Letters to the Editor

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

¹ Centro de Astrobiología, INTA-CSIC, Crta Torrejón km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: jcernicharo@cab.inta-csic.es
² LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France
³ LAOG, Observatoire de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁵ Instituut voor Sterrenkunde, Department of Physics and Astronomy, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 14 February 2011
Accepted: 23 March 2011

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 10 700 K (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 5000 K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of T_vib ≃ 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 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 T_vib = 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 km s^-1 near the 2400 K photosphere, to 19 km 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.