Different Degrees of Nitrogen and Carbon Depletion in the Warm Molecular Layers of Protoplanetary Disks

Observations have revealed that the elemental abundances of carbon and oxygen in the warm molecular layers of some protoplanetary disks are depleted compared to those in the interstellar medium by a factor of similar to 10-100. Meanwhile, little is known about nitrogen. To investigate the time evolution of nitrogen, carbon, and oxygen elemental abundances in disks, we develop a one-dimensional plane-parallel model that incorporates dust settling, turbulent diffusion of dust and ices, as well as gas-ice chemistry including the chemistry driven by stellar UV/X-rays and galactic cosmic rays. We find that gaseous CO in the warm molecular layer is converted to CO2 ice and locked up near the midplane via the combination of turbulent mixing (i.e., the vertical cold finger effect) and ice chemistry driven by stellar UV photons. On the other hand, gaseous N-2, the main nitrogen reservoir in the warm molecular layer, is less processed by ice chemistry and exists as it is. Then, nitrogen depletion occurs solely through the vertical cold finger effect of N-2. As the binding energy of N-2 is lower than that of CO and CO2, the degree of nitrogen depletion is smaller than that of carbon and oxygen depletion, leading to higher elemental abundance of nitrogen than that of carbon and oxygen. This evolution occurs within 1 Myr and proceeds further, when the alpha parameter for the diffusion coefficient is greater than or similar to 10(-3). Consequently, the N2H+/CO column density ratio increases with time. How the vertical transport affects the midplane ice composition is briefly discussed.

Automated summary

Observations have shown that the amounts of carbon and oxygen in the warm molecular layers of some protoplanetary disks are reduced by a factor of 10-100 compared to the interstellar medium. However, little is known about nitrogen in these environments. In order to study the evolution of nitrogen, carbon, and oxygen in these disks, a one-dimensional plane-parallel model incorporating various factors such as dust settling, turbulent diffusion, and gas-ice chemistry is developed. The results indicate that CO is converted to CO2 ice and locked near the midplane due to turbulent mixing and ice chemistry driven by stellar UV photons. On the other hand, gaseous N-2, the main nitrogen reservoir in the warm molecular layer, is less affected by ice chemistry and remains intact. Nitrogen depletion is mainly caused by the vertical cold finger effect of N-2. The nitrogen depletion is less severe than that of carbon and oxygen due to the lower binding energy of N-2. This evolution occurs within 1 million years and continues further when the diffusion coefficient's alpha parameter is greater than or equal to 10(-3). As a result, the ratio of N2H+ to CO column density increases over time. The impact of vertical transport on the midplane ice composition is also briefly discussed.