  We explore the origin of the flat spectrum seen in some T Tauri stars by
considering a three-component structure: a central star, a circumstellar disk,
and a dusty halo. The radiative energy transport is faithfully treated by
solving the angle- and frequency-dependent radiative transfer equation in two
space dimensions assuming axisymmetry, and hence the radiative equilibrium
temperature in the disk and halo is determined simultaneously. The disk is
effectively heated by the scattering and reprocessing of stellar radiation
through the halo. The large mid- to far-infrared excess originates from the
photosphere of the warmed disk, resulting in a flat spectrum, as observed. The
halo which we consider is observed as a compact reflection nebula, and is
discriminated from extended, disk-like envelopes around flat-spectrum T Tauri
stars. We show that the overall spectral shape of flat-spectrum T Tauri stars
can be reproduced by the present disk-halo model.

Astrophysics