The nitrogen carrier in protoplanetary disks. (arXiv:1902.03647v1 [astro-ph.SR])

The nitrogen carrier in protoplanetary disks. (arXiv:1902.03647v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pontoppidan_K/0/1/0/all/0/1">Klaus M. Pontoppidan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Salyk_C/0/1/0/all/0/1">Colette Salyk</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Banzatti_A/0/1/0/all/0/1">Andrea Banzatti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blake_G/0/1/0/all/0/1">Geoffrey A. Blake</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walsh_C/0/1/0/all/0/1">Catherine Walsh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lacy_J/0/1/0/all/0/1">John H. Lacy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Richter_M/0/1/0/all/0/1">Matthew J. Richter</a>

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 ($[{rm NH_3/H_{nuc}}]sim 3times 10^{-6}$). 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.

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 ($[{rm NH_3/H_{nuc}}]sim 3times 10^{-6}$). 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.

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