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.
