Characterising discs around Herbig Ae/Be stars through modelling of low-J 12CO linesO. Panić and M. R. Hogerheijde
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
Received 27 May 2009 / Accepted 6 July 2009
Context. While there has been extensive investigation of the dust emission in discs around young intermediate-mass or Herbig Ae/Be stars at a range of wavelengths, their gas content has been systematically studied mainly via spatially unresolved (sub)millimetre observations of the rotational lines of 12CO.
Aims. We are interested in how the available low-J 12CO spectra compare to the disc properties inferred from the dust emission, and to what extent the gas- and dust-emission approaches to disc modelling are complementary to each other.
Methods. First, we use the disc structure derived from the spectral energy distribution (SED) modelling to produce the synthetic 12CO J = 3-2 spectra for a discrete sample of sources. We then compare these synthetic spectra to observations, to test the existing disc models for each source. In our second approach, we study the dependence of the 12CO J = 3-2 spectrum on disc size, inclination and temperature, for discs around Herbig Ae/Be stars in general. We calculate the spectral line profiles for a grid of parametric disc models. The calculated spectra are compared to the spectra observed towards a large sample of sources. Both methods use a molecular excitation and radiative transfer code for the calculation of the 12CO line emission.
Results. SED models are insensitive to the parameters that dominate the low-J 12CO emission, i.e., the disc size and orientation. To minimise some of the important parameter degeneracies, it is necessary to model a disc's SED with prior knowledge of the disc size and inclination. We show how the spectral profile of low-J 12CO lines can be used to constrain these parameters, and to obtain disc models that are good starting points for the outside-in SED modelling of discs from long to shorter wavelengths.
Conclusions. For a disc gas mass of the order of 0.01 M, the optically thick J = 3-2 12CO line intensity shows that the majority of discs around Herbig Ae/Be stars are smaller than 200 AU, and that the largest and brightest sources thoroughly studied with submillimetre interferometry are not representative of the sample.
Key words: planetary systems: protoplanetary disks -- stars: pre-main-sequence
© ESO 2009