MODELING OF IR EMISSION OF INTERSTELLAR CLOUDS .1. EMISSION OF ISOLATED CLOUDS AND DUST ABUNDANCE VARIATIONS

We have developed a general radiative transfer code for the continuum radiation to compute the incident radiation field inside a non-homogeneous, spherically symmetric cloud. We have included a dust emission model which takes into account IR emission by transiently heated small dust particles. The model has been applied to compute the emission at IR-submm wavelengths for a set of clouds of various extinction and density distributions, containing a dust composition similar to that of the diffuse interstellar medium. Although the model is not restricted to the hypothesis adopted in this paper, we assumed that the clouds are heated only by the diffuse interstellar radiation field. We give, for a wide range of density distributions and total mass, the global emission of the cloud from IR to Submm wavelengths. We also present and discuss the spatial and spectral distribution of this emission and the characteristics of the radiation field through the cloud. At IRAS wavelengths, we find that the Limb Brightening (LB) is expected to appear in all the IRAS bands if the total extinctions toward the cloud center exceeds 4-10 mag. IRAS color as 12/100 or 25/100 are predicted to decrease continuously from the edge to the center of the cloud for constant dust composition into the cloud. Comparison of these results with the IRAS data suggest that the actual color variations can only be explained by variations of dust abundance rather than by radiative transfer effects. This implies that halos with an increased abundance of small dust particles must be present around clouds showing limb brightening at short IRAS wavelengths. Possible influences of halos on the equilibrium temperature of small and bigger dust grains in dense clouds and on the associated FIR-Submm cloud emission is also investigated. The 100 mum surface brightness is predicted to be proportional to the total column density only in the outer regions of the clouds but it can be used to estimate column densities up to extinctions of 5 - 10 mag if the inhomogeneities in the cloud envelope emitting most of the 100 mum radiation are small enough. For higher extinctions (A(V) > 10 mag), the model predicts a significant LB at 100 mum which we attribute to the decrease of the temperature and emission of the bigger grains inside the cloud. As a consequence, the 100 mum surface brightness is limited to congruent-to 20 MJy/sr for any model, independent of column density. In the FIR-Submm, the LB effect progressively disappear and the global emission of the cloud becomes progressively proportional to its mass.