Abstract
The observation of microwave spectra of molecules in interstellar clouds allows separation and detection of the lines of isotopes of many of the more common elements. Comparison of intensities of isotopic lines shows that the relative isotopic abundances for C, O, S, N, and Si are generally rather similar to those found on Earth. However, there are interesting and provocative differences.
Special conditions of opacity, of cloud structure, of excitation, or of chemical fractionation of isotopes can impair the determination of isotopic abundances from the intensities and shapes of molecular lines. High opacity is commonly encountered, and so is chemical fractionation at least in the case of the hydrogen isotopes. However, careful selection and interpretation of measurements and comparison of different spectra seem to allow the determination of relative isotopic abundances to a useful precision and degree of certainty. The 12C/13C ratio is generally about 45, one-half that found on Earth, but is as low as about 20 in the Sgr A and Sgr B clouds, and as high as about 80 in some other clouds. Such apparent variations are probably real, and do not depend simply on distance of the cloud from the Galactic Center, as might be expected if interstellar clouds at a given distance are intermingled. The 170/180 abundance ratio is slightly greater in interstellar clouds than on Earth. Both deuterium and 15N are substantially depleted in the Sagittarius clouds as compared with most other parts of the Galaxy. This provides some evidence that deuterium in the Galaxy is a relict of events other than stellar activity.
Most of these results fit rather well current views of the nucleosynthesis and evolving stellar history of the Galaxy. However, the variation in isotopic ratios seems to show that the large molecular clouds, of mass 105 – 106 M⊙, retain their integrity for approximately 109 years or longer. The Earth may have been formed in a cloud which was somewhat poorer than average in 13C relative to 12C. The long lifetime of massive clouds is not surprising, except in view of possible gravitational collapse, for which important details of the dynamics involved are obscure.
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Work supported in part by NASA Grant NGL 05–003–272 and NSF Grant AST 75–13501.
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© 1977 D. Reidel Publishing Company, Dordrecht, Holland
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Townes, C.H. (1977). Isotopic Abundances in Interstellar Clouds. In: Van Woerden, H. (eds) Topics in Interstellar Matter. Astrophysics and Space Science Library, vol 70. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-1254-6_13
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