Ethynyl (C 2 H) in Massive Star formation: Tracing the Initial Conditions?

APEX single-dish observations at submillimeter wavelengths toward a sample of massive star-forming regions reveal that C 2 H is almost omnipresent toward all covered evolutionary stages from infrared dark clouds via high-mass protostellar objects to ultracompact H II regions. High-resolution data from the Submillimeter Array toward one hot-core-like high-mass protostellar object show a shell-like distribution of C 2 H with a radius of ~9000 AU around the central submillimeter continuum peak position. These observed features are well reproduced by a 1D cloud model with power-law density and temperature distributions and a gas-grain chemical network. The reactive C 2 H radical (ethynyl) is abundant from the onset of massive star formation, but later it is rapidly transformed to other molecules in the core center. In the outer cloud regions the abundance of C 2 H remains high due to constant replenishment of elemental carbon from CO being dissociated by the interstellar UV photons. We suggest that C 2 H may be a molecule well suited to study the initial conditions of massive star formation.