Adsorption of volatiles on dust grains in protoplanetary disks

Abstract

The adsorption of volatile molecules onto dust grain surfaces fundamentally influences dust-related processes, including condensation of gas-phase molecules, dust coagulation, and planet formation in protoplanetary disks. Using advanced ab-initio density functional theory with r${}^2$SCAN+rVV10 van der Waals functionals, we calculate adsorption energies of H${}_2$, H${}_2$O, and CO on carbonaceous (graphene, amorphous carbon) and silicate (MgSiO${}_3$) surfaces. Results reveal fundamentally different adsorption mechanisms: weak physisorption on carbonaceous surfaces ($|\hat{I}”\hat{I}{\mu }_{\mathrm{ad}}|\sim 0.1-0.2\mathrm{eV}$) versus strong chemisorption on silicates ($|\hat{I}”\hat{I}{\mu }_{\mathrm{ad}}|\sim 0.5-1.5\mathrm{eV}$) via coordination bonds. Kinetic Monte Carlo simulations incorporating these energies demonstrate divergent surface evolution: carbonaceous grains exhibit distinct condensation radius compared to silicates, while the cocrystal of H${}_2$O and CO significantly increases the desorption temperature of CO. The actual radii of gas-phase molecule depletion could thus be a comprehensive result of temperatures, chemical compositions, and even evolution tracks. Meanwhile, silicates maintain chemisorbed molecular coatings throughout most disk regions. Such dichotomy in surface coverage could also provide a natural mechanism for carbon depletion in inner planetary systems.

16 pages, 6 figures, Accepted for publication by ApJ