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 On the ubiquity of molecular anions in the dense interstellar medium


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Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B and TMC-1C. The C6H-C6H column density ratio is greater than about 1.0% in every source, with a mean value of 3.0% and standard deviation 0.92%. Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H-C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud. We also report the first detection of C4H- in TMC-1 at 4.8-sigma confidence, and derive an anion-to-neutral ratio C4H-C4H = 1.2 +- 0.4 x 10^-5 = 0.0012 +- 0.0004%. Such a low value compared with C6H- highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H- could be produced as a result of reactions of oxygen atoms with C5H- and C6H-.



Author: M. A. Cordiner; J. V. Buckle; E. S. Wirström; A. O. H. Olofsson; S. B. Charnley

Source: https://archive.org/



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