Isotopic Abundance Ratios from Microwave Observations of Formaldehyde

Extract

This report is a summary of the determination of the ratio of the column density of H₂¹²C¹⁶O (hereafter H₂CO) or H₂¹²C¹⁸O (hereafter H₂C¹⁸O) to that of H₂¹³C¹⁶O (hereafter H₂¹³CO). With one exception, all of the published ratios have been determined from measurements of the 1₁₀−1₁₁ lines. The exception is the Orion Molecular Cloud (see discussion of Kutner et al. 1976). The most complete surveys of H₂¹³CO are those of Wilson et al. (1976) and Gardner and Whiteoak (1979). The ratios obtained from 1₁₀−1₁₁ data are lower limits because of the effect of photon trapping in H₂CO (see the discussion by Henkel et al. 1979b). From measurements of the 1₁₀−1₁₁ and 2₁₁−2₁₂ lines and model calculations, Henkel et al. estimate that the corrections for 10 clouds outside the Galactic center region is 1.6±0.7. This correction gives an average (H₂CO/H₂¹³CO) ratio of 64±17. The models of Henkel et al. also predict the optical depth of the 3₁₂−3₁₃ line and the brightness temperature of the 2₁₂−1 ₁₁ line. Preliminary measurements of Wilson et al. (1979) show that the 3₁₂−3₁₃ results are consistent with the predictions; but more sensitive measurements are required to test the model fully. Unpublished 2₁₂− 1 ₁₁ emission-line spectra are weaker than predicted by the model, but this might be caused by beam dilution, since the observations were made so as to cover a 1.5′ uniformly weighted aperture. The lineshapes of the 1₁₀−1₁₁ and 2 ₁₁−2₁₂ absorption lines agree poorly with the 2₁₂−1₁₁ emission lines, indicating either that self-absorption in the millimeter lines of H₂CO is important, or that there are H₂CO clouds behind the continuum sources. Because of their position relative to the continuum sources these clouds would contribute to the millimeter emission lines but not to the centimeter absorption lines.