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High-resolution infrared spectroscopy in the 2.3-4.6 micron region isreported for the peculiar A supergiant, single-lined spectroscopic binary HR4049. Lines from the CO fundamental and first overtone, OH fundamental, andseveral H2O vibration-rotation transitions have been observed in thenear-infrared spectrum. The spectrum of HR 4049 appears principally in emissionthrough the 3 and 4.6 micron region and in absorption in the 2 micron region.The 4.6 micron spectrum shows a rich 'forest' of emission lines. All thespectral lines observed in the 2.3-4.6 micron spectrum are shown to becircumbinary in origin. The presence of OH and H2O lines confirm theoxygen-rich nature of the circumbinary gas which is in contrast to thepreviously detected carbon-rich material. The emission and absorption lineprofiles show that the circumbinary gas is located in a thin, rotating layernear the dust disk. The properties of the dust and gas circumbinary disk andthe spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolarand M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocityof $\gtrsim$ 1 km/s. The severe depletion of refractory elements but near-solarabundances of volatile elements observed in HR 4049 results from abundancewinnowing. The separation of the volatiles from the grains in the disk and thesubsequent accretion by the star are discussed. Contrary to prior reports, theHR 4049 carbon and oxygen isotopic abundances are typical AGB values:12C/13C=6^{+9}_{-4} and 16O/17O>200.

Infrared High-Resolution Spectroscopy of Post-AGB Circumstellar Disks. I. HR 4049 - The Winnowing Flow Observed?