  We present the results of a three-dimensional Monte Carlo radiative transfer
code for starless molecular cloud cores heated by an external isotropic or
non-isotropic interstellar radiation field. The code computes the dust
temperature distribution inside model clouds with specified but arbitrary
density profiles. In particular we examine in detail spherical (Bonnor-Ebert)
clouds, axisymmetric and non-axisymmetric toroids, and clouds heated by an
external stellar source in addition to the general interstellar field. For
these configurations, the code also computes maps of the emergent intensity at
different wavelengths and arbitrary viewing angle, that can be compared
directly with continuum maps of prestellar cores. In the approximation where
the dust temperature is independent of interactions with the gas and where the
gas is heated both by collisions with dust grains and ionization by cosmic
rays, the temperature distribution of the gas is also calculated. For cloud
models with parameters typical of dense cores, the results show that the dust
temperature decreases monotonically from a maximum value near the cloud's edge
(14-15 K) to a minimum value at the cloud's center (6-7 K). Conversely, the gas
temperature varies in a similar range, but, due to efficient dust-gas coupling
in the inner regions and inefficient cosmic-ray heating in the outer regions,
the gradient is non-monotonic and the gas temperature reaches a maximum value
at intermediate radii. The emission computed for these models (at 350 micron
and 1.3 mm) shows that deviations from spherical symmetry in the density and/or
temperature distributions are generally reduced in the simulated intensity maps
(even without beam convolution), especially at the longer wavelengths.
