HCNJ = 1–0 HFS intensity ratios R₁₂ (red curves) and R₀₂ (blue curves) versus impact parameter for two kind of extended cloud environments (both with n(H₂) = 5 × 10³ cm⁻³ and T_k = 30 K). Upper panel:extended envelope alone, with N(HCN) = 10¹³ cm⁻² along its diameter, resulting in (weakly) collisionally excited HCN emission. Lower panel: same envelope surrounding a dense core, with n(H₂) = 10⁵ cm⁻³, and scattering the bright HCN J = 1–0 line emission arising from the core.