Interstellar molecules

Abstract
Scans were made at the absorption wavelengths of 14 molecules resulting in the detection of three molecules: H2, HD and CO. Lines of H2 are measured in all 11 reddened stars (EB-V > 0.10) analysed with f, the fraction of hydrogen in molecular form, exceeding 10-1. In eight of nine unreddened stars (EB-V < 0.05) no H2 lines are detected limiting f to less than 10-7. Ratios of orthohydrogen (J = 1) to parahydrogen (J = 0) correspond to an average excitation temperature of 81 K. Two HD lines have been measured in nine stars yielding a ratio of HD to H2 of about 10-6. Consideration of the differences in self-shielding between H2 and HD for disruption by ultraviolet photons indicates that one HD molecule is formed and dissociated for about every 200 H2 molecules. Column densities for CO for four reddened stars have been determined. Ratios of the J = 1 to J = 0 rotational levels in CO yield an excitation temperature of 5 K for CO toward three stars.

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