The Nitrogen Carrier in Inner Protoplanetary Disks

Abstract

The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH_3, 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 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 NH_3 have been obtained. We present new Gemini-TEXES high-resolution spectroscopy of the 10.75 μm band of warm NH_3, and use two-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to [NH_3/H_(nuc)] < 10^(−7) at 1 au. These NH_3 abundances are significantly lower than those typical for ices in circumstellar envelopes ([NH_3/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)]/[H_2O_(ice)] < 1.5%–9%). We identify the NH_3/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 NH_3 to N_2, significantly increasing the volatility of nitrogen in planet-forming regions.