  The thermal structure of protoplanetary disks is a fundamental characteristic
of the system that has wide reaching effects on disk evolution and planet
formation. In this study, we constrain the 2D thermal structure of the
protoplanetary disk TW Hya structure utilizing images of seven CO lines. This
includes new ALMA observations of 12CO J=2-1 and C18O J=2-1 as well as archival
ALMA 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 CO
radial depletion in TW Hya. These observations were modeled using the
thermochemical code RAC2D, and our best fit model reproduces all spatially
resolved CO surface brightness profiles. The resulting thermal profile finds a
disk mass of 0.025 Msun and a thin upper layer of gas depleted of small dust
with a thickness of approx 1.2% of the corresponding radius. Using our final
thermal structure, we find that CO alone is not a viable mass tracer as its
abundance is degenerate with the total H2 surface density. Different mass
models can readily match the spatially resolved CO line profiles with disparate
abundance assumptions. Mass determination requires additional knowledge and, in
this work, HD provides the additional constraint to derive the gas mass and
supports the inference of CO depletion in the TW Hya disk. Our final thermal
structure confirms the use of HD as a powerful probe of protoplanetary disk
mass. Additionally, the method laid out in this paper is an employable strategy
for extraction of disk temperatures and masses in the future.
