PROPERTIES OF DUST GRAINS PROBED WITH EXTINCTION CURVES

Modern data of the extinction curve from the ultraviolet to the near-infrared are revisited to study properties of dust grains in the Milky Way (MW) and the Small Magellanic Cloud (SMC). We confirm that the graphite-silicate mixture of grains yields the observed extinction curve with the simple power-law distribution of the grain size but with a cutoff at some maximal size: the parameters are tightly constrained to be q = 3.5 +/- 0.2 for the size distribution a(-q) and the maximum radius a(max) = 0.24 +/- 0.05 mu m, for both MW and SMC. The abundance of grains, and hence the elemental abundance, is constrained from the reddening versus hydrogen column density, E(B - V)/N-H. If we take the solar elemental abundance as the standard for the MW, >56% of carbon should be in graphite dust, while it is <40% in the SMC using its available abundance estimate. This disparity and the relative abundance of C to Si explain the difference of the two curves. We find that 50%-60% of carbon may not necessarily be in graphite but in the amorphous or glassy phase. Iron may also be in the metallic phase or up to similar to 80% in magnetite rather than in silicates, so that the Mg/Fe ratio in astronomical olivine is arbitrary. With these substitutions, the parameters of the grain size remain unchanged. The mass density of dust grains relative to hydrogen is rho(dust)/rho(H)=1/(120(-16)(+10)) for the MW and 1/(760(-90)(+70)) for the SMC under the elemental abundance constraints. We underline the importance of the wavelength dependence of the extinction curve in the near-infrared in constructing the dust model: if A(lambda) proportional to lambda(-gamma) with gamma similar or equal to 1.6, the power-law grain-size model fails, whereas it works if gamma similar or equal to 1.8-2.0.