Dust in the Small Magellanic Cloud: Interstellar Polarization and Extinction

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Abstract
The typical extinction curve for the Small Magellanic Cloud (SMC), in contrast to that for the Galaxy, has no bump at 2175 Å and a steeper rise into the far-ultraviolet. For the Galaxy, the interpretation of the extinction, and therefore the dust content of the interstellar medium, has been greatly assisted by measurements of the wavelength dependence of the polarization. Up to the present, no such measurements existed for the SMC. Therefore, to further elucidate the dust properties in the SMC, we have measured, for the first time, linear polarization in five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the IUE in order to study the extinction. With the help of parameterization, we attempted to correlate the SMC extinction and polarization data. In addition, we performed dust model fits to both extinction and polarization using silicate and graphite, or amorphous carbon spheres and silicate cylinders. The size distribution for the cylinders is taken from a fit to the polarization, and we introduce the notion of volume continuity between this and the silicate sphere size distribution. The main results are (1) the wavelength of maximum polarization, λmax, in the SMC is typically smaller than that in the Galaxy; (2) however, AZV 456, which shows the UV extinction bump, has a λmax typical of that in the Galaxy, its polarization curve is narrower, its bump is shifted to shorter wavelengths as compared with the Galaxy, and its UV extinction does not conform to the Galactic analytical interpolation curve based on the ratio of total to selective extinction; (3) the "typical," monotonic SMC extinction curve can be best fitted with amorphous carbon and silicate grains; (4) the extinction toward AZV 456 may be explained only by assuming a larger gas-to-dust ratio than the observed N(H I)/A(V) value, with a small amount of the available carbon in graphite form; and (5) from an analysis of both the extinction and polarization data and our model fits, it appears that the SMC has typically smaller grains than those in the Galaxy. The absence of the extinction bump in the SMC generally has been thought to imply a lower amount or even an absence of carbon in solid form in the SMC compared with the Galaxy. Our results show that the size distribution, and not only the carbon abundance, is different in the SMC as compared with the Galaxy. In addition, and contrary to previous findings, another component besides silicates is indeed needed to provide a sizable part of the observed extinction toward the SMC. Using the SMC as a laboratory for studying the solid component of the interstellar medium, we also discuss some of the implications of our results in view of proposed interstellar dust models.
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