2018ApJ...868L...2B

We have performed a 5-8 µm spectral line survey of the hot molecular core associated with the massive protostar AFGL 2591, using the Echelon-Cross-Echelle Spectrograph (EXES) on board 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 µm using the iSHELL instrument on the Infrared Telescope Facility (IRTF), and the full N-band window from 8 to 13 µm using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the IRTF. Here we present the first detection of rovibrational transitions of CS in this source. The absorption lines are centered on average around -10 km s^–1 and the line widths of CS compare well with the hot component of ¹³CO (around 10 km s^–1). Temperatures for CS, hot ¹³CO, and ¹²CO v = 1-2 agree well and are around 700 K. We derive a CS abundance of 8 x 10^–3 and 2 x 10^–6 with respect to CO and H₂, respectively. This enhanced CS abundance with respect to the surrounding cloud (1 x 10^–8) may reflect sublimation of H₂S ice followed by gas-phase reactions to form CS. Transitions are in local thermodynamic equilibrium and we derive a density of >10⁷ 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.