Changing disc compositions via internal photoevaporation: II. M dwarf systems

The chemical evolution of the inner regions of protoplanetary discs is a complex process influenced by several factors, including the inward drift and evaporation of volatile-rich pebbles, which can enrich the inner disc with vapour. During the evolution of the disc, its inner part is first enriched with evaporating water-ice, resulting in a low C/O ratio. Subsequently, carbon-rich gas from the outer disc, originating from the evaporation of CO, CO₂, and CH₄ ice, is viscously transported inwards, while the supply of water-rich pebbles ceases and the water vapour in the inner disc is accreted onto the star. Consequently, the C/O ratio of the inner disc increases again after 2 Myr. Previously, we studied how internal photoevaporation influences the chemical composition and evolution of discs around Sun-like stars by carrying away gas and opening gaps that block inward drifting pebbles. We now extend our study to lower-mass stars (M_⊙ = 0.1-0.5 M), where the time evolution of the disc's C/O ratio is different due to the closer-in position of the evaporation fronts and differences in disc mass, size, and structure. Our simulations were carried out with a semi-analytical 1D disc model. The code chemcomp includes viscous evolution and heating, pebble growth and drift, pebble evaporation and condensation, as well as a simple chemical partitioning model for the disc. We show that internal photoevaporation plays a major role in the evolution of protoplanetary discs and their chemical composition: As for solar-mass stars, photoevaporation opens a gap, which stops the inward drift of pebbles. As a result, they can no longer contribute to the volatile content of the gas in the inner disc. In addition, volatile-rich gas from the outer disc, originating from evaporated CO, CO₂, or CH₄ ice, is carried away by the photoevaporative winds. Consequently, the C/O ratio in the inner disc remains low, contradicting observations of the composition of discs around low-mass stars. Our model implies that inner discs at young ages (<2 Myr) should be oxygen-rich and carbon-poor, while older discs (>2 Myr) should be carbon-rich. The survival of discs to this age can be attributed to lower photoevaporation rates. These lower rates could either originate from the large spread of observed X-ray luminosities or from the photoevaporation model used in this study, which likely overestimates the photoevaporation efficiency at a given X-ray luminosity, leading to discrepancies with the observed C/O ratios in discs around low-mass stars. A reduction of the photoevaporation rate brings the calculated elemental abundances into better agreement with observations.