Photoelectric spectrophotometry of gaseous nebulae IV, interaction of dust, gas and radiation

Abstract

Additional evidence for the existence of interstellar grains in H II regions is presented from observations of galactic nebulae. The removal of dust by means of radiation pressure is quantitatively evaluated theoretically and rejected as being important in producing the dust deficiency in the Orion Nebula. Evaporation of the grains by means of absorption of stellar continuum radiation is tested and also rejected, while evaporation by absorption of resonance-line Lyman-α radiation can cause evaporation in the Orion Nebula even if the grains are as stable as H₂0 ice. In the case of ionization-bounded nebulae, absorption by grains can destroy the majority of Lyman-α photons. The oxygen-to-hydrogen ratio is determined in NGC 6514 and is found to be similar to that in the Orion Nebula, arguing against a depletion of atomic oxygen by adsorption onto the very abundant grains in this particular nebula. The dynamical force of the grains on the gas as the particles are being pushed by radiation pressure is demonstrated. This strong central force can be important in the determination of the density structure of the inner regions of H II regions.