2011A&A...529L...3C

We report the detection in IRC +10216 of 63 rotational transitions of HCN, most of them with quantum numbers J=3-2, pertaining to 28 different vibrational states with energies up to 10700K (ν₁+3ν₂+ν₃). Some of the transitions were also observed for the rare isotopologue H¹³CN. The observations were carried out with the IRAM 30-m telescope. The HCN lines with level energies above 5000K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of T_vib≃2400K and a fractional HCN abundance relative to H₂, x(HCN)=5-7x10^–5. These high-energy levels are mainly populated by photospheric radiation, and their vibrational temperature yields a direct measurement of the stellar photospheric temperature. The lines with energy levels between 2000 and 5000K yield T_vib≃1300K and x(HCN)≃0.8-2x10^–5. They are radiatively excited and arise from a shell extending between 1.5 and 5 stellar radii. Finally, the lines from the low-energy vibrational states (<2000K) trace a larger region of the envelope (5-28 stellar radii) with T_vib=400-600K and x(HCN)=4-6x10^–6. They are strongly affected by mid and near-infrared pumping. The line widths increase from 5km/s near the 2400K photosphere, to 19km/s in the warm 400K shell. This provides a unique insight into the physical conditions of the gas acceleration and dust formation region.