  Class 0 sources are objects representing the earliest phase of the
protostellar evolution. Since they are highly obscured by an extended dusty
envelope, these objects emit mainly in the far-infrared to millimetre
wavelength range. The analysis of their spectral energy distributions with wide
wavelength coverage allows to determine the bolometric temperature and
luminosity. However, a more detailed physical interpretation of the internal
physical structure of these objects requires radiative transfer modelling. We
present modelling results of spectral energy distributions of a sample of nine
Class 0 sources in the Perseus and Orion molecular clouds. The SEDs have been
simulated using a radiative transfer code based on the Monte Carlo method. We
find that a spherically symmetric model for the youngest Class 0 sources allows
to reproduce the observed SEDs reasonably well. From our modelling we derive
physical parameters of our sources, such as their mass, density distribution,
size, etc. We find a density structure of $\rho \sim r^{-2}$ for the collapsing
cores at young ages, evolving to $\rho \sim r^{-3/2}$ at later times.

Astrophysics