The first multidimensional view of mass loss from externally FUV irradiated protoplanetary discs

Computing the flow from externally far-ultraviolet (FUV) irradiated protoplanetary discs requires solving complicated and expensive photodissociation physics iteratively in conjunction with hydrodynamics. Previous studies have therefore been limited to 1D models of this process. In this paper, we compare 2D axisymmetric models of externally photoevaporating discs with their 1D analogues, finding that mass-loss rates are consistent to within a factor of 4. The mass-loss rates in 2D are higher, in part because half of the mass loss comes from the disc surface (which 1D models neglect). 1D mass-loss rates used as the basis for disc viscous evolutionary calculations are hence expected to be conservative. We study the anatomy of externally driven winds including the streamline morphology, kinematic, thermal, and chemical structure. A key difference between the 1D and 2D models is in the chemical abundances. For instance in the 2D models CO can be dissociated at smaller radial distances from the disc outer edge than in 1D calculations because gas is photodissociated by radiation along trajectories that are assumed infinitely optically thick in 1D models. Multidimensional models will hence be critical for predicting observable signatures of environmentally photoevaporating protoplanetary discs.