Projection of circumstellar disks on their environments

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 (1000-10 000 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, CK3/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. The presence of a disk shadow may break a number of degeneracies encountered when fitting to the SED only. Specifically, the inclination, flaring properties and extinction toward the central star may be independently determined from near-infrared images of disk shadows. Constraints on the disk mass and size can be extracted from a simultaneous fit of SEDs and images. We find that the CK3 disk must have a very low mass in opacity-producing, small (less than or similar to 10 mu m) dust grains ( corresponding to a total mass of similar to 7 x 10(-6) M-., assuming a gas-to-dust ratio of 100) to simultaneously reproduce the very strong silicate emission features and the near-infrared edge-on morphology. Ced 110 IRS 4 requires that a roughly spherical cavity with radius similar to 500 AU centered on the central star-disk system is carved out of the envelope to reproduce the near-infrared images. We show that in some cases the bipolar nebulosity created by a disk shadow may resemble the effect of a physical bipolar cavity where none exists. 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. Selection criteria are given for distinguishing disk shadows from genuine large disks. 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. Finally, it is shown that disk shadows can be used to select edge-on disks suitable for observing ices located inside the disk.