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The origin of the elevated C/O ratios in discs around late M dwarfs comparedto discs around solar-type stars is not well understood. Here we endeavour toreproduce the observed differences in the disc C/O ratios as a function ofstellar mass using a viscosity-driven disc evolution model and study thecorresponding atmospheric composition of planets that grow inside the water-iceline in these discs. We carried out simulations using a coupled disc evolutionand planet formation code that includes pebble drift and evaporation. We used achemical partitioning model for the dust composition in the disc midplane.Inside the water-ice line, the disc's C/O ratio initially decreases tosub-stellar due to the inward drift and evaporation of water-ice-rich pebblesbefore increasing again to super-stellar values due to the inward diffusion ofcarbon-rich vapour. We show that this process is more efficient for verylow-mass stars compared to solar-type stars due to the closer-in ice lines andshorter disc viscous timescales. In high-viscosity discs, the transition fromsub-stellar to super-stellar takes place faster due to the fast inwardadvection of carbon-rich gas. Our results suggest that planets accreting theiratmospheres early (when the disc C/O is still sub-stellar) will have lowatmospheric C/O ratios, while planets that accrete their atmospheres late (whenthe disc C/O has become super-stellar) can obtain high C/O ratios. Our modelpredictions are consistent with observations, under the assumption that allstars have the same metallicity and chemical composition, and that the verticalmixing timescales in the inner disc are much shorter than the radial advectiontimescales. This further strengthens the case for considering stellarabundances alongside disc evolution in future studies that aim to link planet(atmospheric) composition to disc composition.

Close-in ice lines and the super-stellar C/O ratio in discs around very low-mass stars