Changing disc compositions via internal photoevaporation II: M dwarf systems

Abstract

The chemical evolution of the inner regions of protoplanetary discs is a complex process. Several factors influence it, one being the inward drift and evaporation of volatile-rich pebbles. During the disc's evolution, its inner part is first enriched with evaporating water-ice, resulting in a low C/O ratio. Afterwards, C-rich gas from the outer disc is transported inwards. Consequently, the C/O ratio of the inner disc increases again after 2 Myr. Previously, we studied how internal photoevaporation influences these processes in discs around Sun-like stars. We now extend our study to lower-mass stars, 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 are carried out with the semi-analytical 1D disc model chemcomp, which includes viscous evolution and heating, pebble growth and drift, pebble evaporation and condensation, as well as a simple chemical partitioning model. We show that internal photoevaporation plays a major role in the evolution of protoplanetary discs: As for Sun-like stars, photoevaporation opens a gap, which stops inward drifting pebbles. In addition, volatile-rich gas from the outer disc is carried away by the photoevaporative winds. Consequently, the C/O ratio in the inner disc remains low, contradicting observations of discs around low-mass stars. Our model implies that young inner discs (< 2 Myr) should be O-rich and C-poor, while older discs (> 2 Myr) should be C-rich. The survival of discs to this age can be attributed to lower photoevaporation rates, which either originate from a large spread of observed X-ray luminosities or from the photoevaporation model used here, which likely overestimates the photoevaporation efficiency. A reduction of the latter brings the calculated elemental abundances into better agreement with observations.