The Helium Spectrum and Dust in the Orion Nebula

Abstract

view Abstract Citations (40) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Helium Spectrum and Dust in the Orion Nebula Robbins, R. R. Abstract Capture-cascade intensities appropriate to diffuse nebulae are calculated and then modified for selfabsorption in the helium triplets by the construction and solution of an integral equation of transfer. Comparison between these calculations and observed helium intensities in the nebula show that AT(He)/ ]\T(H) = 0.12 in Orion and that the optical thickness of the triplets is ro(3889) 15 if large-scale motions or expansion are not important in the transfer of radiation, but is closer to 25 if they are. The corresponding optical depths in the 10830 A line would be 370 and 615. The fit between theory and observation is sensitive to possible inhomogeneities in optical depth in the nebula. The results from the transfer solution are found to be consistent wfth the population of the metastable level that is implied by its equilibrium equation if the nebular material is assumed to exist in condensations occupying approximately one-twentieth to one-thirtieth of the volume of the nebula. It further agrees with the population implied by the collisional excitation of 10830 if account is taken of the destruction of these photons upon dust grains in the nebula. A significant fraction of the X10830 photons appear to be extinguished on grains, which suggests in turn that hydrogen La is probably completely extinguished in this fashion. The dust opacity at X10830 derived from the emergent intensity of 10830 agrees roughly with estimates from visual photometry and is relatively insensitive to assumed conditions. Under certain conditions, it should be a useful point for normalizing dust calculations. The above three methods of estimating the metastable population appear to be in fair agreement with the population implied by the 3889 absorption features seen in the spectrum of the Trapezium stars, when the rate of magnetic-dipole depopulation for the 23S level recently calculated by Griem is employed. Publication: The Astrophysical Journal Pub Date: May 1970 DOI: 10.1086/150451 Bibcode: 1970ApJ...160..519R full text sources ADS |