SMA observations of C2H in High-Mass Star Forming Regions
  C$_2$H is a representative hydrocarbon that is abundant and ubiquitous in the
interstellar medium (ISM). To study its chemical properties, we present
Submillimeter Array (SMA) observations of the C$_2$H $N=3-2$ and HC$_3$N
$J=30-29$ transitions and the 1.1 mm continuum emission toward four OB
cluster-forming regions, AFGL 490, ON 1, W33 Main, and G10.6-0.4, which cover a
bolometric luminosity range of $\sim$10$^3$--10$^6$ $L_{\odot}$. We found that
on large scales, the C$_2$H emission traces the dense molecular envelope.
However, for all observed sources, the peaks of C$_2$H emission are offset by
several times times 10$^4$ AU from the peaks of 1.1 mm continuum emission,
where the most luminous stars are located. By comparing the distribution and
profiles of C$_2$H hyperfine lines and the 1.1 mm continuum emission, we find
that the C$_2$H column density (and abundance) around the 1.1 mm continuum
peaks is lower than those in the ambient gas envelope. Chemical models suggest
that C$_2$H might be transformed to other species owing to increased
temperature and density; thus, its reduced abundance could be the signpost of
the heated molecular gas in the $\sim$10$^4$ AU vicinity around the embedded
high-mass stars. Our results support such theoretical prediction for centrally
embedded $\sim10^3$--$10^6L_{\odot}$ OB star-forming cores, while future
higher-resolution observations are required to examine the C$_2$H
transformation around the localized sites of high-mass star formation.
