THE GAS-RICH CIRCUMBINARY DISK OF HR 4049. II. A DETAILED STUDY OF THE NEAR-INFRARED SPECTRUM.

HR 4049 is a peculiar evolved binary surrounded by a circumbinary disk. Mid-infrared observations show that the disk is rich in molecular gas and radially extended. To study the properties of this disk, we re-analyzed a set of near-infrared observations at high spectral resolution obtained with Gemini-Phoenix. These data cover absorption lines originating from the first overtone of CO and from H₂O in the 2.3 μm region as well as more complex emission-absorption profiles from H₂O and the fundamental mode of CO near 4.6 μm. By using an excitation diagram and from modeling the spectrum, we find that most of the CO overtone and H₂O absorption originates from hot gas (T_ex ≈ 1000 K) with high column densities, consistent with the mid-infrared data. The strong emission in the wavelength range of the CO fundamental furthermore suggests that there is a significant quantity of gas in the inner cavity of the disk. In addition, there is a much colder component in the line of sight to the disk. A detailed analysis of the overtone line profiles reveals variations in the line widths that are consistent with a radially extended disk in Keplerian rotation with hotter gas closer to the central star. We estimate the mass of the primary to be ∼0.34 M_☼ and discuss the implications for its evolutionary status. [ABSTRACT FROM AUTHOR]