Aims.Finding tracers of the innermost regions of prestellar cores is important for understanding their chemical and dynamical evolution before the onset of gravitational collapse. While classical molecular tracers, such as CO and CS, have been shown to be strongly depleted in cold, dense gas by condensation on grain mantles, it has been a subject of discussion to what extent nitrogen-bearing species, such as ammonia, are affected by this process. As deuterium fractionation is efficient in cold, dense gas, deuterated species are excellent tracers of prestellar cores. A comparison of the spatial distribution of neutral and ionized deuterated species with the dust continuum emission can thus provide important insights into the physical and chemical structure of such regions.
Methods: .We study the spatial distribution of the ground-state 335.5 GHz line of ND2H in LDN 1689N, using APEX, and compare it with the distribution of the DCO⁺(3-2) line, as well as the 350 mum dust continuum emission observed with the SHARC II bolometer camera at CSO.
Results: .While the distribution of the ND2H emission in LDN 1689N is generally similar to that of the 350 mum dust continuum emission, the peak of the ND2H emission is offset by ~10'' to the East from the dust continuum and DCO⁺ emission peak. ND2H and ND3 share the same spatial distribution. The observed offset between the ND2H and DCO⁺ emission is consistent with the hypothesis that the deuterium peak in LDN 1689N is an interaction region between the outflow shock from IRAS 16293-2422 and the dense ambient gas. We detect the J = 4 --> 3 line of H¹³CO⁺ at 346.998 GHz in the image side band serendipitously. This line shows the same spatial distribution as DCO⁺(3-2), and peaks close to the 350 mum emission maximum which provides further support for the shock interaction scenario.