Infrared Detection of Abundant CS in the Hot Core AFGL 2591 at High
  Spectral Resolution with SOFIA/EXES
  We have performed a 5-8 $\mu$m spectral line survey of the hot molecular core
associated with the massive protostar AFGL 2591, using the
Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory for
Infrared Astronomy (SOFIA). We have supplemented these data with a ground based
study in the atmospheric M band around 4.5 $\mu$m using the iSHELL instrument
on the Infrared Telescope Facility (IRTF), and the full N band window from 8-13
$\mu$m using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the IRTF.
  Here we present the first detection of ro-vibrational transitions of CS in
this source. The absorption lines are centred on average around -10 kms$^{-1}$
and the line widths of CS compare well with the hot component of $^{13}$CO
(around 10 kms$^{-1}$). Temperatures for CS, hot $^{13}$CO and $^{12}$CO v=1-2
agree well and are around 700 K. We derive a CS abundance of 8$\times$10$^{-3}$
and 2$\times$10$^{-6}$ with respect to CO and H$_2$ respectively. This enhanced
CS abundance with respect to the surrounding cloud (1$\times$10$^{-8}$) may
reflect sublimation of H$_2$S ice followed by gas-phase reactions to form CS.
  Transitions are in LTE and we derive a density of $>$10$^7$ cm$^{-3}$, which
corresponds to an absorbing region of $<$0.04$''$. EXES observations of CS are
likely to probe deeply into the hot core, to the base of the outflow.
Submillimeter and infrared observations trace different components of the hot
core as revealed by the difference in systemic velocities, line widths and
temperatures, as well as the CS abundance.
