Herschel/HIFI observation of highly excited rotational lines of HNC toward IRC +10 216^⋆,⋆⋆

¹ Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, 28850 Madrid, Spain
e-mail: danielf@cab.inta-csic.es
² LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
⁵ Laboratoire d’Astrophysique de l’Observatoire de Grenoble, 38041 Grenoble, France
⁶ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’ Hères, France
⁷ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 14 November 2011
Accepted: 16 April 2012

Abstract

We report the detection in emission of various highly excited rotational transitions of HNC (J = 6–5 through J = 12–11) toward the carbon-star envelope IRC +10 216 using the HIFI instrument on-board the Herschel Space Observatory. Observations of the J = 1–0 and J = 3–2 lines of HNC with the IRAM 30-m telescope are also presented. The lines observed with HIFI have upper level energies corresponding to temperatures between 90 and 340 degrees Kelvin, and trace a warm and smaller circumstellar region than that seen in the interferometric maps of the J = 1–0 transition, whose emission extends up to a radius of 20′′. After a detailed chemical and radiative transfer modeling, we find that the presence of HNC in the circumstellar envelope of IRC +10 216 is consistent with formation from the precursor ion HCNH⁺, which in turn is produced through several proton transfer reactions which are triggered by cosmic-ray ionization. We also find that the radiative pumping through λ 21 μm photons to the first excited state of the bending mode ν₂ plays a crucial role to populate the high-J HNC levels involved in the transitions observed with HIFI. Emission in these high-J rotational transitions of HNC is expected to be strong in regions which are warm and dense and/or have an intense infrared flux at wavelengths around 21 μm.