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, CCH2, 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 (∼ 10 cm) 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. Subject headings: Infrared: ISM – ISM: molecules – HII regions: NGC 7538 IRS 1