How the gradient of <i>M</i> d versus UV field strength yields insights into the ages of protoplanetary disc populations

ABSTRACT Far-ultraviolet (UV) radiation from massive stars launches photoevaporative winds from the outer regions of protoplanetary discs around other stars, removing gas and dust. Observations have identified a relation between the median dust disc mass and the external UV field strength. Here, we use disc evolutionary models to explore how this relation evolves over time, and with respect to other stellar and disc properties. We find that the slope for the relationship $\lambda _{\rm UV}$ flattens over time as populations age, possibly explaining the differences seen between the L1641-N and L1641-S clusters in Orion A. We determine that $\lambda _{\rm UV}$ depends on the stellar mass, where more massive stars exhibit steeper gradients than their lesser counterparts, in agreement with the differences seen between Herbig and T Tauri stars. Additionally, the strength of the mechanism for angular momentum transport, either viscosity or magnetohydrodynamic disc winds, is found to significantly affect $\lambda _{\rm UV}$ with stronger $\alpha$ values reducing $\lambda _{\rm UV}$ due to more material accreting onto the central stars in weaker UV environments. Estimates of $\lambda _{\rm UV}$ from observations of L1641 place preliminary constraints on $\alpha$ to be between $10^{-3.5}$ and $10^{-2.5}$, consistent with literature estimates. Further observations in different regions and better classifications of stellar masses will allow us to place stringent constraints on disc evolution properties, improving our understanding of how protoplanetary discs evolve.