On the ubiquity of molecular anions in the dense interstellar medium

TL;DR

This study surveys molecular anions in dense interstellar clouds, detecting C6H- in all seven sources and C4H- in one, revealing their widespread presence and correlations with cloud properties, and suggesting new formation pathways.

Contribution

It provides the first detections of C6H- and C4H- in multiple star-forming regions, demonstrating the ubiquity of anions and exploring their formation mechanisms in dense clouds.

Findings

C6H- detected in all seven sources, including four new detections.

C6H-/C6H ratio averages 3.0%, correlating with density and age.

First detection of C4H- in TMC-1, with a low anion-to-neutral ratio.

Abstract

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-.