Probing the dust formation region in IRC +10216 with the high vibrational states of hydrogen cyanide

Abstract

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 10700 K (ν1+3ν 2+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 5000 K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of Tvib ≃ 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 T vib 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 (<2000 K) trace a larger region of the envelope (5-28 stellar radii) with Tvib = 400-600 K and x(HCN) = 4-6 × 10-6. They are strongly affected by mid and near-infrared pumping. The line widths increase from 5 s -1 near the 2400 K photosphere, to 19 s-1 in the warm 400 K shell. This provides a unique insight into the physical conditions of the gas acceleration and dust formation region. © 2011 ESO.