On the Ubiquity of Molecular Anions in the Dense Interstellar Medium

Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C 6 H − is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B, and TMC-1C. The C 6 H − /C 6 H column density ratio is ≳ 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 C 6 H is present. The C 6 H − /C 6 H 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 C 4 H − in TMC-1 (at 4.8σ confidence), and derive an anion-to-neutral ratio C 4 H − /C 4 H =(1.2 ± 0.4) × 10 −5 (= 0.0012% ± 0.0004%). Such a low value compared with C 6 H − highlights the need for a revised radiative electron attachment rate for C 4 H. Chemical model calculations show that the observed C 4 H − could be produced as a result of reactions of oxygen atoms with C 5 H − and C 6 H − .