RESOLVED DEPLETION ZONES AND SPATIAL DIFFERENTIATION OF N₂H⁺AND N₂D⁺

Abstract

We present a study on the spatial distribution of N 2 D + and N 2 H + in 13 protostellar systems. Eight of thirteen objects observed with the IRAM 30 m telescope show relative offsets between the peak N 2 D + (J = 2 → 1) and N 2 H + (J = 1 → 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 → 2) and N 2 H + (J = 1 → 0) transitions, respectively. Depletion of N 2 D + in L1157 is clearly observed inside a radius of ∼2000 AU (7 ) and the N 2 H + emission is resolved into two peaks at radii of ∼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 H + due to destruction of H 2 D + 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 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.

The Vla Nascent Disk and Multiplicity Survey: First Look at Resolved Candidate Disks Around Class 0 and I Protostars in the Perseus Molecular Cloud

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A molecular line study towards massive extended green object clumps in the southern sky: chemical properties

“…In the chemical models of low mass star formation, a relative enhancement of N 2 H + abundance is expected in the cold prestellar phase, as CO is thought to be depleted in starless cores (e.g. Lee et al 2003;Bergin & Tafalla 2007 central star evolves, the gas gets warm and CO should be evaporated from the dust grains if the dust temperature exceeds about 20 K (Tobin et al 2013). Thus the N 2 H + abundance should decrease as a function of evolutionary stage.…”

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. I. Identifying and Characterizing the Protostellar Content of the OMC-2 FIR4 and OMC-2 FIR3 Regions

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