ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N{sub 2}H{sup +} RING IN PROTOPLANETARY DISKS
- Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8577 (Japan)
- Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands)
- Department of Earth and Planetary Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo (Japan)
- Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)
We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of the N{sub 2}H{sup +} ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N{sub 2}H{sup +} is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N{sub 2}. Using the full gas-grain network model, we reproduced the N{sub 2}H{sup +} ring in a disk model with millimeter grains. The chemical conversion of CO and N{sub 2} to less volatile species (sink effect hereinafter) is found to affect the N{sub 2}H{sup +} 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 N{sub 2} 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 N{sub 2}H{sup +} ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N{sub 2}H{sup +}, however, is different among the full-network model and no-sink model. The column density of N{sub 2}H{sup +} in the no-sink model depends sensitively on the desorption rate of CO and N{sub 2} and the cosmic-ray flux. We also found that N{sub 2}H{sup +} abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N{sub 2} are the same.
- OSTI ID:
- 22522187
- Journal Information:
- Astrophysical Journal, Vol. 807, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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