Letter to the Editor

Silicon in the dust formation zone of IRC +10216 ^*,**

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: Leen.Decin@ster.kuleuven.be
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
³ Laboratory of Molecular Astrophysics, Department of Astrophysics, CAB, INTA-CSIC, Ctra de Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain
⁴ Dept of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
⁵ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
⁶ Department of Physics, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada
⁷ LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France
⁸ Radio Astronomy Laboratory, University of California at Berkeley, CA 94720, USA
⁹ Max-Planck-Institut für extraterrestrische Physik, 85748 Giessenbachstrasse, Germany
¹⁰ Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
¹¹ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24 3AA, UK
¹² Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
¹³ Blue Sky Spectroscopy, 9/740 4 Ave S, Lethbridge, Alberta T1J 0N9, Canada
¹⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria
¹⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁷ Dept of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden

Received: 30 March 2010
Accepted: 27 April 2010

Abstract

The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflows from these stars create a circumstellar envelope in which a rich gas-phase and dust-nucleation chemistry takes place. We observed the nearest carbon-rich evolved star, IRC +10216, using the PACS (55–210 μm) and SPIRE (194–672 μm) spectrometers on board Herschel. We find several tens of lines from SiS and SiO, including lines from the v = 1 vibrational level. For SiS these transitions range up to J = 124–123, corresponding to energies around 6700 K, while the highest detectable transition is J = 90–89 for SiO, which corresponds to an energy around 8400 K. Both species trace the dust formation zone of IRC +10216, and the broad energy ranges involved in their detected transitions permit us to derive the physical properties of the gas and the particular zone in which each species has been formed. This allows us to check the accuracy of chemical thermodynamical equilibrium models and the suggested depletion of SiS and SiO due to accretion onto dust grains.