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Paper: Molecular Absorption in the Local Diffuse Interstellar Medium
Volume: 156, Highly Redshifted Radio Lines
Page: 188
Authors: Liszt, H.; Lucas, R.
Abstract: A study of mm-wave absorption lines shows that molecules common to dark clouds have surprisingly high abundances even in local diffuse gas with AV <= 1. Because relative abundances of trace molecules are so similar in dark and diffuse gas, and because diffuse clouds may also produce heavily saturated lines of CO, hcop, and HCN, it may be difficult to distinguish the two cases or to infer N(HH). For CO and its isotopes, either the lines are too optically thick or the CO/HH ratios are too uncertain to provide a useful indicator of N(HH). There are some distinguishing characteristics of diffuse gas, notably an absence of nnhp and high N(coth)/N(coei). Of all the molecules seen in absorption, OH and hcop have the best-determined and most stable abundances relative to HH.
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