White, Glenn J. and Phillips, J. P.
(1988).
CO J = 2-1 mapping of NGC 1976.
Astronomy and Astrophysics, 197(1-2) pp. 253–260.
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Abstract
The authors present 77 CO J = 2-1 spectra centred upon four areas about the periphery of the Orion nebula NGC 1976. These reveal evidence of extensive line splitting, with primary components at VLSR ~ 12 km s-1 and VLSR ~ 7 km s-1. The spatial structure of the regions responsible for these components appears to differ appreciably, and various arguments are outlined to suggest that two distinct segments of a single enveloping cloud are responsible; one to the rear of the H II region (VLSR ~ 7 km s-1), associated with various star formation zones, the other in front of NGC 1976, and responsible for localised foreground extinction. This contrasts with previous models, which have proposed compressive shock regions ahead of local ionisation fronts; an hypothesis which is shown not to be necessary, and may be inconsistent with the observed width and conformity of the line splitting zone.
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