2016A&A...591A.122S

Context. Nitrogen chemistry in protoplanetary disks and the freeze-out on dust particles is key for understanding the formation of nitrogen-bearing species in early solar system analogs. In dense cores, 10% to 20% of the nitrogen reservoir is locked up in ices such as NH₃, NH₄⁺ and OCN^–. So far, ammonia has not been detected beyond the snowline in protoplanetary disks.
Aims. We aim to find gas-phase ammonia in a protoplanetary disk and characterize its abundance with respect to water vapor.
Methods. Using HIFI on the Herschel Space Observatory, we detected for the first time the ground-state rotational emission of ortho-NH₃ in a protoplanetary disk around TW Hya. We used detailed models of the disk's physical structure and the chemistry of ammonia and water to infer the amounts of gas-phase molecules of these species. We explored two radial distributions (extended across the disk and confined to <60au like the millimeter-sized grains) and two vertical distributions (near the midplane and at intermediate heights above the midplane, where water is expected to photodesorb off icy grains) to describe the (unknown) location of the molecules. These distributions capture the effects of radial drift and vertical settling of ice-covered grains.
Results. The NH₃1₀-0₀ line is detected simultaneously with H₂O 1₁₀-1₀₁ at an antenna temperature of 15.3mK in the Herschel beam; the same spectrum also contains the N₂H⁺ 6-5 line with a strength of 18.1mK. We use physical-chemical models to reproduce the fluxes and assume that water and ammonia are cospatial. We infer ammonia gas-phase masses of 0.7-11.0x10²¹g, depending on the adopted spatial distribution, in line with previous literature estimates. For water, we infer gas-phase masses of 0.2-16.0x10²²g, improving upon earlier literature estimates This corresponds to NH₃/H₂O abundance ratios of 7%-84%, assuming that water and ammonia are co-located. The inferred N₂H⁺ gas mass of 4.9x10²¹g agrees well with earlier literature estimates that were based on lower excitation transitions. These masses correspond to a disk-averaged abundances of 0.2-17.0x10^–11, 0.1-9.0x10^–10 and 7.6x10^–11 for NH₃, H₂O and N₂H⁺ respectively.
Conclusions. Only in the most compact and settled adopted configuration is the inferred NH₃/H₂O consistent with interstellar ices and solar system bodies of ∼5%-10%; all other spatial distributions require additional gas-phase NH₃ production mechanisms. Volatile release in the midplane may occur through collisions between icy bodies if the available surface for subsequent freeze-out is significantly reduced, for instance, through growth of small grains into pebbles or larger bodies.