Chemical segregation toward massive hot cores: The AFGL2591 star forming
  region
  We present high angular resolution observations (0.5"x0.3") carried out with
the Submillimeter Array (SMA) toward the AFGL2591 high-mass star forming
region. Our SMA images reveal a clear chemical segregation within the AFGL2591
VLA 3 hot core, where different molecular species (Type I, II and III) appear
distributed in three concentric shells. This is the first time that such a
chemical segregation is ever reported at linear scales <3000 AU within a hot
core. While Type I species (H2S and 13CS) peak at the AFGL2591 VLA 3 protostar,
Type II molecules (HC3N, OCS, SO and SO2) show a double-peaked structure
circumventing the continuum peak. Type III species, represented by CH3OH, form
a ring-like structure surrounding the continuum emission. The excitation
temperatures of SO2, HC3N and CH3OH (185+-11 K, 150+-20 K and 124+-12 K,
respectively) show a temperature gradient within the AFGL2591 VLA 3 envelope,
consistent with previous observations and modeling of the source. By combining
the H2S, SO2 and CH3OH images, representative of the three concentric shells,
we find that the global kinematics of the molecular gas follow Keplerian-like
rotation around a 40 Mo-star. The chemical segregation observed toward AFGL2591
VLA 3 is explained by the combination of molecular UV photo-dissociation and a
high-temperature (~1000 K) gas-phase chemistry within the low extinction
innermost region in the AFGL2591 VLA 3 hot core.
