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. To study its chemical properties, we present Submillimeter Array observations of the C₂H N = 3–2 and HC₃N 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 ∼10³–10⁶ $L_{\odot }$. We found that on large scales, the C₂H emission traces the dense molecular envelope. However, for all observed sources, the peaks of C₂H emission are offset by several times 10⁴ AU from the peaks of 1.1 mm continuum emission, where the most luminous stars are located. By comparing the distribution and profiles of C₂H hyperfine lines and the 1.1 mm continuum emission, we find that the C₂H column density (and abundance) around the 1.1 mm continuum peaks is lower than those in the ambient gas envelope. Chemical models suggest that C₂H 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 ∼10⁴ AU vicinity around the embedded high-mass stars. Our results support such theoretical prediction for centrally embedded ∼10³–10⁶ $L_{\odot }$ OB star-forming cores, while future higher-resolution observations are required to examine the C₂H transformation around the localized sites of high-mass star formation.