2022ApJ...935..165B

We have performed a high-resolution 4-13 μm spectral survey of the hot molecular gas associated with the massive protostars AFGL 2591 and AFGL 2136. Here we present the results of the analysis of the ν₂ band of H₂O, detected with the Echelon Cross Echelle Spectrograph on board the Stratospheric Observatory for Infrared Astronomy between wavelengths of 5 and 8 μm. All lines are seen in absorption. Rotation diagrams indicate that the gas is optically thick and lines are observed to saturate at 40% and 15% relative to the continuum for AFGL 2136 and AFGL 2591, respectively. We applied two curve of growth analyses to derive the physical conditions, one assuming a foreground origin and one a circumstellar disk origin. We find temperatures of 400-600 K. A foreground origin would require the presence of externally heated clumps that are smaller than the continuum source. The disk analysis is based on stellar atmosphere theory, which takes into consideration the temperature gradient in the disk. We discuss the challenges with each model, taking into consideration the properties of other species detected in the spectral survey, and conclude that further modeling efforts are required to establish whether the absorption has a disk or foreground origin. The main challenge to the foreground model is that molecules are expected to be observed in emission. The main challenges to the disk model are the midplane heating mechanism and the presence of narrow absorption lines shifted from the systemic velocity.