High-resolution infrared spectroscopy in the 2.3-4.6 \mu m region is reported for the peculiar A supergiant , single-lined spectroscopic binary HR 4049 .
Lines from the CO fundamental and first overtone , OH fundamental , and several H _ { 2 } O vibration-rotation transitions have been observed in the near-infrared spectrum .
The spectrum of HR 4049 appears principally in emission through the 3 and 4.6 \mu m region and in absorption in the 2 \mu m region .
The 4.6 \mu m spectrum shows a rich ‘ forest ’ of emission lines .
All the spectral lines observed in the 2.3-4.6 \mu m spectrum are shown to be circumbinary in origin .
The presence of OH and H _ { 2 } O lines confirm the oxygen-rich nature of the circumbinary gas which is in contrast to the previously detected carbon-rich material .
The emission and absorption line profiles show that the circumbinary gas is located in a thin , rotating layer near the dust disk .
The properties of the dust and gas circumbinary disk and the spectroscopic orbit yield masses for the individual stars , M _ { A~ { } I } \sim 0.58 M _ { \odot } and M _ { M~ { } V } \sim 0.34 M _ { \odot } .
Gas in the disk also has an outward flow with a velocity of > \atop \sim 1 km s ^ { -1 } .
The severe depletion of refractory elements but near-solar abundances of volatile elements observed in HR 4049 results from abundance winnowing .
The separation of the volatiles from the grains in the disk and the subsequent accretion by the star are discussed .
Contrary to prior reports , the HR 4049 carbon and oxygen isotopic abundances are typical AGB values : ^ { 12 } C/ ^ { 13 } C=6 ^ { +9 } _ { -4 } and ^ { 16 } O/ ^ { 17 } O > 200 .