Dust evolution during protoplanetary disk buildup enhances CO ice relative to water

Abstract

Water ice is expected to be the dominant volatile component of bodies formed in the outer Solar System. However, recent observations of comets and trans-Neptunian objects suggest that the relative abundances of ices can vary substantially, with some bodies exhibiting unusually high CO/H$_2$O ratios. We study the prospects of producing CO-rich pebbles and planetesimals. We use a one-dimensional protoplanetary disk model with dust evolution including coagulation, fragmentation, and radial drift, water and CO ice and vapors evolution, and planetesimal formation via the streaming instability. We compare models with and without the disk formation stage. CO-rich pebbles can be formed at the CO snow line due to the cold finger effect, regardless of whether the disk buildup is included. Models including disk buildup show stronger CO enhancement relative to water in the outer disk. However, CO-rich planetesimals do not form in the smooth disk models. The formation of CO-rich planetesimals likely requires mechanisms that preserve the CO-enriched ice reservoir, such as pressure traps or gas removal processes. Models concerning the chemical evolution of protoplanetary disks and its impact on the atmospheric C/O ratio of forming planets should consider the disk buildup stage.

Figures (8)

• Figure 1: Dust surface density evolution in the fiducial model without disk buildup (left) and with disk buildup taken into account (right). Time zero refers to the moment the gas disk reaches maximum mass in the model including its buildup, corresponding to the time the dust is initialized in the model without disk buildup.
• Figure 2: Evolution of the CO to H$_2$O ice ratio as a function of time and space in the fiducial model without disk buildup (left) and with disk buildup taken into account (right). Note the x-axis is different from Fig. \ref{['fig:sigmad']} as this figure is centered at the CO snow line.
• Figure 3: Difference between the evolution of the surface density of H$_2$O ice (left), CO ice (middle) and dust grain size (right) between the runs with and without disk buildup. The vertical blue line marks the location of the CO snow line.
• Figure 4: Radial gas velocity in the fiducial model with disk buildup as a function of radial distance and time. Blue colors mean the velocity vector is directed towards and red colors away from the central star.
• Figure 5: Time evolution of the total mass of CO ice and water ice (as indicated by the different line styles) in the models with and without disk buildup (indicated with blue and black lines, respectively) for the fiducial set of models.