Resolved Depletion Zones and Spatial Differentiation of N2H+ and N2D+
Résumé
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 ~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 N2H+ 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. Based on observations carried out with the IRAM 30 m Telescope, the IRAM Plateau de Bure Interferometer, and the Submillimeter Array. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).