Chemistry in Protoplanetary Disks: the gas-phase CO/H2 ratio and the Carbon reservoir

TL;DR

This study investigates how CO gas abundance in protoplanetary disks is influenced by chemical reactions, temperature, and cosmic rays, revealing that CO is a poor tracer of disk gas content due to complex surface processes.

Contribution

It demonstrates that CO gas-phase abundance is heavily affected by grain surface chemistry and temperature, challenging its use as a gas mass tracer in disks.

Findings

CO converts to less volatile forms on grain surfaces at high densities.

Gas-phase CO abundance is only high above 30-35 K.

CO abundance depends on disk temperature, age, and cosmic ray ionization.

Abstract

The gas mass of protoplanetary disks, and the gas-to-dust ratio, are two key elements driving the evolution of these disks and the formation of planetary system. We explore here to what extent CO (or its isotopologues) can be used as a tracer of gas mass. We use a detailed gas-grain chemical model and study the evolution of the disk composition, starting from a dense pre-stellar core composition. We explore a range of disk temperature profiles, cosmic rays ionization rates, and disk ages for a disk model representative of T Tauri stars. At the high densities that prevail in disks, we find that, due to fast reactions on grain surfaces, CO can be converted to less volatile forms (principally s-CO$_2$, and to a lesser extent s-CH$_4$) instead of being evaporated over a wide range of temperature. The canonical gas-phase abundance of 10$^{-4}$ is only reached above about 30-35 K. The dominant Carbon bearing entity depends on the temperature structure and age of the disk. The chemical evolution of CO is also sensitive to the cosmic rays ionization rate. Larger gas phase CO abundances are found in younger disks. Initial conditions, such as parent cloud age and density, have a limited impact. This study reveals that CO gas-phase abundance is heavily dependent on grain surface processes, which remain very incompletely understood so far. The strong dependence on dust temperature profile makes CO a poor tracer of the gas-phase content of disks.