  APEX single-dish observations at sub-millimeter wavelengths toward a sample
of massive star-forming regions reveal that C2H is almost omni-present toward
all covered evolutionary stages from Infrared Dark Clouds via High-Mass
Protostellar Objects to Ultracompact HII regions. High-resolution data from the
Submillimeter Array toward one hot-core like High-Mass Protostellar Object show
a shell-like distribution of C2H with a radius of ~9000AU around the central
submm 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 C2H 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 C2H
remains high due to constant replenishment of elemental carbon from CO being
dissociated by the interstellar UV photons. We suggest that C2H may be a
molecule well suited to study the initial conditions of massive star formation.
