SOFIA/EXES OBSERVATIONS OF WATER ABSORPTION IN THE PROTOSTAR AFGL 2591 AT HIGH SPECTRAL RESOLUTION

We present high spectral resolution (∼3 km s−1) observations of the &ngr; 2 ?> ro-vibrational band of H2O in the 6.086–6.135 μm range toward the massive protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption features are detected in total, with seven caused by transitions in the &ngr; 2 ?> band of H2O, two by transitions in the first vibrationally excited &ngr; 2 ?> band of H2O, and one by a transition in the &ngr; 2 ?> band of H 2 18 ?> O. Among the detected transitions is the &ngr; 2 ?> 1 1,1 ?> –0 0,0 ?> line that probes the lowest-lying rotational level of para-H2O. The stronger transitions appear to be optically thick, but reach maximum absorption at a depth of about 25%, suggesting that the background source is only partially covered by the absorbing gas or that the absorption arises within the 6 μm emitting photosphere. Assuming a covering fraction of 25%, the H2O column density and rotational temperature that best fit the observed absorption lines are N ( H 2 O ) = ( 1.3 ± 0.3 ) × 10 19 ?> cm−2 and T = 640 ± 80 K ?> .