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We present a detailed analysis of the mid-infrared spectrum of the peculiarevolved object HR 4049. The full Spitzer-IRS high-resolution spectrum shows awealth of emission with prominent features from CO$_2$ and H$_2$O and possiblecontributions from HCN and OH. We model the molecular emission and find that itoriginates from a massive ($M \gtrsim 8 \times 10^{-3}$ M$_{\odot}$), warm($T_{\rm ex} \approx 500$ K) and radially extended gas disk that is opticallythick at infrared wavelengths. We also report less enrichment in $^{17}$O and$^{18}$O than previously found and a comparison of the Spitzer observations toearlier data obtained by ISO-SWS reveals that the CO$_2$ flux has more thandoubled in 10 years time, indicating active and ongoing chemical evolution inthe circumbinary disk. If the gas originates from interaction between thestellar wind and the dust, this suggests that the dust could be oxygen-rich innature. The molecular gas plays a crucial role in the thermal properties of thecircumbinary disk by allowing visible light to heat the dust and then trappingthe infrared photons emitted by the dust. This results in higher temperaturesand a more homogeneous temperature structure in the disk.

The Gas-Rich Circumbinary Disk of HR 4049 I: A Detailed Study of the Mid-Infrared Spectrum