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 N-2. 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 N-2 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 mu m 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] similar to 3 x 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 mu m spectroscopy ([HCNice]/[H2Oice] < 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 N-2, significantly increasing the volatility of nitrogen in planet-forming regions.