ranges? The regions of photo-dissociation near newly forming stars may provide such mechanisms, and a clear understanding of the physics of photo-dissociative regions is critical to understanding star formation. In external galaxies, particularly starburst galaxies, photo-dissociative regions may dominate, or be the only regions accessible to observation. The interpretation of such observations must depend on a clear understanding of photo-dissociative regions in the Milky Way. [Figure not included] A clearer understanding of the interaction between newly forming stars and the interstellar medium is beginning to come from observations of bright, nearby regions of active star formation such as M17 (the Horseshoe Nebula, shown at 3.3 fm in the picture) and M42 (the Orion Nebula). Infrared images obtained by I. Gatley and M. Merrill (NOAO) with new, large format array detectors and instruments such as COB and SQIID help to reveal the structural and thermal state in the material from which stars are forming. Diagnostic features available with this instrumentation include Brackett series hydrogen recombination emission strengths to map the distribution of, and extinction to, the ionized gas; molecular hydrogen emission line ratios to map the distribution and excitation of the molecular gas; and images at 3.3 fm at the emission band normally attributed to poly-aromatic hydrocarbons to trace the dust emission. Together these features trace the dense ionization fronts in the nebulosity surrounding young stars, revealing the structure and state of the gas and dust. The excitation mechanism of the molecular gas, whether UV fluorescence or mechanical shock from stellar winds, can also be determined from these diagnostics. Of further concern is the effect of patchy extinction on the on the line ratios used to deredden unresolved star forming regions in external galaxies. A series of related high resolution spectroscopic studies have been carried out by C.R. O'Dell (Rice U.) and his collaborators (H.O. Castanada (Inst. Astrofisica de Canarias), D. Meyer (Northwestern U.), and M.R. Jones, Z. Wen, and D.K. Walters (Rice U.)). Coude Feed spectra across the face of the Orion nebula have revealed the presence of multiple emitting layers. When these are combined with 21 cm radio data, maps of the interstellar extinction, and new absorption line spectra of the associated bright stars, a detailed three dimensional picture of the region emerges. The nebula is revealed to be a thin layer imposed near the front of the Orion Molecular Cloud. A thin layer of neutral material from the molecular cloud also lies in front of the entire region.
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