The existence of dust in gaseous nebulae, its location within the nebula, and its effects on nebular structure and evolution are problems that are critical to our understanding and interpretation of the spectra of these objects. We have calculated depletions of Al and Ca in a planetary nebula and an H II region based on observations and photoionization models. These calculations were aided by computation of charge exchange rate coefficients for several ions of Al and Ca with H, derived using the Landau-Zener approximation. The resulting depletions provide indirect evidence for the presence of dust grains within the nebulae. Further, comparison of Ca depletions derived from (Ca II) and (Ca V) lines suggest the existence of a depletion gradient, in the sense that the higher ionization stages show less depletion. We have run a photoionization model of a test planetary nebula to examine several grain properties and effects as a function of nebular radius. Our most important result is that heating by grains through photoejection can amount to a significant fraction of the total nebular heating, especially in the innermost regions. One important effect of dust grains within nebulae is the destruction of resonance lines. We have examined the effect of dust on the intensity of He I lambda 10830 (2^3S-2^3 P), which is an important diagnostic of the 2 ^3S level population. This line has been observed to be weaker than predicted by roughly a factor of 2. The resolution of this discrepancy is critical for our understanding of collisional effects in He I, which in turn has important ramifications for He abundances. We find that a combination of destruction by dust, telluric absorption in the Earth's atmosphere, and observational uncertainties can resolve the discrepancy. We have also made observations of optical He I lines in two planetary nebulae to further examine collisional effects. Our results suggest that current theoretical predictions are correct, and that the uncertainties acquired in observationally determining collisional contributions to lines are appreciable. We therefore conclude that there is no compelling evidence for any depopulation of the 2 ^3S state of He I. This implies that He abundances have previously been overestimated.
- Pub Date:
- January 1993
- Physics: Astronomy and Astrophysics