  We present three-dimensional Monte Carlo radiative transfer models of a very
young (<10^5 years old) low mass (50 Msun) stellar cluster containing 23 stars
and 27 brown dwarfs. The models use the density and the stellar mass
distributions from the large-scale smoothed particle hydrodynamics (SPH)
simulation of the formation of a low-mass stellar cluster by Bate, Bonnell and
Bromm. We compute the spectral energy distributions of the whole cluster and
the individual objects, simulated far-infrared Spitzer Space Telescope (SST)
images and construct colour-colour diagrams (near-infrared HKL and SST
mid-infrared bands). The presence of accretion discs around the light sources
influences the morphology of the dust temperature structure on a large scale
(up to a several 10^4 au). A considerable fraction of the interstellar dust is
underheated compared to a model without the accretion discs because the
radiation from the light sources is blocked/shadowed by the discs. The spectral
energy distribution (SED) of the model cluster with accretion discs shows
excess emission at lambda = 3-30 micron and lambda > 500 micron compared to
that without accretion discs. Our model with accretion discs around each object
shows a similar distribution of spectral index (2.2-20 micron) values (i.e.
Class 0--III sources) as seen in the Rho Ophiuchus cloud. We confirm that the
best diagnostics for identifying objects with accretion discs are mid-infrared
(lambda = 3-10 micron) colours (e.g. SST IRAC bands) rather than HKL colours.
