Abstract
Nearly 41,000 radial velocity points across the H II region S142 have been measured using an efficient Fabry-Perot interferometer camera and image processing techniques. The locus of the most negative H-alpha velocities coincides with the position of the 'hot' component of the CO molecular cloud to the east of the nebula. The observed velocity field is explained as a systematic expansion of the ionized gas away from the molecular cloud and from the observer. There is a striking match between the most negative velocities and the shape of the CO emission contours. Some neutral material acts as a wall across the face of the H II region, inhibiting the flow of ionized gas in the earth's direction. The resulting flow configuration explains the radial velocity gradient across the nebula and the fact that the mean V(LSR) for the whole nebula is redshifted by +5 km/s with respect to the -41 km/s of the molecular cloud.
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