tokens: what is the driving mechanism of photoevaporation between FUV, EUV, X-ray, and what are the exact values of mass-loss rate is still open. In order for the photoevaporation to start the accretion rate have to drop below the photoevaporation rate. As soon as it starts, a cavity free from gas and dust is generated, with the very large grains rapidly migrating inward (Alexander Armitage 2007 ). CQ Tau was considered by Donehew Brittain ( 2011 ) and Mendigutía et al. ( 2012 ) to have a high accretion rate ( yr −1 ), which can be considered high for disc dispersal to be the main mechanism acting in the formation of the CQ Tau cavity (Owen 2016 ). A combination between the clearing mechanism and photoevaporation can, instead, be considered as a possible option (e.g. Williams Cieza 2011 ; Rosotti et al. 2013 ; Rosotti, Ercolano Owen 2015 ) to be further investigated. Dead zones . The suppression of magnetorotational instability inside the disc can create regions of low disc ionization, the so-called ‘dead zones’ (e.g. Flock, Henning Klahr 2012 ). In these regions the rate of gas flow decreases with an accumulation of gas in the outer edge. At the same time the pressure maxima is able to trap large particles with a consequent ring-like feature in both gas and dust components (Flock et al. 2015 ). The ring is in general located at about the same radius (Pinilla et al. 2016 ) in both the components. However, when dead zones and MHD wind are present at the same time, a difference in the inner radii position can be observed. The presence of dead zones can increase the turbulence present in the disc and generate asymmetric vortices in the disc (Ruge et al. 2016 ) at (sub)mm wavelengths. Such structures can be also created by disc–planet interaction if the α viscosity is low. We cannot exclude this mechanism by our observations. Higher resolution data are need to better constrain the profile of the ring-like disc, mainly its symmetry and variability (Pinilla et al. 2016 ). However, dead zones can explain ring-like structures, but not cavities such as the one clearly present in the CQ Tau system. A combination between the mechanism described above can be also a possible solution, which, however, we will not address in this paper. 7 CONCLUSIONS Discs where it is possible to clearly distinguish a cavity are of particular interest in the study of protoplanetary disc evolution and of the planet formation process. In particular, we have studied the disc around the CQ Tau pre-main-sequence star. We have made use of spatially resolved ALMA observations of the dust continuum and of three CO isotopologues, 12 CO, 13 CO, and C 18 O, together with the observed SED of CQ Tau. We employed the chemical–physical code DALI to model the CQ Tau disc and to derive the surface density profile of the dust (small and large grains separately) and of the gas. Finally, we ran 3D SPH simulations to test whether an embedded planet could be able to create the observed disc structure.
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