The nitrogen carrier in protoplanetary disks
  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 $\rm [NH_3/H_{nuc}]<10^{-7}$ at 1 au.
These NH$_3$ abundances are significantly lower than those typical for ices in
circumstellar envelopes ($[{\rm NH_3/H_{nuc}}]\sim 3\times 10^{-6}$). 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$_{\rm
ice}$]/[H$_2$O$_{\rm ice}$]$<1.5-9$\%). 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.
