Isotopic Abundance Ratios from Microwave Observations of Formaldehyde

Abstract

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 Unpublished 212− 1 11 emission-line spectra are weaker than predicted by the model, but this might be caused by beam dilution, since the observations were made so as to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.