The structure of brown dwarf circumstellar discs

We present synthetic spectra for circumstellar discs that are heated by radiation from a central brown dwarf. Under the assumption of vertical hydrostatic equilibrium, our models yield scaleheights for brown dwarf discs in excess of three times those derived for classical T Tauri (CTTS) discs. If the near-IR excess emission observed from brown dwarfs is indeed due to circumstellar discs, then the large scaleheights we find could have a significant impact on the optical and near-IR detectability of such systems. Our radiation transfer calculations show that such highly flared discs around brown dwarfs will result in a large fraction of obscured sources due to extinction of direct starlight by the disc over a wide range of sightlines. The obscured fraction for a ‘typical’ CTTS is less than 20 per cent. We show that the obscured fraction for brown dwarfs may be double that for CTTS, but this depends on stellar and disc mass. We also comment on possible confusion in identifying brown dwarfs via colour‐magnitude diagrams: edge-on CTTS display similar colours and magnitudes to face-on brown dwarf plus disc systems.