First detection of gas-phase ammonia in a planet-forming disk NH_3, N_2H^+, and H_2O in the disk around TW Hydrae

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

First detection of gas-phase ammonia in a planet-forming disk. NH₃, N₂H⁺, and H₂O in the disk around TW Hydrae

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 <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 NH310–00 line is detected simultaneously with H2O 110–101 at an antenna temperature of 15.3 mK in the Herschel beam; the same spectrum also contains the N2H+ 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 × 1021 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 × 1022 g, 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 4.9 × 1021 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 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

Steepening of the 820

Context. Grain growth in planet-forming disks is the first step toward the formation of planets. The growth of grains and their inward drift leaves a distinct imprint on the dust surface density distribution and the resulting surface brightness profile of the thermal continuum emission. Aims. We determine the surface brightness profile of the continuum emission using resolved observations at millimeter wavelengths of the disk around TW Hya, and infer the signature of dust evolution on the surface density and dust opacity. Methods. Archival ALMA observations at 820 μm on baselines up to 410 kλ are compared to parameterized disk models to determine the surface brightness profile. Results. Under the assumption of a constant dust opacity, a broken radial power law best describes the dust surface density with a slope of −0.53 ± 0.01 from the 4.1 au radius of the already known inner hole to a turn-over radius of 47.1 ± 0.2 au, steepening to −8.0 ± 0.1 at larger radii. The emission drops below the detection limit beyond ~60 au. Conclusions. The shape of the dust surface density is consistent with theoretical expectations for grain growth, fragmentation, and drift, but its total dust content and its turn-over radius are too large for TW Hya’s age of 8–10 Myr even when taking into account a radially varying dust opacity. Higher resolution imaging with ALMA of TW Hya and other disks is required to establish whether unseen gaps associated with, e.g., embedded planets trap grains at large radii or whether locally enhanced grain growth associated with the CO snow line explains the extent of the millimeter continuum surface brightness profile. In the latter case, population studies should reveal a correlation between the location of the CO snow line and the extent of the millimeter continuum. In the former case, and if CO freeze-out promotes planet formation, this correlation should extend to the location of gaps as well