The fractional ionization in dark molecular clouds

Aims. We have studied the mechanisms which govern the degree of ionization of the gas in molecular clouds and prestellar cores, with a view to interpreting the relative abundances of the carbon-chain species CnH and their negative ions, CnH-. Methods. We followed the chemical evolution of a medium comprising gas and dust as it evolves towards its steady- state composition. Various assumptions were made concerning the grain size-distribution and the fraction of very small grains (in practice, PAH), as well as the cosmic ray ionization rate. Particular attention was paid to reactions which determine the fractional ionization of the gas and the charge of the grains. Results. We found that the abundance ratio n(C6H-)/n(C6H) is determined essentially by the ratio of the free electron density to the density of atomic hydrogen. A model with a high fractional abundance of PAH and a low fractional abundance of electrons yields agreement to a factor of 2 with the value of the ratio C6H-:C6H observed recently in TMC-1. However, the fractional abundances of the molecular ions HCO+ and DCO+ are then higher than observed. The best overall fit with the observations of TMC-1 is obtained when the cosmic ray ionization rate is reduced, together with the rate of removal of atomic hydrogen from the gas phase (owing to adsorption on to grains).