SOFIA/EXES Observations of Water Absorption in the Protostar AFGL 2591
  at High Spectral Resolution
  We present high spectral resolution (~3 km/s) observations of the nu_2
ro-vibrational band of H2O in the 6.086--6.135 micron 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 nu_2
band of H2O, two by transitions in the first vibrationally excited nu_2 band of
H2O, and one by a transition in the nu_2 band of H2{18}O. Among the detected
transitions is the nu_2 1(1,1)--0(0,0) line which 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 micron 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)*10^{19}
cm^{-2} and T=640+-80 K.
