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We have performed a 5-8 $\mu$m spectral line survey of the hot molecular coreassociated with the massive protostar AFGL 2591, using theEchelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory forInfrared Astronomy (SOFIA). We have supplemented these data with a ground basedstudy in the atmospheric M band around 4.5 $\mu$m using the iSHELL instrumenton 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 inthis 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=1-2agree 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 enhancedCS abundance with respect to the surrounding cloud (1$\times$10$^{-8}$) mayreflect sublimation of H$_2$S ice followed by gas-phase reactions to form CS.  Transitions are in LTE and we derive a density of $>$10$^7$ cm$^{-3}$, whichcorresponds to an absorbing region of $<$0.04$''$. EXES observations of CS arelikely to probe deeply into the hot core, to the base of the outflow.Submillimeter and infrared observations trace different components of the hotcore as revealed by the difference in systemic velocities, line widths andtemperatures, as well as the CS abundance.

Infrared Detection of Abundant CS in the Hot Core AFGL 2591 at High Spectral Resolution with SOFIA/EXES