Physical conditions in the Orion Nebula and an assessment of its helium abundance

摘要: New long-slit spectrophotometric observations were obtained to redetermine the helium abundance of Orion Nebula. The ionic ratio He(+)/H(+) is found remain nearly constant at 0.088 {plus minus} 0.006. In bright innermost region, errors are largely systematic due uncertainties in reddening curve for grains and deviations from case B emissivity. outer regions, dominated by statistical errors. Photoionization models computed determine correction unobserved presence neutral regions where hydrogen ionized. 114 refs.