Model (a) assuming the standard CO radial abundance (top) and model (b) assuming a centrally depleted CO radial abundance likely caused by an enhanced CO dissociation driven by X-ray radiation (bottom). ¹²CO (J=1-0) (left) and ¹²CO (J=2-1) (middle) observed and synthetic spectra in the C-rich X-AGB T Dra. Observed spectra are shown in black (velocity resolution is δv=1.6 km s⁻¹). Synthetic spectra are shown in blue. Right panels: the different adopted CO abundance profiles for each model. As discussed in Sect. 5, model (b) fits the line shapes better than model (a).