Synthetic infrared images and spectral energy distributions of a young low-mass stellar cluster

We present three-dimensional Monte Carlo radiative-transfer models of a very young (<10(5) yr old) low-mass (50 M.) 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. Using adaptive mesh refinement, the SPH density is mapped to the radiative-transfer grid without loss of resolution. The temperature of the ISM and the circumstellar dust is computed using Lucy's Monte Carlo radiative equilibrium algorithm. Based on this temperature, we compute the spectral energy distributions of the whole cluster and the individual objects. We also compute simulated far-infrared Spitzer Space Telescope (SST) images (24-, 70-, and 160-mu m bands) and construct colour-colour diagrams (near-infrared HKL and mid-infrared SST bands). The presence of accretion discs around the light sources influences the morphology of the dust temperature structure on a large scale (up to several 10(4) au). A considerable fraction of the interstellar dust is underheated compared with 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 = 3-0 mu m and lambda > 500 mum, compared with that without accretion discs. While the former excess is caused by the warm dust present in the discs, the latter is caused by the presence of the underheated (shadowed) dust. Our model with accretion discs around each object shows a similar distribution of spectral index (2.2-20 mum) values (i.e. Class 0-III sources) to that seen in the rho Ophiuchus cloud. We confirm that the best diagnostics for identifying objects with accretion discs are mid-infrared (lambda = 3-10 mum) colours (e.g. SST IRAC bands) rather than HKL colours.