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

TL;DR

This study uses high-resolution observations of CH3CN emission to reveal a dense, hot, and centrally concentrated molecular cocoon around the ultracompact HII region G5.89-0.39, showing a temperature decrease with distance from Feldt's star and indicating localized heating and abundance enhancement.

Contribution

First detailed temperature and density profile of the molecular cocoon around G5.89-0.39 using population diagram analysis of CH3CN emission.

Findings

Temperature decreases from 150 to 40 K with distance from Feldt's star.

Dense gas distribution follows a power-law with exponents 5.5 and 2.0.

Heating primarily supplied by Feldt's star.

Abstract

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 Submillimeter Array. The integrated CH3CN emission reveals dense and hot molecular cocoon in the periphery of the HII region G5.89-0.39, with a CH3CN deficient region roughly centered at G5.89-0.39. By analyzing the CH3CN emission using population diagram analysis, we find, for the first time, a decreasing temperature structure from 150 to 40 K with the projected distance from Feldt's star, which is thought to be responsible for powering the HII 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 CH3CN 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 CH3CN abundance in the close vicinity of Feldt's star.