  The attenuation of starlight by dust in galactic environments is investigated
through models of radiative transfer in a spherical, clumpy ISM. Extinction
properties for MW, LMC, and SMC dust types are considered. It is illustrated
that the attenuation curves are primarily determined by the wavelength
dependence of absorption rather than by the underlying extinction
(absorption+scattering) curve. Attenuation curves consistent with the "Calzetti
attenuation curve" are found by assuming the silicate-carbonaceous dust model
for the MW, but with the 2175A absorption bump suppressed or absent. The
discrepancy between our results and previous work that claimed the SMC-type
dust to be the most likely origin of the Calzetti curve is ascribed to the
difference in adopted albedos; this study uses the theoretically calculated
albedos whereas the previous ones adopted empirically derived albedos from
observations of reflection nebulae. It is also found that the model attenuation
curves calculated with the MW dust are well represented by a modified Calzetti
curve with a varying slope and UV bump strength. The strong correlation between
the slope and UV bump strength, with steeper curves having stronger bumps, as
found in star-forming galaxies at 0.5<z<2.0, is well reproduced by our models
if the abundance of the UV bump carriers or PAHs is assumed to be 30% or 40% of
that of the MW-dust. The trend is explained by radiative transfer effects which
lead to shallower attenuation curves with weaker UV bumps as the ISM is more
clumpy and dustier. We also argue that at least some of the IUE local starburst
galaxies may have a UV bump feature in their attenuation curves, albeit much
weaker than that of the MW extinction curve.
