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Inferring the evolutionary stages of the internal structures of NGC 7538 S and IRS1 from chemistry

To unambiguously diagnose the detailed physical mechanisms and the evolutionary status of high-mass star-forming regions (HMSFRs), we have performed 0.4" ($\sim1000$ AU) resolution observations towards NGC7538S and IRS1, using the Plateau de Bure Interferometer (PdBI). This paper presents a joint analysis of the 1.37 mm continuum emission and the line intensity of 15 molecular species. Assuming local thermal equilibrium, we derived molecular column densities and molecular abundances for each internal gas substructure which is spatially resolved. These derived quantities are compared with a suite of 1-D gas-grain models. NGC7538S is resolved into at least three dense gas condensations. Despite the comparable continuum intensity of these condensations, their differing molecular line emission is suggestive of an overall chemical evolutionary trend from the northeast to the southeast. Since these condensations are embedded within the same parent gas core, their differing chemical properties are the most likely due to the different warm-up histories, rather than the different dynamic timescales. Despite remaining spatially unresolved, in IRS1 we detect abundant complex organic molecules, indicating that IRS1 is the most chemically evolved hot molecular core presented here. We observe a continuum that is dominated by absorption features with at least three strong emission lines, potentially from $\rm CH_3OH$. The $\rm CH_3OH$ lines which are purely in emission have higher excitation than the ones showing purely absorption. Potential reasons for that difference are discussed. This is the first comprehensive comparison of observations of the high-mass cores NGC7538S and IRS1 and a chemical model. We have found that different chemical evolutionary stages can coexist in the same natal gas core. Our achievement illustrates the strength of chemical analysis for understanding HMSFRs.