ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+ RING IN PROTOPLANETARY DISKS

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We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of the N2H+ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N2H+ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N2. Using the full gas-grain network model, we reproduced the N2H+ ring in a disk model with millimeter grains. The chemical conversion of CO and N2 to less volatile species (sink effect hereinafter) is found to affect the N2H+ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain-surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulae, which are derived by analyzing the full-network model. The N2H+ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N2H+, however, is different among the full-network model and no-sink model. The column density of N2H+ in the no-sink model depends sensitively on the desorption rate of CO and N2 and the cosmic-ray flux. We also found that N2H+ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N2 are the same.

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