Deuterium fractionation in cold dense cores in the low-mass star forming region L1688

In this work, we study deuterium fractionation in four starless cores in thelow-mass star-forming region L1688 in the Ophiuchus molecular cloud. We studyhow the deuterium fraction ($R_D$) changes with environment, comparedeuteration of ions and neutrals, core centre and its envelope, and attempt toreproduce the observed results with a gas-grain chemical model. We chose highand low gas density tracers to study both core centre and the envelope. Withthe IRAM 30m antenna, we mapped N$_2$H$^+$(1-0), N$_2$D$^+$(1-0),H$^{13}$CO$^+$ (1-0) and (2-1), DCO$^+$(2-1), and$p$-NH$_2$D(1$_{11}$-1$_{01}$) towards the chosen cores. The missing $p$-NH$_3$and N$_2$H$^+$(1-0) data were taken from the literature. To measure themolecular hydrogen column density, dust and gas temperature within the cores,we used the Herschel/SPIRE dust continuum emission data, the GAS survey data(ammonia), and the COMPLETE survey data to estimate the upper limit on COdepletion. We present the deuterium fraction maps for three species towardsfour starless cores. Deuterium fraction of the core envelopes traced byDCO$^+$/H$^{13}$CO$^+$ is one order of magnitude lower ($\sim$0.08) than thatof the core central parts traced by the nitrogen-bearing species ($\sim$0.5).Deuterium fraction increases with the gas density as indicated by highdeuterium fraction of high gas density tracers and low deuterium fraction oflower gas density tracers and by the decrease of $R_D$ with core radii,consistent with the predictions of the chemical model. Our model results show agood agreement with observations for $R_D$(N$_2$D$^+$/N$_2$H$^+$) andR$_D$(DCO$^+$/HCO$^+$) and underestimate the $R_D$(NH$_2$D/NH$_3$).