Impact of vertical gas accretion on the carbon-to-oxygen ratio of gas giant atmospheres

Abstract

Recent theoretical, numerical, and observational works have suggested that when a growing planet opens a gap in its disk the flow of gas into the gap is dominated by gas falling vertically from a height of at least one gas scale height. Our primary objective is to include, for the first time, the chemical impact that accreting gas above the midplane will have on the resulting carbon-to-oxygen ratio (C/O). We compute the accretion of gas onto planetary cores beginning at different disk radii and track the chemical composition of the gas and small icy grains to predict the resulting C/O in their atmospheres. In our model, all of the planets which began their evolution inward of 60 AU open a gap in the gas disk, and hence are chemically affected by the vertically accreting gas. Two important conclusions follow from this vertical flow: (1) more oxygen-rich icy dust grains become available for accretion onto the planetary atmosphere; (2) the chemical composition of the gas dominates the final C/O of planets in the inner (<20 AU) part of the disk. This implies that with the launch of the James Webb Space Telescope we can trace the disk material that sets the chemical composition of exoplanetary atmospheres.