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We report the results of a spectroscopic study of the high-mass protostellarobject NGC 7538 IRS 9 and compare our observations to published data on thenearby object NGC 7538 IRS 1. Both objects originated in the same molecularcloud and appear to be at different points in their evolutionary histo- ries,offering an unusual opportunity to study the temporal evolution of envelopechemistry in objects sharing a presumably identical starting composition.Observations were made with the Texas Echelon Cross Echelle Spectrograph(TEXES), a sensitive, high spectral resolution (R = {\lambda}/{\Delta}{\lambda}\simeq 100,000) mid-infrared grating spectrometer. Forty-six individual linesin vibrational modes of the molecules C2H2, CH4, HCN, NH3 and CO were detected,including two isotopologues (13CO, 12C18O) and one combination mode ({\nu}4 +{\nu}5 C2H2). Fitting synthetic spectra to the data yielded the Doppler shift,excitation temperature, Doppler b parameter, column density and covering factorfor each molecule observed; we also computed column density upper limits forlines and species not detected, such as HNCO and OCS. We find differences amongspectra of the two objects likely attributable to their differing radiation andthermal environments. Temperatures and column densities for the two objects aregenerally consistent, while the larger line widths toward IRS 9 result in lesssaturated lines than those toward IRS 1. Finally, we compute an upper limit onthe size of the continuum-emitting region (\sim2000 AU) and use this constraintand our spectroscopy results to construct a schematic model of IRS 9.

A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 and IRS 1