Resolved depletion zones and spatial differentiation of N₂H ⁺ and N₂D⁺

We present a study on the spatial distribution of N₂D ⁺ and N₂H⁺ in 13 protostellar systems. Eight of thirteen objects observed with the IRAM 30 m telescope show relative offsets between the peak N₂D⁺ (J = 2 → 1) and N ₂H⁺ (J = 1 → 0) emission. We highlight the case of L1157 using interferometric observations from the Submillimeter Array and Plateau de Bure Interferometer of the N₂D⁺ (J = 3 → 2) and N₂H⁺ (J = 1 → 0) transitions, respectively. Depletion of N₂D⁺ in L1157 is clearly observed inside a radius of 2000 AU (7″) and the N₂H⁺ emission is resolved into two peaks at radii of 1000 AU (3.″5), inside the depletion region of N₂D⁺. Chemical models predict a depletion zone in N₂D⁺ and N₂H⁺ due to destruction of H₂D⁺ at T 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 N₂D⁺ to N₂H⁺ have been shown to decrease as protostars evolve by Emprechtinger et al., but this is the first time depletion zones of N₂D⁺ have been spatially resolved. We suggest that the difference in depletion zone radii for N ₂H⁺ and N₂D⁺ is caused by either the CO evaporation temperature being above 20 K or an H₂ ortho-to-para ratio gradient in the inner envelope.