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

ABSTRACT We present high spectral resolution (∼3 km s−1) observations of the ro-vibrational band of H2O in the 6.086-6.135 m 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 band of H2O, two by transitions in the first vibrationally excited band of H2O, and one by a transition in the band of H O. Among the detected transitions is the 1 -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 m emitting photosphere. Assuming a covering fraction of 25%, the H2O column density and rotational temperature that best fit the observed absorption lines are cm−2 and .