Detecting nitrogen-carriers in the inner regions of protoplanetary disks

TL;DR

This study models and observes nitrogen-bearing molecules in protoplanetary disks, predicting detectable NO fluxes and exploring the challenges of detecting NH3 with JWST, to better understand nitrogen's role in planet formation.

Contribution

It combines thermo-chemical modeling with observational data to assess N-molecule detectability and the nitrogen budget in planet-forming disks, highlighting NO as a promising tracer.

Findings

NO flux may be detectable in disks, providing insights into N-budget.

NH3 detection remains challenging with current instruments like JWST.

Future FIR facilities could improve NH3 detection prospects.

Abstract

Nitrogen is a key element for building habitable worlds, yet only a small fraction of the available N-budget of planet-forming disks has been detected. In particular, the lack of any IR NH$_3$ detection is striking, as this molecule is predicted to be rather abundant in the warm, inner regions of protoplanetary disks, and therefore potentially readily incorporated into (giant) planets' atmospheres. We present a combined modeling and observational study of N-bearing molecules in planet-forming disks, using detailed thermo-chemical disk models that investigate the sensitivity of N-containing molecules to the bulk elemental composition of the disk. Our models predict a strong increase in HCN flux with high C/H, and conversely a strong increase in flux from NO when O/H is high. The flux from NH$_3$ is not very sensitive to O/H, but does decrease at high C/H due to competition with HCN. However, the absolute NH$_3$ flux predicted by our model is not large enough to be detected with JWST-MIRI, even when N/H is enhanced by an order of magnitude. The flux from NO, on the other hand, is potentially detectable, and could therefore provide further insights into the N-budget of the inner disk. Using a cross-correlation technique, we search for NH$_3$ and NO detections in three disks, GW Lup, Sz 98, and V1094 Sco. We do not find any NH$_3$ detections, and only one tentative NO detection in V1094 Sco, though this needs further study to be confirmed. Additionally, we demonstrate that future facilities in the FIR may provide a better opportunity to detect NH$_3$ and thereby draw a comparison to the NH$_3$ budget known to be present in interstellar ices.