The nitrogen carrier in 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 N${}_2$ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exo-planets. 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 $\mu$m band of warm NH${}_3$, and use 2-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to $[\mathrm{N}{\mathrm{H}}_3/{\mathrm{H}}_{\mathrm{nuc}}]<10^{-7}$ at 1 au. These NH${}_3$ abundances are significantly lower than those typical for ices in circumstellar envelopes ($[\mathrm{N}{\mathrm{H}}_3/{\mathrm{H}}_{\mathrm{nuc}}]\sim 3\times 10^{-6}$). 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 $\mu$m spectroscopy ([HCN${}_{\mathrm{ice}}$]/[H${}_2$O${}_{\mathrm{ice}}$]$<1.5-9$\%). We identify the NH${}_3$/HCN ratio as an indicator of chemical evolution in the disk, and 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.

Accepted for publication in the Astrophysical Journal

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