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

The fractionation of D and 13C in HCO+ was investigated in the R Coronae Australis molecular cloud core. The distributions of H13CO+ and DCO+ were found to be morphologically similar but their column density maxima were found to lie in different locations. The H13CO+/HC18O+ abundance ratio was found to vary little from 10 within the mapped region, in excellent agreement with the 13CO/C18O abundance ratios derived earlier towards the cloud by Harjunpää & Mattila (1996). This corroborates the close relationship between HCO+ and CO predicted by the chemistry models. The DCO+/HCO+ abundance ratio ranges from 0.006 to 0.04, being lowest towards two locations near the embedded infrared source IRS 7 where the kinetic temperature, as derived from methyl acetylene (CH3CCH) observations, is somewhat elevated. The variation of the degree of deuterium fractionation within the core is due to an increase in the kinetic temperature near the cluster of newly born stars. This temperature rise results in two effects: Firstly, the reaction H2D → H3 becomes faster; and secondly, an intensified desorption from grain surfaces increases the abundance of neutral atoms and molecules in the gas phase leading to the destruction of H3 and H2D + ions. Both processes decrease the DCO+/HCO+ abundance ratio. Far from the active region the derived abundances of neutral species indicate the presence of depletion onto grain surfaces. The observations suggest furthermore that the fractional electron abundance, χ(e−), is lowest in the dense clump near IRS 7. This region also exhibits a low degree of gas phase depletion. In fact, increased fractional abundances of neutral species such as atomic oxygen and CO lead to a decrease in the [H3 ]/[HCO +] abundance ratio which is directly proportional to χ(e−).