  We use a 3D Monte Carlo radiative transfer code to study the projection of
large shadows by circumstellar disks around young stellar objects on
surrounding reflection nebulosity. It is shown that for a wide range of
parameters a small (10-100 AU) circumstellar disk can project a large (1 000-10
000 AU) dark band in the near-infrared that often resembles a massive edge-on
disk. The disk shadows are divided into two basic types, depending on the
distribution of the reflecting material and the resulting morphology of the
shadows in the near-infrared. Two YSOs associated with bipolar nebulosity, CK
3/EC 82 illuminating the Serpens Reflection Nebula (SRN) and Ced 110 IRS 4 in
the Chamaeleon I molecular cloud, are modelled in detail as disk shadows.
Spectral energy distributions of the two sources are collected using both
archival ISO data and new Spitzer-IRS data. An axisymmetric model consisting of
a small disk and a spherically symmetric envelope can reproduce the
near-infrared images and full spectral energy distributions of the two disk
shadow candidates. It is shown that the model fits can be used to constrain the
geometry of the central disks due to the magnifying effect of the projection.
We find that a disk unresolved in near-infrared images, but casting a large
disk shadow, can be modelled at a level of sophistication approaching that of
an edge-on disk with resolved near-infrared images. It is found that the most
obvious observable difference between a disk shadow and a large optically thick
disk is that the disk shadows have a compact near-infrared source near the
center of the dark band. High resolution imaging and/or polarimetry should
reveal the compact source in the center of a disk shadow as an edge-on disk.
[Abstract abridged]
