The state of ionization in dense molecular clouds.

Observations of five isotopes of HCO(plus), including the first detection in an astronomical source of the very rare isotope (D-13)/CO(plus), and extensive mappings of three of them, have allowed a careful determination of the DCO(plus)/HCO(plus) abundance ratio throughout regions of the cool molecular clouds TMC 1, TMC 2, L 183 (L 134N), L 1450 (NGC 1333), and NGC 2264. The ratio is found to be large and almost constant in the interior of the clouds, implying that the fractional electron abundance is smaller than 1-2 x 10 to the -7th throughout the cloud centers. Lower limits to the electron abundance, only a factor of 15 smaller than the upper limits, are derived from the observed HCO(plus) column density. The low fractional ionization of the gas inside the clouds suggests that metals, such as Fe and Mg, may be severely depleted.