Letter to the Editor

Herschel-PACS observation of the 10 Myr old T Tauri disk TW Hya ^*,**

Constraining the disk gas mass

¹ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, UK e-mail: wfdt@roe.ac.uk
² Université Joseph-Fourier Grenoble 1/CNRS, Laboratoire d'Astrophysique de Grenoble (LAOG) UMR 5571, BP 53, 38041 Grenoble Cedex 09, France
³ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴ UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
⁶ Dep. de Física Teórica, Fac. de Ciencias, UAM Campus Cantoblanco, 28049 Madrid, Spain
⁷ LAEX, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
⁸ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁹ SOFIA-USRA, NASA Ames Research Center, Mailstop 211-3 Moffett Field CA 94035, USA
¹⁰ Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD, 20771, USA
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Eureka Scientific and Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD, 20771, USA
¹³ ESO-ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
¹⁴ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁵ Astronomy Department, University of California, Berkeley CA 94720-3411, USA
¹⁶ CEA/IRFU/SAp, AIM UMR 7158, 91191 Gif-sur-Yvette, France
¹⁷ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁸ Calar Alto Observatory, Centro Astronómico Hispano-Alemán C/Jesús Durbán Remón, 2-2, 04004 Almería, Spain
¹⁹ Unidad de Archivo de Datos, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
²⁰ Spanish Virtual Observatory
²¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA
²² NASA Herschel Science Center, California Institute of Technology, Pasadena, CA, USA
²³ Clemson University, Clemson, SC, USA
²⁴ NASA Exoplanet Science Institute/Caltech 770 South Wilson Avenue, Mail Code: 100-22, Pasadena, CA USA 91125, USA
²⁵ NASA Goddard Space Flight Center, Exoplanets & Stellar Astrophysics, Code 667, Greenbelt, MD 20771, USA
²⁶ Departamento de Fisica Teórica, Facultad de Ciencias, Universidad Autónomade Madrid, Cantoblanco, 28049 Madrid, Spain
²⁷ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁸ Johns Hopkins University Dept. of Physics and Astronomy, 3701 San Martin drive Baltimore, MD 21210, USA
²⁹ Research and Scientific Support Department-ESA/ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
³⁰ Astrophysikalisches Institut und Universitätssternwarte, Friedrich-Schiller-Universität, Schillergäßchen 2-3, 07745 Jena, Germany
³¹ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³² ESA-ESAC Gaia SOC, PO Box 78. 28691 Villanueva de la Cañada, Madrid, Spain
³³ Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
³⁴ European Southern Observatory, Karl-Schwarzschild-Strasse, 2, 85748 Garching bei München, Germany
³⁵ The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQL, UK
³⁶ 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: 28 April 2010

Abstract

Planets are formed in disks around young stars. With an age of ~10 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TW Hya disk, making TW Hya the perfect test case in a large survey of disks with Herschel–PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of [O i] and [C ii] as part of the open-time large program GASPS. We complement this with continuum data and ground-based ¹² CO 3–2 and ¹³CO 3–2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code MCFOST and the thermo-chemical code ProDiMo to derive the gas and dust masses. We detect the [O i] line at 63 μm. The other lines that were observed, [O i] at 145 μm and [C ii] at 157 μm, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming [ ¹²CO] /[ ¹³CO]  = 69 suggests a dust mass for grains with radius <1 mm of ~1.9 × 10^-4  (total solid mass of 3 × 10^-3 ) and a gas mass of (0.5–5) × 10^-3 . The gas-to-dust mass may be lower than the standard interstellar value of 100.