Gas lines from the 5-Myr old optically thin disk around HD 141569A

Herschel observations and modeling^{⋆,⋆⋆,⋆⋆⋆}

¹ UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: Wing-Fai.Thi@obs.ujf-grenoble.fr
² Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, IRFU/SAP, 91191 Gif-sur-Yvette, France
³ Dep. de Física Teórica, Fac. de Ciencias, UAM Campus Cantoblanco, 28049 Madrid, Spain
⁴ UMI – LFCA, CNRS / INSU France, and Dept. de Astronomia y Obs. Astronomico Nacional, Universidad de Chile, Casilla 36-D, Correo Central, Santiago, Chile ( UMI 3386 )
⁵ SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
⁶ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ SOFIA-USRA, NASA Ames Research Center, Mail Stop N211-3, Building N211/Rm. 249, Moffett Field, CA 94035, USA
⁸ ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
⁹ Astrophysics Group, Department of Physics & Astronomy, The Open University, UK
¹⁰ RAL Space, The Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
¹¹ NASA Herschel Science Center, California Institute of Technology, Mail Code 100-22, Pasadena, CA 91125, USA
¹² Department of Physics and Astronomy, 118 Kinard Laboratory, Clemson University, Clemson, SC 29634, USA

Received: 26 June 2013
Accepted: 10 September 2013

Abstract

Context. The gas- and dust dissipation processes in disks around young stars remain uncertain despite numerous studies. At the distance of ~99–116 pc, HD 141569A is one of the nearest HerbigAe stars that is surrounded by a tenuous disk, probably in transition between a massive primordial disk and a debris disk. Atomic and molecular gases have been found in the structured 5-Myr old HD 141569A disk, making HD 141569A the perfect object within which to directly study the gaseous atomic and molecular component.

Aims. We wish to constrain the gas and dust mass in the disk around HD 141569A.

Methods. We observed the fine-structure lines of O i at 63 and 145 μm and the C ii line at 157 μm with the PACS instrument onboard the Herschel Space Telescope as part of the open-time large program GASPS. We complemented the atomic line observations with archival Spitzer spectroscopic and photometric continuum data, a ground-based VLT-VISIR image at 8.6 μm, and ¹²CO fundamental ro-vibrational and pure rotational J = 3–2 observations. We simultaneously modeled the continuum emission and the line fluxes with the Monte Carlo radiative transfer code MCFOST and the thermo-chemical code ProDiMo to derive the disk gas- and dust properties assuming no dust settling.

Results. The models suggest that the oxygen lines are emitted from the inner disk around HD 141569A, whereas the [C ii] line emission is more extended. The CO submillimeter flux is emitted mostly by the outer disk. Simultaneous modeling of the photometric and line data using a realistic disk structure suggests a dust mass derived from grains with a radius smaller than 1 mm of ~2.1 × 10^-7M_⊙ and from grains with a radius of up to 1 cm of 4.9 × 10^-6M_⊙. We constrained the polycyclic aromatic hydrocarbons (PAH) mass to be between 2 × 10^-11 and 1.4 × 10^-10M_⊙ assuming circumcircumcoronene (C₁₅₀H₃₀) as the representative PAH. The associated PAH abundance relative to hydrogen is lower than those found in the interstellar medium (3 × 10^-7) by two to three orders of magnitude. The disk around HD 141569A is less massive in gas (2.5 to 4.9 × 10^-4M_⊙ or 67 to 164 M_⊕) and has a flat opening angle (<10%).

Conclusions. We constrained simultaneously the silicate dust grain, PAH, and gas mass in a ~5-Myr old Herbig Ae disk. The disk-averaged gas-to-dust-mass is most likely around 100, which is the assumed value at the disk formation despite the uncertainties due to disagreements between the different gas tracers. If the disk was originally massive, the gas and the dust would have dissipated at the same rate.