Letter to the Editor

Gas in the protoplanetary disc of HD 169142: Herschel's view ^*

¹ Dep. de Física Teórica, Fac. de Ciencias, UAM Campus Cantoblanco, 28049 Madrid, Spain e-mail: gwendolyn.meeus@uam.es
² School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
³ Université Joseph Fourier – Grenoble 1/CNRS, Laboratoire d'Astrophysique de Grenoble (LAOG) UMR 5571, BP 53, 38041 Grenoble Cedex 09, France
⁴ UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, UK; Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁶ School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
⁷ LAEX, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
⁸ Institute for Astronomy (IfA), University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
⁹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Leuven, Belgium
¹⁰ SOFIA-USRA, NASA Ames Research Center, MS 211-3, Moffett Field, CA 94035, USA
¹¹ NASA Goddard Space Flight Center, Exoplanets & Stellar Astrophysics lab, Code 667, Greenbelt, MD 20771, USA
¹² Astronomy Department, University of California, Berkeley, CA 94720-3411, USA
¹³ Eureka Scientific, 2452 Delmer, Suite 100, Oakland, CA 96002, USA
¹⁴ ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
¹⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁶ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁷ Spanish Virtual Observatory, Unidad de Archivo de Datos, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
¹⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA
¹⁹ NASA Herschel Science Center, California Institute of Technology, Pasadena, USA
²⁰ Calar Alto Observatory, Centro Astronómico Hispano-Alemán, c/ Jesús Durbán Remón, 2-2, 04004 Almería, Spain
²¹ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634-0978, USA
²² NASA Exoplanet Science Institute/Caltech 770 South Wilson Avenue, Mail Code: 100-22, Pasadena, CA 91125, USA
²³ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁴ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
²⁵ Research and Scientific Support Department of ESA, ESTEC/SRE-C, Postbus 299, 2200 AG Noordwijk, The Netherlands
²⁶ Astrophysikalisches Institut und Universitätssternwarte, Friedrich-Schiller-Universität, Schillergäßchen 2-3, 07745 Jena, Germany
²⁷ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
²⁸ ESA-ESAC Gaia SOC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
²⁹ Spitzer Science Center, California Institute of Technology, 1200 E California Blvd, 91125 Pasadena, USA
³⁰ Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
³¹ CEA/IRFU/SAP, AIM UMR 7158, 91191 Gif-sur-Yvette, France
³² European Southern Observatory, Karl-Schwarzschild-Strasse, 2, 85748 Garching bei München, Germany
³³ The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQL, UK
³⁴ Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK and The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQL, UK

Received: 30 March 2010
Accepted: 12 April 2010

Abstract

In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD 169142, we present far-IR observations obtained with the PACS instrument onboard the Herschel Space Observatory. This work is part of the open time key program GASPS, which is aimed at studying the evolution of protoplanetary discs. To constrain the gas properties in the outer disc, we observed the star at several key gas-lines, including [OI] 63.2 and 145.5 μm, [CII] 157.7 μm, CO 72.8 and 90.2 μm, and o-H₂O 78.7 and 179.5 μm. We only detect the [OI] 63.2 μm line in our spectra, and derive upper limits for the other lines. We complement our data set with PACS photometry and ^12/13CO data obtained with the Submillimeter Array. Furthermore, we derive accurate stellar parameters from optical spectra and UV to mm photometry. We model the dust continuum with the 3D radiative transfer code MCFOST and use this model as an input to analyse the gas lines with the thermo-chemical code ProDiMo. Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU, but this is not a unique solution. Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions. Our results are inconsistent with a gas-poor disc with a large UV excess; a gas mass of 5.0 ± 2.0 × 10^-3  is still present in this disc, in agreement with earlier CO observations.