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 NH_3, NH_4^+ 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-NH_3 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 <60 au 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 NH_31_0–0_0 line is detected simultaneously with H_2O 1_(10)–1_(01) at an antenna temperature of 15.3 mK in the Herschel beam; the same spectrum also contains the N_2H^+ 6–5 line with a strength of 18.1 mK. We use physical-chemical models to reproduce the fluxes and assume that water and ammonia are cospatial. We infer ammonia gas-phase masses of 0.7−11.0 × 10^(21) g, depending on the adopted spatial distribution, in line with previous literature estimates. For water, we infer gas-phase masses of 0.2−16.0 × 10^(22) g, improving upon earlier literature estimates This corresponds to NH_3/H_2O abundance ratios of 7%−84%, assuming that water and ammonia are co-located. The inferred N_2H^+ gas mass of 4.9 × 10^(21) g agrees well with earlier literature estimates that were based on lower excitation transitions. These masses correspond to a disk-averaged abundances of 0.2−17.0 × 10^(-11), 0.1−9.0 × 10^(-10) and 7.6 × 10^(-11) for NH_3, H_2O and N_2H^+ respectively.
Conclusions. Only in the most compact and settled adopted configuration is the inferred NH_3/H_2O consistent with interstellar ices and solar system bodies of ~5%–10%; all other spatial distributions require additional gas-phase NH_3 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
