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 N2H+ (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 similar to 2000 AU (7 '') and the N2H+ emission is resolved into two peaks at radii of similar to 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 similar to 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 N2H+ and N2D+ 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.