Proto-planetary gas-disc sizes: preliminary results from the ALMA DECO Large Program

The field of planet formation is undergoing rapid development, mostly thanks to the transformational capabilities of ALMA. The disk gas radius (Rgas) is one of the fundamental properties needed for constraining disk evolution all the way to planet formation. For example, disk radii can be used to disentangle viscous and MHD wind accretion. Rgas can be measured from molecular emission, mostly CO, observed with ALMA, but it is critical to have analysis tools capable of retrieving all the information hidden in the data in a reliable way. Usually, disk radii have been measured from the CO images and only for a limited number of bright and extended disks. In interferometry, however, images are highly processed data products, and this introduces systematic biases which are difficult to quantify. I will present a new strategy which retrieves the disk size directly in the visibilities, the native quantity measured by the interferometer. The analysis, implemented in the code CSALT (Andrews et al., in prep.), is based on a parametric model of the disc structure and emission; this approach allows a robust statistical inference of the model parameters. In this talk, I will present preliminary results obtained with this new technique for a subset of protoplanetary disks from the DECO Large Program (P.I. Ilse Cleeves). I will show the results on my analysis for the three isotopologues 12CO, 13CO, C18O, and I will outline the improvements with respect to other approaches.