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We have studied the extinction properties of 38 starburst and Blue Compact galaxies covering the metallicity range $8.3\le\log(O/H)\le9.2$, by analyzing their UV+optical spectra. The UV spectra come from the compilation of IUE spectra by Kinney et al. (Kinney, Bohlin, Calzetti, Panagia, \& Wyse, 1993, ApJS, to appear on the May issue). The optical spectra, spanning the wavelength range 3200-7500 \AA, have been observed in a IUE-matched aperture. Following standard techniques, we have derived the selective extinction E(B-V) from the Balmer decrement and the metallicity from the [OII] and [OIII] lines.
In order to clarify the properties of dust extinction in the UV for starburst galaxies, we have fitted the observed UV fluxes of our galaxies in the wavelength range 1250-2600 \AA\ according to the power law $F(\lambda)\propto\lambda^{\beta}$ and studied the behaviour of $\beta$ as function of the selective extinction E(B-V). We find that these two quantities are correlated and that there is no difference between the loci occupied by the low and high metallicity galaxies in the plane $\beta$-vs-E(B-V). The correlation indicates that a higher metallicity does not change the characteristics of individual grains, but merely increases their number. On this ground, our conclusion is that the shape of the extinction law in the UV does not depend on metallicity, for extinctions E(B-V)$>0.2$.
Although the low metallicity galaxies in our sample follow neither the Large Magellanic Cloud nor the Small Magellanic Cloud extinction laws, the absence in our spectra of prominent 2200 \AA\ dust features illustrates that a simple application of a galactic extinction law may be inadequate to properly correct the UV spectra. Models of clumpy dust layers and of dust mixed with the ionized gas are currently under analysis.
