Lifetimes of ultracompact H II regions: High-resolution methyl cyanide observations

We observed the dense molecular cores associated with two ultracompact H II regions, G5.89+-0.4 and G34.3+0.2, in the J = 6-5 transitions of CH3CN to probe the temperature and density of the gas. Simultaneously, each region was observed in the J = 1-0 transition of a CO isotope to probe the molecular column density. The molecular cores are found to have densities similar to 10(6) cm(-3) and kinetic temperatures of 90 and 250 K, respectively. We also find possible evidence for infall of molecular gas toward the H II regions. The lifetime of the H II region ultracompact phase is critically dependent on the properties of the surrounding molecular gas. Based on the number of ultracompact H II regions versus the total number of O stars, it has been suggested that the lifetime of the ultracompact phase is inconsistent with spherical expansion cofined only by the ambient medium. We show that for the molecular cloud parameters given above and including the effect of the shell of neutral material formed by the expansion, pressure confinement by the ambient medium may significantly increase the lifetime of the ultracompact phase of the H II regions.