We present high angular resolution observations ( 0.5 " \times 0.3 " ) 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 \leq 3000 AU within a hot core .
While Type I species ( H _ { 2 } S and ^ { 13 } CS ) peak at the AFGL2591 VLA 3 protostar , Type II molecules ( HC _ { 3 } N , OCS , SO and SO _ { 2 } ) show a double-peaked structure circumventing the continuum peak .
Type III species , represented by CH _ { 3 } OH , form a ring-like structure surrounding the continuum emission .
The excitation temperatures of SO _ { 2 } , HC _ { 3 } N and CH _ { 3 } OH ( 185 \pm 11 K , 150 \pm 20 K and 124 \pm 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 H _ { 2 } S , SO _ { 2 } and CH _ { 3 } OH images , representative of the three concentric shells , we find that the global kinematics of the molecular gas follow Keplerian-like rotation around a 40 M _ { \odot } -star .
The chemical segregation observed toward AFGL2591 VLA 3 is explained by the combination of molecular UV photo-dissociation and a high-temperature ( \sim 1000 K ) gas-phase chemistry within the low extinction innermost region in the AFGL2591 VLA 3 hot core .