The H$_2$O Spectrum of the Massive Protostar AFGL 2136 IRS 1 from 2 to
  13 $\mu$m at High Resolution: Probing the Circumstellar Disk
  We have observed the massive protostar AFGL 2136 IRS 1 in multiple wavelength
windows in the near-to-mid-infrared at high ($\sim3$ km s$^{-1}$) spectral
resolution using VLT+CRIRES, SOFIA+EXES, and Gemini North+TEXES. There is an
abundance of H$_2$O absorption lines from the $\nu_1$ and $\nu_3$ vibrational
bands at 2.7 $\mu$m, from the $\nu_2$ vibrational band at 6.1 $\mu$m, and from
pure rotational transitions near 10-13 $\mu$m. Analysis of state-specific
column densities derived from the resolved absorption features reveals that an
isothermal absorbing slab model is incapable of explaining the relative depths
of different absorption features. In particular, the strongest absorption
features are much weaker than expected, indicating optical depth effects
resulting from the absorbing gas being well-mixed with the warm dust that
serves as the "background" continuum source at all observed wavelengths. The
velocity at which the strongest H$_2$O absorption occurs coincides with the
velocity centroid along the minor axis of the compact disk in Keplerian
rotation recently observed in H$_2$O emission with ALMA. We postulate that the
warm regions of this dust disk dominate the continuum emission at near-to-mid
infrared wavelengths, and that H$_2$O and several other molecules observed in
absorption are probing this disk. Absorption line profiles are not symmetric,
possibly indicating that the warm dust in the disk that produces the infrared
continuum has a non-uniform distribution similar to the substructure observed
in 1.3 mm continuum emission.
