The circumstellar disc in the Bok globule CB 26

Context. Circumstellar discs are expected to be the nursery of planets. Grain growth within such discs is the first step in the planet formation process. The Bok globule CB 26 harbours such a young disc. Aims. We present a detailed model of the edge-on circumstellar disc and its envelope in the Bok globule CB 26. Methods. The model is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA radio maps at 1.1 mm, 1.3 mm and 2.7 mm, and the spectral energy distribution (SED) from 0.9 μm to 3 mm. New photometric and spectroscopic data from the Spitzer Space Telescope and the Caltech Submilimeter Observatory are also part of our analysis. Using the self-consistent radiative transfer code MC3D, the model we construct is able to discriminate between parameter sets and dust properties of both envelope and disc. Results. We find that the data are fit by a disc that has an inner hole with a radius of 45 ± 5 AU. Based on a dust model including silicate and graphite, the maximum grain size needed to reproduce the spectral millimetre index is 2.5 μm. Features seen in the nearinfrared images, dominated by scattered light, can be described as a result of a rotating envelope. Conclusions. Successful employment of ISM dust in both the disc and envelope hint that grain growth may not yet play a significant role for the appearance of this system. A large inner hole implies that CB 26 is a circumbinary disc.