Observations and a model for the infrared continuum of Centaurus A

We present ISOSWS, ISOPHOT_S and 8-13 mu m observations of Centaurus A whic h show prominent PAH and silicate features. These and other data are used t o construct a model for the infrared continuum. We find that, in a small nu clear aperture (similar to 4 arcsec, similar to 60 pc), the spectral energy distribution is characteristic of emission from a starburst and dusty AGN torus; in larger apertures, additional components of cirrus and starburst e mission are required. The model components are based on the radiative transfer models of Efstathi ou et al. which include multiple scattering and the radiative effects of a dust-embedded source with a distribution of grain species and sizes. The to rus component is modelled in terms of a tapered dusty disc centrally illumi nated by a quasar-like source. The cirrus and starburst components are, res pectively, modelled in terms of diffuse dust illuminated by the interstella r medium and an ensemble of optically thick molecular clouds centrally illu minated by hot stars. These latter components additionally include emission from small graphite particles and PAHs. Based on our overall model, the torus diameter is estimated to be 3.6 pc an d the best inclination angle of the torus is 45 degrees. We present indepen dent observational evidence for this structure. This result has implication s for the detectability of tori in low-power AGN and for the use of the IRA S 60/25-mu m flux ratio as an indicator of the torus inclination.