Measuring proto-planetary gas-disc sizes: general considerations and preliminary results from the DECO Large Program

Observations of planet formation are undergoing rapid development, mostly thanks to the transformational capabilities of the ALMA observatory. ALMA relies on molecular spectroscopy to determine the spatially resolved physical conditions in the gas reservoirs of discs. To explore this new frontier, analysis tools capable of retrieving all the information hidden in the data are essential. Usually, this kind of analysis relies on analysis of images. In interferometry these are however highly processed data products, and this introduces systematic biases which are difficult to quantify. I will present a different strategy which instead analyses the data directly in the visibilities, the native quantity measured by the interferometer. The analysis, implemented in the code CSALT, is based on a parametric model of the disc structure and emission; this approach allows a robust statistical inference of the model parameters and allows to infer a size also for discs that in the image plane are only marginally resolved. Measuring gas-disc sizes is of fundamental importance to understand how proto-planetary discs form and evolve. The viscous evolutionary paradigm predict that discs enlarge as a function of time, while the MHD wind-driven one states that they shrink due to angular momentum loss. Until now, it was impossible to test these predictions on a statistically significant scale, but with the advent of the DECO Large Program (P.I. Ilse Cleeves), which observed 80 “common” discs in 4 regions with different ages, we now have enough data to constrain this key prediction. In this talk, I will present preliminary results obtained for a subset of proto-planetary discs with new analysis techniques. I will show the models obtained for discs in the three isotopologues 12CO, 13CO, C18O coming from the Large Program DECO, and I will outline the developments that I obtained with respect to other analysis approaches. This new method leads to an unprecedented possibility of parametric-modelling inference, opening the possibility to rely on robust interpretation of physical properties also for barely resolved discs.