Observations of Herbig Ae/Be stars with Herschel/PACS^⋆

The atomic and molecular contents of their protoplanetary discs

¹ Universidad Autonoma de Madrid, Dpt. Fisica Teorica, Campus Cantoblanco Spain
e-mail: gwendolyn.meeus@uam.es
² Dept. of Astrophysics, CAB (CSIC-INTA), ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Spain
³ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
⁴ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique (IPAG) UMR 5274, 38041 Grenoble, France
⁵ Eureka Scientific, 2452 Delmer, Suite 100, Oakland CA 96002, USA
⁶ ExoPlanets and Stellar Astrophysics Laboratory, Code 667, Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Goddard Center for Astrobiology, Greenbelt, MD 20771, USA
⁸ Institute for Astronomy (IfA), University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
⁹ SOFIA-USRA, NASA Ames Research Center, Building N232, Rm 146, PO Box 1, Moffett Field, CA 94035, USA
¹⁰ Department of Physics & Astronomy, 118 Kinard Laboratory, Clemson University, Clemson, SC 29634, USA
¹¹ ALMA SCO, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹² Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
¹³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium

Received: 14 March 2012
Accepted: 14 June 2012

Abstract

We observed a sample of 20 representative Herbig Ae/Be stars and 5 A-type debris discs with PACS onboard Herschel, as part of the GAS in Protoplanetary Systems (GASPS) project. The observations were done in spectroscopic mode, and cover the far-infrared lines of [O i], [C ii], CO, CH⁺, H₂O, and OH. We have a [O i] 63 μm detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. The [O i] 145 μm line is only detected in 25% and CO J = 18–17 in 45% (and fewer cases for higher J transitions) of the Herbig Ae/Be stars, while for [C ii] 157 μm, we often find spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. First seen in HD 100546, CH⁺ emission is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and either the stellar or disc parameters, such as stellar luminosity, ultraviolet and X-ray flux, accretion rate, polycyclic aromatic hydrocarbon (PAH) band strength, and flaring. We find that the stellar ultraviolet flux is the dominant excitation mechanism of [O i] 63 μm, with the highest line fluxes being found in objects with a large amount of flaring and among the largest PAH strengths. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of [O i] 63 μm and [O i] 145 μm, CO J = 18–17 and [O i] 6300 Å, and between the continuum flux at 63 μm and at 1.3 mm, while we find weak correlations between the line flux of [O i] 63 μm and the PAH luminosity, the line flux of CO J = 3–2, the continuum flux at 63 μm, the stellar effective temperature, and the Brγ luminosity. Finally, we use a combination of the[O i] 63 μm and ¹²CO J = 2–1 line fluxes to obtain order of magnitude estimates of the disc gas masses, in agreement with the values that we find from detailed modelling of two Herbig Ae/Be stars, HD 163296 and HD 169142.