We present a study on the spatial distribution of N _ { 2 } D ^ { + } and N _ { 2 } H ^ { + } in thirteen protostellar systems .
Eight of thirteen objects observed with the IRAM 30m telescope show relative offsets between the peak N _ { 2 } D ^ { + } ( J = 2 \rightarrow 1 ) and N _ { 2 } H ^ { + } ( J = 1 \rightarrow 0 ) emission .
We highlight the case of L1157 using interferometric observations from the Submillimeter Array and Plateau de Bure Interferometer of the N _ { 2 } D ^ { + } ( J = 3 \rightarrow 2 ) and N _ { 2 } H ^ { + } ( J = 1 \rightarrow 0 ) transitions respectively .
Depletion of N _ { 2 } D ^ { + } in L1157 is clearly observed inside a radius of \sim 2000 AU ( 7″ ) and the N _ { 2 } H ^ { + } emission is resolved into two peaks at radii of \sim 1000 AU ( 3.5″ ) , inside the depletion region of N _ { 2 } D ^ { + } .
Chemical models predict a depletion zone in N _ { 2 } D ^ { + } and N _ { 2 } D ^ { + } due to destruction of H _ { 2 } D ^ { + } at T \sim 20 K and the evaporation of CO off dust grains at the same temperature .
However , the abundance offsets of 1000 AU between the two species are not reproduced by chemical models , including a model that follows the infall of the protostellar envelope .
The average abundance ratios of N _ { 2 } D ^ { + } to N _ { 2 } H ^ { + } have been shown to decrease as protostars evolve by Emprechtinger et al. , but this is the first time depletion zones of N _ { 2 } D ^ { + } have been spatially resolved .
We suggest that the difference in depletion zone radii for N _ { 2 } H ^ { + } and N _ { 2 } D ^ { + } is caused by either the CO evaporation temperature being above 20 K or an H _ { 2 } ortho-to-para ratio gradient in the inner envelope .