H I aperture synthesis observations towards galactic H II regions and molecular clouds - I. NGC 7538 and S159

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Abstract
Aperture synthesis observations at λ 21 cm with an angular resolution of 1.9 × 2.2 arcmin (α × δ respectively) are presented of H I emission and absorption towards the H II region/molecular cloud complex containing NGC 7538 and SI59. Bright H I emission features are associated with both NGC 7538 and S 159. H I absorption has also been mapped across NGC 7538; when combined with the emission observations, this absorption map shows evidence for a shocked neutral shell, expanding at about 10 km s−1, around the H II region, presumably related to the ionization front. These features are consistent with a modified form of the ‘blister’ model for H II regions, in which the ‘Strömgren sphere’ is highly elongated towards the low-density edge of the parent molecular cloud. This elongation is probably caused by the more rapid expansion of ionization fronts into regions of lower density. Strong absorption features are related to the compact H II regions; they may represent H2 gas dissociated by ultraviolet radiation from the exciting stars. H I self-absorption features are associated with some of the molecular cloud components of the complex. The atomic hydrogen abundance in the largest cloud appears to be at least 0.5 per cent that of molecular hydrogen if Ts ∼ 15 K. It is difficult to understand how cosmic ray dissociation could produce so much atomic gas, and the implications of this result for molecular cloud models are discussed.

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