CO ro-vibrational lines in HD 100546
Kapteyn Astronomical Institute Rijks-universiteit Groningen
Landleven 12, 9747,
2 Universitäts-Sternwarte München, Scheinerstr. 1, Department of Physics and Astronomy, 3400 N, 8167 9 Munich, Germany
3 Departamento de Astronomía, Universidad de Chile, 36-D Casilla, Santiago, Chile
4 UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique (IPAG) UMR 5274, 38041 Grenoble, France
5 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
6 Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
7 European Southern Observatory, Karl-Schwarzschild-Str.2, 85748 Garching bei München, Germany
8 SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
Accepted: 13 November 2013
Aims. We have studied the emission of CO ro-vibrational lines in the disc around the Herbig Be star HD 100546 to determine physical properties, disc asymmetries, the CO excitation mechanism, and the spatial extent of the emission, with the final goal of using the CO ro-vibrational lines as a diagnostic to understand inner disc structure in the context of planet formation.
Methods. High-spectral-resolution infrared spectra of CO ro-vibrational emission at eight different position angles were taken with the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) at the Very Large Telescope (VLT). From these spectra flux tables, line profiles for individual CO ro-vibrational transitions, co-added line profiles, and population diagrams were produced. We have investigated variations in the line profile shapes and line strengths as a function of slit position angle. We used the thermo-chemical disc modelling code ProDiMo based on the chemistry, radiation field, and temperature structure of a previously published model for HD 100546. We calculated line fluxes and profiles for the whole set of observed CO ro-vibrational transitions using a large CO model molecule that includes the lowest two electronic states, each with 7 vibrational levels and within them 60 rotational levels. Comparing observations and the model, we investigated the possibility of disc asymmetries, the excitation mechanism (UV fluorescence), the geometry, and physical conditions of the inner disc.
Results. The observed CO ro-vibrational lines are largely emitted from the inner rim of the outer disc at 10–13 AU. The line shapes are similar for all v levels and line fluxes from all vibrational levels vary only within one order of magnitude. All line profile asymmetries and variations can be explained with a symmetric disc model to which a slit correction and pointing offset is applied. Because the angular size of the CO emitting region (10–13 AU) and the slit width are comparable the line profiles are very sensitive to the placing of the slit. The model reproduces the line shapes and the fluxes of the v = 1–0 lines as well as the spatial extent of the CO ro-vibrational emission. It does not reproduce the observed band ratios of 0.5–0.2 with higher vibrational bands. We find that lower gas volume densities at the surface of the inner rim of the outer disc can make the fluorescence pumping more efficient and reproduce the observed band ratios.
Key words: protoplanetary disks / line: profiles / stars: individual: HD 100546 / stars: variables: T Tauri / circumstellar matter / stars: variables: Herbig Ae/Be
Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 084.C-0605.
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014