High Resolution Infrared Spectroscopy of Hot Molecular Gas in AFGL 2591
  and AFGL 2136: Accretion in the Inner Regions of Disks Around Massive Young
  Stellar Objects
  We have performed a high resolution 4-13 ${\mu}m$ spectral survey of the hot
molecular gas associated with the massive protostars AFGL 2591 and AFGL 2136,
utilising the Echelon-Cross-Echelle-Spectrograph (EXES) on-board the
Stratospheric Observatory for Infrared Astronomy (SOFIA), and the iSHELL
instrument and Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA
Infrared Telescope Facility (IRTF). Here we present results of this survey with
analysis of CO, HCN, C$_2$H$_2$, NH$_3$ and CS, deriving the physical
conditions for each species. Also from the IRTF, iSHELL data at 3 ${\mu}m$ for
AFGL 2591 are presented that show HCN and C$_2$H$_2$ in emission. In the EXES
and TEXES data, all species are detected in absorption, and temperatures and
abundances are found to be high (600 K and 10$^{-6}$, respectively).
Differences of up to an order of magnitude in the abundances of transitions
that trace the same ground state level are measured for HCN and C$_2$H$_2$. The
mid-infrared continuum is known to originate in a disk, hence we attribute the
infrared absorption to arise in the photosphere of the disk. As absorption
lines require an outwardly decreasing temperature gradient, we conclude that
the disk is heated in the mid-plane by viscous heating due to accretion. We
attribute the near-IR emission lines to scattering by molecules in the upper
layers of the disk photosphere. The absorption lines trace the disk properties
at 50 AU where a high temperature gas-phase chemistry is taking place.
Abundances are consistent with chemical models of the inner disk of Herbig
disks.
