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

We present a detailed analysis of the mid-infrared spectrum of the peculiar evolved object HR 4049. The full Spitzer-IRS high-resolution spectrum shows a wealth of emission with prominent features from CO$_2$ and H$_2$O and possible contributions from HCN and OH. We model the molecular emission and find that it originates 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 optically thick at infrared wavelengths. We also report less enrichment in $^{17}$O and $^{18}$O than previously found and a comparison of the Spitzer observations to earlier data obtained by ISO-SWS reveals that the CO$_2$ flux has more than doubled in 10 years time, indicating active and ongoing chemical evolution in the circumbinary disk. If the gas originates from interaction between the stellar wind and the dust, this suggests that the dust could be oxygen-rich in nature. The molecular gas plays a crucial role in the thermal properties of the circumbinary disk by allowing visible light to heat the dust and then trapping the infrared photons emitted by the dust. This results in higher temperatures and a more homogeneous temperature structure in the disk.