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The thermal structure of protoplanetary disks is a fundamental characteristicof the system that has wide reaching effects on disk evolution and planetformation. In this study, we constrain the 2D thermal structure of theprotoplanetary disk TW Hya structure utilizing images of seven CO lines. Thisincludes new ALMA observations of 12CO J=2-1 and C18O J=2-1 as well as archivalALMA observations of 12CO J=3-2, 13CO J=3-2, 6-5, C18O J= 3-2, 6-5.Additionally, we reproduce a Herschel observation of the HD J=1-0 line flux,the spectral energy distribution, and utilize a recent quantification of COradial depletion in TW Hya. These observations were modeled using thethermochemical code RAC2D, and our best fit model reproduces all spatiallyresolved CO surface brightness profiles. The resulting thermal profile finds adisk mass of 0.025 Msun and a thin upper layer of gas depleted of small dustwith a thickness of approx 1.2% of the corresponding radius. Using our finalthermal structure, we find that CO alone is not a viable mass tracer as itsabundance is degenerate with the total H2 surface density. Different massmodels can readily match the spatially resolved CO line profiles with disparateabundance assumptions. Mass determination requires additional knowledge and, inthis work, HD provides the additional constraint to derive the gas mass andsupports the inference of CO depletion in the TW Hya disk. Our final thermalstructure confirms the use of HD as a powerful probe of protoplanetary diskmass. Additionally, the method laid out in this paper is an employable strategyfor extraction of disk temperatures and masses in the future.

The TW Hya Rosetta Stone Project III: Resolving the Gaseous Thermal Profile of the Disk