The external photoevaporation of structured protoplanetary disks

Abstract

Context. The dust in planet-forming disks is known to evolve rapidly through growth and radial drift. In the high irradiation environments of massive star-forming regions where most stars form, external photoevaporation also contributes to rapid dispersal of disks. This raises the question of why we still observe quite high disk dust masses in massive star-forming regions. Aims. We test whether the presence of substructures is enough to explain the survival of the dust component and observed millimeter continuum emission in protoplanetary disks located within massive star-forming regions. We also characterize the dust content removed by the photoevaporative winds. Methods. We performed hydrodynamical simulations (including gas and dust evolution) of protoplanetary disks subject to irradiation fields of FUV = 102, 103, and 104 G0, with different dust trap configurations. We used the FRIED grid to derive the mass loss rate for each irradiation field and disk properties, and then proceed to measure the evolution of the dust mass over time. For each simulation we estimated the continuum emission at λ = 1.3 mm along with the radii encompassing 90% of the continuum flux, and characterized the dust size distribution entrained in the photoevaporative winds, in addition to the resulting far-ultraviolet (FUV) cross section. Results. Our simulations show that the presence of dust traps can extend the lifetime of the dust component of the disk to a few millionyears if the FUV irradiation is FUV ≲ 103 G0, but only if the dust traps are located inside the photoevaporative truncation radius. The dust component of a disk will be quickly dispersed if the FUV irradiation is strong (104 G0) or if the substructures are located outside the photoevaporation radius. We do find however, that the dust grains entrained with the photoevaporative winds may result in an absorption FUV cross section of σ ≈ 10−22 cm2 at early times of evolution (<0.1 Myr), which is enough to trigger a self-shielding effect that reduces the total mass loss rate, and slow down the disk dispersal in a negative feedback loop process.