2002MNRAS.334..705S

Numerical radiative transfer models are used to discuss the observed flat infrared spectrum of T Tauri stars showing moderate dust obscuration. The models consist of a central young star surrounded by a static cocoon of gas and dust, but with no equatorial accretion disc. Here the cocoon is treated as a spherical dust shell undergoing no systematic inflow or outflow motion. The static cocoon is supported against the gravity of the central star by a large-scale magnetic field system that anchors in the star and threads the cocoon material. Using an approximation in which magnetic force densities inside the cocoon are averaged over all polar and azimuthal angles, the cocoon may be treated as a spherical-like configuration for the purpose of numerical radiative transfer calculations. It is shown that the model infrared spectral energy distributions of a spherical static cocoon produce both the Lada spectral types and the flat infrared spectrum of T Tauri stars as well as do models based upon discs or infalling clouds with a bipolar cavity.

The static dust cocoon model is applied to the young T Tau binary system. The cocoon is illuminated by radiation emitted from the optical primary (T Tau N), which is a classical T Tauri star. The infrared secondary (T Tau S) is identified as a magnetic brown dwarf in binary orbital motion outside the cocoon, but still embedded in a large nebula of cold gas and dust distributed around the binary system. Both T Tau N and T Tau S are magnetically linked to each other. The radiative transfer model for the cocoon infrared energy distribution is compared with observational flux data for T Tau N. The model appears in good agreement with the data from the optical to submillimetre wavebands. The model shows an almost flat infrared spectrum for T Tau N, except for the strong 10-µm silicate band in emission as observed. It is suggested that the magnetic brown dwarf (T Tau S) undergoes episodic and luminous non-thermal infrared and radio outburst emissions. Such a possibility is well in accord with a non-thermal origin for the circular polarized radio emission observed from the T Tau system. Nevertheless, if future high-resolution observations prove that the blue continuum emission from T Tauri stars originates from stellar chromospheric activity or boundary-layer emission from an equatorial disc, then a spherical cocoon model for the T Tauri phenomenon is wrong.