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We present observations of CH3CN (12-11) emission at a resolution of 2"toward the shell-like ultracompact HII region G5.89-0.39 with the SubmillimeterArray. The integrated CH3CN emission reveals dense and hot molecular cocoon inthe periphery of the HII region G5.89-0.39, with a CH3CN deficient regionroughly centered at G5.89-0.39. By analyzing the CH3CN emission usingpopulation diagram analysis, we find, for the first time, a decreasingtemperature structure from 150 to 40 K with the projected distance from Feldt'sstar, which is thought to be responsible for powering the HII region. Ourresults further indicate that the majority of the heating energy in theobserved dense gas is supplied by the Feldt's star. From the derived CH3CNcolumn density profile, we conclude that the dense gas is notuniformly-distributed but centrally-concentrated, with a power-law exponent of5.5 for r < 8000 AU, and 2.0 for 8000 AU < r < 20000 AU, where r is thedistance to Feldt's star. The estimated large power index of 5.5 can beattributed to an enhancement of CH3CN abundance in the close vicinity ofFeldt's star.

The Hot and Clumpy Molecular Cocoon Surrounding the Ultracompact HII Region G5.89-0.39