Vibrational temperature diagram derived from the HCN J = 3–2 lines given in Table 1. Upper level population vs. its energy above the ground state. The data is fitted by 3 line segments corresponding to regions with temperatures T_vib ~ 400 K, 1200 K, and 2400 K, respectively. HCN column densities are averaged over a source size of 1′′ diameter. A conservative calibration error of 30% has been applied to all intensities. Values in parentheses correspond to 3σ errors on the derived vibrational temperatures and column densities. Filled triangles correspond to lines with half-power widths Δv < 7 km s^-1, filled squares correspond to lines with 7 < Δv < 9 km s^-1, empty triangles correspond to lines with 9 < Δv < 12 km s^-1, and empty squares to lines with Δv > 12 km s^-1.