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Interstellar radiation field and dust temperatures in the diffuse interstellar matter and in giant molecular clouds.

In Paper I (Mezger et al., 1982) we have investigated the emission spectrum of dust in both the diffuse interstellar matter (ISM) and in Giant Molecular Clouds (GMC's). This included an estimation of dust absorption cross sections from the Lyman continuum to the submm range and a redetermination of the interstellar radiation field (ISRF) using the most recent observations. In this paper we reevaluate the ISRF as a function of galactocentric distance D G , taking into account the results of recent surveys of the galactic 2.4 μm and 3.4 μm emission as well as of FIR surveys of the galactic plane as discussed in Paper I, and using an improved model of the variation of dust opacity in the galactic plane. We then determine the radiation field inside GMC's, including the radiation from the GMC itself, as a function of both D G and the extinction A V measured from the surface of the cloud. Next, we calculate the dust temperatures of the MRN composite graphite/silicate dust model (Mathis et al., 1977) in the radiation field within the cloud. The main results are: (i) The ISRF between 0.09 and 8 μm is dominated by stellar radiation and between 8 and 1000 μm is dominated by reemitted radiation from dust grains. The total energy density of the ISRF between D G = 10 and 5 kpc increases by a factor seven. The ISRF attains its intensity maximum at ∼ 1 μm. (ii) The dominant sources of heating in GMC's are stellar radiation for graphite grains and FIR heating for silicate grains. The contribution of normal field stars embedded in GMC's to the heating of dust is normally negligible. This may be different in a cloud which contains many low-mass pre-MS stars. While the temperatures of graphite grains in the diffuse ISM and in the outer sheaths of GMC's are twice as high as the temperatures of silicate grains, both types of grains attain approximately equal temperatures between 5 and 7 K deep inside GMC's. (iii) The stellar radiation absorbed in the outer layers of GMC's is practically all converted into FIR radiation whose integrated mean intensity is about five times that of the diffuse galactic FIR emission.