Missing protoplanetary carbon monoxide was hiding in ice

A young star is often surrounded by a protoplanetary disk—a dense mass of dust and gas that can form into solid, rocky planets. To track a disk’s evolution, astronomers routinely measure the emission signal of carbon monoxide because it is abundant and easily observed at millimeter wavelengths, particularly by the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile.

Although it’s a useful tracer, CO depletes more rapidly in observations than in protoplanetary model simulations. That mismatch has made estimates of a disk’s mass, and other properties derived from CO observations, highly uncertain. Previous efforts to understand why have focused on various mechanisms, including possible chemical reactions of CO with other gases, its ionization by cosmic rays into volatile hydrocarbons, and its transport in the protoplanetary disk, although none have explained the inconsistency.

Now Diana Powell of the Center for Astrophysics|Harvard & Smithsonian in Cambridge, Massachusetts, and her colleagues appear to have found the missing CO trapped in icy dust particles. In their ice-formation model, they demonstrate how a surface energy effect results in heterogeneous ice nucleation that then holds CO molecules, rendering them invisible to telescopes.

The missing piece of the ice-formation model came from a phenomenon well known to meteorologists studying cloud physics in Earth’s atmosphere. At the microphysical level, the surface energy of liquid droplets depends, in part, on their curvature—a phenomenon known as the Kelvin effect. Molecules of water experience hydrogen bonding with one another, and that strong interaction is one of the reasons that water has a relatively high boiling point. It also affects, for example, the condensation of vapor into liquid water and ice nucleation in clouds. If a surface between a liquid and a gas is flat, more molecules can be hydrogen bonded to one another, and the molecules thus require more energy to evaporate.

But the curved surfaces of droplets decrease the number of molecules that can hydrogen bond to one another, which means that less energy is required for a phase change. Powell and her colleagues reasoned that the Kelvin effect would accelerate the rate of gaseous CO depletion, and they explicitly included the effect in their ice-formation model of protoplanetary disks.

The model reproduced the same concentrations of CO as the observations of four protoplanetary disks collected by ALMA. Powell and her colleagues found that individual CO molecules can act as nucleation sites for ice particles. As they grow, the ice particles can become coated in more CO and eventually drift from the outer parts of the protoplanetary disk toward the host star. The researchers suspect that a similar depletion process affects water and carbon dioxide, so future work should provide a better understanding of additional building blocks of planets and their early bulk composition. (D. Powell et al., Nat. Astron., 2022, doi:10.1038/s41550-022-01741-9 .)