Letter to the Editor

A high-resolution line survey of IRC +10216 with Herschel/HIFI ^*,**

First results: Detection of warm silicon dicarbide (SiC)

¹ Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain e-mail: jcernicharo@cab.inta-csic.es
² Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park XH, Amsterdam, The Netherlands
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁵ Leiden observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ LUTH, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92190 Meudon, France
⁷ I. Physikalisches Institut, University of Cologne, Germany
⁸ Dept of Physics & Astronomy, University College London, Gower St., London WC1E 6BT, UK
⁹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹⁰ INAF – Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Department of Physics and Astronomy, University of Waterloo, Waterloo, ON Canada N2L 3G1, Canada
¹² Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 168-314, Pasadena, CA 91109, USA
¹³ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁴ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC, Utrecht, The Netherlands
¹⁵ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁶ University of Massachusetts, Astronomy Dept., 710 N. Pleasant St., LGRT-619E, Amherst, MA 01003-9305, USA
¹⁷ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands

Received: 4 June 2010
Accepted: 26 July 2010

Abstract

We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5–636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-to-noise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC₂). We identified 55 SiC₂ transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC₂ is produced in the inner dust formation zone, with an abundance of ~ 2 × 10^-7 relative to molecular hydrogen. These SiC₂ lines have been observed for the first time in space and have been used to derive an SiC₂ rotational temperature of ~204 K and a source-averaged column density of ~ 6.4 × 10¹⁵ cm^-2. Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC₂.