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We investigate the origin of the strong radial gradient in theultraviolet-to-infrared ratio in the spiral galaxy NGC 300, and emphasize theimportance of local variations in the interstellar medium geometry, concludingthat they cannot be neglected with respect to metallicity effects. Thisanalysis is based upon a combination of maps from GALEX and Spitzer, and fromthe ground (UBVRI, Halpha and Hbeta). We select ionizing stellar clustersassociated with HII regions of widely varying morphologies, and derive theirfundamental parameters from population synthesis fitting of their spectralenergy distributions, measured to eliminate local backgrounds accurately. Fromthese fits, we conclude that the stellar extinction law is highly variable inthe line of sight of young clusters of similar ages. In the particular modelgeometry that we consider most appropriate to the sampled regions, we checkedthat our findings are not significantly altered by the correct treatment ofradiative transfer effects. The variations are systematic in nature: extinctionlaws of the Milky Way or LMC type are associated with compact HII regions (thecompacity being quantified in two different ways), while clusters surrounded bydiffuse HII regions follow extinction laws of the 30 Doradus or SMC type. TheCalzetti starburst attenuation law, although most often degenerate with the 30Doradus extinction law, overpredicts ionizing photon fluxes by large amounts.We also find that the extinction law variations are correlated with the columndensity of dust species emitting in the near- and mid-infrared. Finally, webriefly discuss the nebular to stellar extinction ratios, and the excitation ofaromatic band carriers, invalidating their claimed association with cold dust.Comment: accepted for publication in ApJ -- figure 6 abridged her

Extinction Law Variations and Dust Excitation in the Spiral Galaxy NGC 300