Limits on the D:H ratio in the interstellar medium from molecular observations

Frost, R. L.; Beckman, J. E.; Watt, G. D.; White, G. J. and Phillips, J. P. (1982). Limits on the D:H ratio in the interstellar medium from molecular observations. In: Beckman, J. E. and Phillips, J. P. eds. Submillimetre Wave Astronomy. Cambridge: Cambridge University Press, pp. 253–259.



The abundance ratio of D:H in the interstellar medium can be derived either more directly from the UV absorption measurements of HD and H2 in the spectra of early-type stars, or less directly from millimeter-wave emission in the rotational lines of a variety of molecules. The latter method is applicable over a much larger portion of the galaxy, but is subject to the uncertainty inherent in chemical fractionation. It is shown here how the basic assumptions of temperature distribution and geometry within a single source can cause the derived ratio HDO:H2O to vary between 0.05 and 0.0025, a comparable range to that found with three different molecules DCN:HCN, DNC:HNC, and DCO+:HCO+ in the same source. This result should lead to caution in the interpretation of such abundance ratios and of their gradients within the galaxy.

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