2019ApJ...874...92P

The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH₃, and N₂. The relative abundances of these carriers determine the composition of planetesimals as a function of disk radius due to strong differences in their volatility. A significant sequestration of nitrogen in carriers less volatile than N₂ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exoplanets. While HCN has been detected in small amounts in inner disks (<10 au), so far only relatively insensitive upper limits on inner disk NH₃ have been obtained. We present new Gemini-TEXES high-resolution spectroscopy of the 10.75 µm band of warm NH₃, and use two-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to [NH₃/H_nuc]< 10^–7 at 1 au. These NH₃ abundances are significantly lower than those typical for ices in circumstellar envelopes ([NH₃/H_nuc]∼3×10^–6). We also consistently retrieve the inner disk HCN gas abundances using archival Spitzer spectra, and derive upper limits on the HCN ice abundance in protostellar envelopes using archival ground-based 4.7 µm spectroscopy ([HCN_ice]/[H₂O_ice] < 1.5%-9%). We identify the NH₃/HCN ratio as an indicator of chemical evolution in the disk, and we use this ratio to suggest that inner disk nitrogen is efficiently converted from NH₃ to N₂, significantly increasing the volatility of nitrogen in planet-forming regions.