Probing the dust formation region in IRC +10216 with the high vibrational states of hydrogen cyanide
Résumé
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 10 700 K (nu1+3nu2+nu3). 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 5000 K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of Tvib ~= 2400 K and a fractional HCN abundance relative to H2, x(HCN) = 5-7 × 10-5. 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 5000 K yield Tvib ~= 1300 K and x(HCN) ~= 0.8-2 × 10-5. 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 (
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