Deuterium fractionation and the degree of ionization in the R Coronae Australis molecular cloud core

The fractionation of D and C-13 in HCO+ was investigated in the R Coronae A ustralis molecular cloud core. The distributions of (HCO+)-C-13 and DCO+ we re found to be morphologically similar but their column density maxima were found to lie in different locations. The (HCO)-C-13+/(HCO+)-O-18 abundance ratio was found to vary little from 1 0 within the mapped region, in excellent agreement with the (CO)-C-13/(CO)- O-18 abundance ratios derived earlier towards the cloud by Harjunpaa & Matt ila (1996). This corroborates the close relationship between HCO+ and CO pr edicted by the chemistry models. The DCO+/HCO+ abundance ratio ranges from 0.006 to 0.04, being lowest towar ds two locations near the embedded infrared source IRS 7 where the kinetic temperature, as derived from methyl acetylene (CH3CCH) observations, is som ewhat elevated. The variation of the degree of deuterium fractionation with in the core is due to an increase in the kinetic temperature near the clust er of newly born stars. This temperature rise results in two effects: First ly, the reaction H2D+ --> H-3(+) becomes faster; and secondly, an intensifi ed desorption from grain surfaces increases the abundance of neutral atoms and molecules in the gas phase leading to the destruction of H-3(+) and H2D + ions. Both processes decrease the DCO+/HCO+ abundance ratio. Far from the active region the derived abundances of neutral species indicate the prese nce of depletion onto grain surfaces. The observations suggest furthermore that the fractional electron abundance , chi(e(-)), is lowest in the dense clump near IRS 7. This region also exhi bits a low degree of gas phase depletion. In fact, increased fractional abu ndances of neutral species such as atomic oxygen and CO lead to a decrease in the [H-3(+)]/[HCO+] abundance ratio which is directly proportional to ch i(e(-)).