Heavy element enrichment of gas in surface-accretion disks: A possible origin of the mass-metallicity anti-correlation in exoplanets

Abstract

Recent observations, including those by JWST, suggest that the atmospheres of many gas giant exoplanets have super-stellar metallicity that is anti-correlated with planetary mass. Several studies suggest that the super-stellar metallicity can be explained by accretion of vapor-enriched disk gas produced by the sublimation of rapidly drifting icy pebbles. However, recent disk observations and experiments suggest that icy dust is fragile at low temperatures, calling into question the conventional picture that icy grains grow efficiently and drift rapidly. We present a new scenario for heavy-element enrichment in the inner disk by fragile, slowly drifting icy dust, assuming that magnetohydrodynamical disk winds drive gas accretion near the disk surface rather than at the midplane. We simulate the evolution of gas and dust in a surface-accretion disk, taking into account the radial transport of gas and dust, collision growth and fragmentation of fragile dust, and the condensation and sublimation of H2O. Two accretion disk models are presented, in which gas accretion flows are assumed to be either vertically uniform or narrowly concentrated near the disk surface. In the uniform accretion disk model, fragile icy grains enhance the water vapor abundance inside the snow line only by a factor of ${\sim}3$ due to their slow drift. In contrast, in the surface-accretion disk model, the slow drift of icy dust leads to water vapor enrichment that is higher by an order of magnitude, owing to the selective removal of ice-free gas from the disk. Furthermore, surface accretion yields an anti-correlation between the water vapor concentration in the inner disk and the residual disk gas mass, analogous to the anti-correlation between atmospheric metallicity and planet mass observed in extrasolar giant planets.