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 \gtrsim 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 4.8 \sigma confidence ) , and derive an anion-to-neutral ratio C _ { 4 } H ^ { - } /C _ { 4 } H = ( 1.2 \pm 0.4 ) \times 10 ^ { -5 } ( = 0.0012 \pm 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 ^ { - } .