Isotope Fractionation in Interstellar Molecules

Abstract

Fractionation of carbon isotopes in interstellar clouds is treated in some detail. Acting together, the exchange reaction ¹³c⁺+¹²co = ¹²c⁺+¹³co + ΔE and the high vapor pressure of CO may cause a preferential loss of ¹²C from the gas due to freezing onto dust grains. The ¹³c/¹²c ratio for the carbon in the gas is then raised by factors ∼ 2 to 4. Fractionation of deuterium is also discussed since the proposed process is similar to the carbon isotope exchange and here there is no question that fractionation is a major effect. Important information about physical conditions in dense clouds — the fractional electron density and CO abundance — can be derived from the fractionation process producing the enhanced DCO⁺/HCO⁺ ratio. Possible fractionation reactions for isotopes of other elements (oxygen, sulfur) are mentioned briefly.

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Supported in part by the National Science Foundation of the U. S., Grant MPS 73-04781.

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Alfred P. Sloan Foundation Fellow.