  Photoevaporative disc winds play a key role in our understanding of
circumstellar disc evolution, especially in the final stages, and they might
affect the planet formation process and the final location of planets. The
study of transition discs (i.e. discs with a central dust cavity) is central
for our understanding of the photoevaporation process and disc dispersal.
However, we need to distinguish cavities created by photoevaporation from those
created by giant planets. Theoretical models are necessary to identify possible
observational signatures of the two different processes, and models to find the
differences between the two processes are still lacking. In this paper we study
a sample of transition discs obtained from radiation-hydrodynamic simulations
of internally photoevaporated discs, and focus on the dust dynamics relevant
for current ALMA observations. We then compared our results with gaps opened by
super Earths/giant planets, finding that the photoevaporated cavity steepness
depends mildly on gap size, and it is similar to that of a 1 Jupiter mass
planet. However, the dust density drops less rapidly inside the photoevaporated
cavity compared to the planetary case due to the less efficient dust filtering.
This effect is visible in the resulting spectral index, which shows a larger
spectral index at the cavity edge and a shallower increase inside it with
respect to the planetary case. The combination of cavity steepness and spectral
index might reveal the true nature of transition discs.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics