2013ApJ...770...48C

Result. 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^–/C₆ 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 C₆ H 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 4.8σ confidence), and derive an anion-to-neutral ratio C₄H^–/C₄ H =(1.2 ± 0.4)x10^–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^–.