2009ApJ...704L...5S

We present observations of CH₃ CN (12-11) emission at a resolution of ∼2'' toward the shell-like ultracompact H II region G5.89-0.39 with the Submillimeter Array. The integrated CH₃ CN emission reveals a dense and hot-molecular cocoon in the periphery of the H II region G5.89-0.39, with a CH₃ CN deficient region roughly centered at G5.89-0.39. By analyzing the CH₃ CN emission using population diagram analysis, we find, for the first time, a decreasing temperature structure from 150 K to 40 K with the projected distance from Feldt's star, which is thought to be responsible for powering the H II region. Our results further indicate that the majority of the heating energy in the observed dense gas is supplied by the Feldt's star. From the derived CH₃ CN column density profile, we conclude that the dense gas is not uniformly distributed but centrally concentrated, with a power-law exponent of 5.5 for r ≲ 8000 AU, and 2.0 for 8000 AU ≲ r ≲ 20000 AU, where r is the distance to Feldt's star. The estimated large power index of 5.5 can be attributed to an enhancement of CH₃ CN abundance in the close vicinity of Feldt's star.