A thermochemical model for PDS 70

ALMA revealed several features in the submillimeter spectrum towards PDS 70 which are associated with molecules in the coldest regions of the disk. Since the gas component dominates the disk’s mass, inferring its distribution across the disk is important to understand some of the most prominent features such as the planets in the cavity. Accordingly, the gas column density is a key parameter on which several predictions are built upon. A physically motivated model for the gas phase is needed to understand the observations and to test theoretical predictions. Our goal is to develop a thermochemical model to explain the submillimeter molecular emission observed from PDS 70 and use it to estimate the mass of the gap-carving planets.

Using ALMA data of three CO isotopologues (CO J=2-1, 13CO J=2-1, and C18O J=2-1), I constructed a thermochemical model using the Protoplanetary Disk Model code ProDiMo. The first part of the modelling was a parametric exploration to fit the signal from the spatially unresolved inner disk. Then, I used an iterative procedure to reproduce the signal from the spatially resolved outer disk. The models were post-proceded with CASA and with the GoFish and ProDiMopy packages in order to obtain the radial brightness distribution that was directly compared to the observed profiles.

We found a peak value for the gas column density of \(\sim 0.13 \ \mathrm{g}\ \mathrm{cm}^{-2}\) at ~75 au and a minimum value of \(\sim 0.003 \ \mathrm{g}\ \mathrm{cm}^{-2}\) at a semi-major axis equal to the orbital separation of the planet PDS 70 b, ~20 au. I use the level of gas depletion in the cavity that is directly inferred from our model as an input to the scaling relation presented in Duffel & Dong 2015 and estimated the mass of each planet to be equal to \(\sim 4 M_\mathrm{jup}\). This is consistent within the uncertainties of the value reported by Wang et al. 2021 that was obtained using infrared spectroscopy.

Stay tuned for the manuscript!