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 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 approx. > 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)10(exp 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()