The Nitrogen Carrier in Inner Protoplanetary Disks

Klaus M. Pontoppidan1, Colette Salyk2, Andrea Banzatti3, Geoffrey A. Blake4, Catherine Walsh5, John H. Lacy6, and Matthew J. Richter7
Astrophysical Journal 874, 92 Link to Article [DOI: 10.3847/1538-4357/ab05d8 ]
1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
2Vassar College Physics and Astronomy Department, 124 Raymond Avenue, Poughkeepsie, NY 12604, USA
3Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA
4Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, 1200 E California Boulevard, Pasadena, CA 91125, USA
5School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
6Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
7Department of Physics, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA

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 $[{\mathrm{NH}}_{3}/{{\rm{H}}}_{\mathrm{nuc}}]\lt {10}^{-7}$ at 1 au. These NH3 abundances are significantly lower than those typical for ices in circumstellar envelopes ($[{\mathrm{NH}}_{3}/{{\rm{H}}}_{\mathrm{nuc}}]\sim 3\times {10}^{-6}$). 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 ([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 N2, significantly increasing the volatility of nitrogen in planet-forming regions.

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