  We report on a study of the thermal dust emission of the circumstellar
envelopes of a sample of Class 0 sources. The physical structure (geometry,
radial intensity profile, spatial temperature and spectral energy distribution)
and properties (mass, size, bolometric luminosity (L_bol) and temperature (T_
bol), and age) of Class 0 sources are derived here in an evolutionary context.
This is done by combining SCUBA imaging at 450 and 850 microm of the thermal
dust emission of envelopes of Class 0 sources in the Perseus and Orion
molecular cloud complexes with a model of the envelope, with the implementation
of techniques like the blackbody fitting and radiative transfer calculations of
dusty envelopes, and with the Smith evolutionary model for protostars. The
modelling results obtained here confirm the validity of a simple spherical
symmetric model envelope, and the assumptions about density and dust
distributions following the standard envelope model. The spherically model
reproduces reasonably well the observed SEDs and the radial profiles of the
sources. The implications of the derived properties for protostellar evolution
are illustrated by analysis of the L_bol, the T_bol, and the power-law index p
of the density distribution for a sample of Class 0 sources.
