The Nitrogen Carrier in Inner Protoplanetary Disks.

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 at 1 au. These NH₃ abundances are significantly lower than those typical for ices in circumstellar envelopes (). 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. [ABSTRACT FROM AUTHOR]