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 {{ν }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 {{ν }₂} band of H₂O, two by transitions in the first vibrationally excited {{ν }₂} band of H₂O, and one by a transition in the {{ν }₂} band of H₂¹⁸O. Among the detected transitions is the {{ν }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.