1999A&A...349..276E

We present heliocentric radial velocities for a large number of emission lines covering all the optical spectral range for two slit positions of the Galactic HII regions M42 and M17 and one slit position of M8. We obtain a fairly similar relation between the heliocentric velocity and the ionization potential of the ions in all the observed slit positions. This general result indicates a similar behaviour of the velocity stratification along the line of sight in the three objects. We suggest that flows of ionized gas streaming away from the molecular clouds toward the observer can explain qualitatively the observations.