2015ApJ...802L..14I

We present high spectral resolution (∼3 km/s) observations of the ν₂ ro-vibrational band of H₂ O 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 ν₂ band of H₂O, two by transitions in the first vibrationally excited ν₂ band of H₂O, and one by a transition in the ν₂ band of H₂¹⁸O. Among the detected transitions is the ν₂ 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(H₂O) = (1.3 ± 0.3) x 10¹⁹/cm2 and T = 640 ± 80K.