High Resolution Mid-Infrared Spectroscopy of NGC 7538 IRS 1: Probing
  Chemistry in a Massive Young Stellar Object
  We present high resolution (R = 75,000-100,000) mid-infrared spectra of the
high-mass embedded young star IRS 1 in the NGC 7538 star-forming region.
Absorption lines from many rotational states of C2H2, 13C12CH2, CH3, CH4, NH3,
HCN, HNCO, and CS are seen. The gas temperature, column density, covering
factor, line width, and Doppler shift for each molecule are derived. All
molecules were fit with two velocity components between -54 and -63 km/s. We
find high column densities (~ 10e16 cm^2) for all the observed molecules
compared to values previously reported and present new results for CH3 and
HNCO. Several physical and chemical models are considered. The favored model
involves a nearly edge-on disk around a massive star. Radiation from dust in
the inner disk passes through the disk atmosphere, where large molecular column
densities can produce the observed absorption line spectrum.
