FUSE Measurements of Extragalactic Interstellar Extinction

Observations of interstellar extinction provide a fundamental experimental basis for understanding the characteristics of dust, a primary constituent of the ISM. Interactions between dust grains and light depend in part upon the grain size relative to the wavelength of light extinguished; in particular, far-ultraviolet observations are useful for investigating the properties of small dust grains. We have acquired a sample of extragalactic sight lines that, in addition to possessing infrared, optical, and ultraviolet data, have also been observed by FUSE. Using extinction curves constructed from this sample we characterise the properties of Magellanic Cloud dust grains with special emphasis on how data at shorter wavelengths affect our current understanding of the differences and similarities between Galactic and Magellanic Cloud dust, particularly with respect to composition and size. 1. Assessing Molecular Hydrogen We have collected LWRS observations of Small and Large Magellanic Cloud (SMC and LMC, respectively) stars acquired with FUSE in 2000 and 2001; this presentation reports initial findings for the SMC extinction curves resulting from these observations. The SMC data were calibrated using CALFUSE v2.2.1 and a curve-of-growth-based analysis was applied in order to derive H2 column densities N(J ≥ 2); a curve-fitting procedure produced the J= 0, 1 populations. 418 SMC FUV Extinction 419 Only the SMC measurements are presented in Table 1, since the Galactic foreground components for each star are relatively small. It should be noted that the uncertainties given for levels J = 2, 3 are advisory in nature and that the level J = 4, 5 populations will be included when the full dataset is published (Cartledge et al. 2004; in preparation). Table 1. The Molecular Hydrogen Models log10[N(H2)] b log10[N(J = 0)] log10[N(1)] log10[N(2)] log10[N(3)] Reddened Star (cm) (km s) (cm) (cm) (cm) (cm) AzV 18 20.36±0.07 8.1 20.13±0.10 19.97±0.10 17.58±0.45 17.79±0.37 AzV 456 20.93±0.09 6.0 20.85±0.10 20.15±0.10 17.67±1.01 15.62±0.31 2. The Extinction Pairs The AzV18/AzV462 pairing represents sight lines in the SMC bar, whose extinction properties include a very small 2175 A bump and steep UV rise. The AzV456/AzV70 pairing is an example of an SMC extinction curve similar to Galactic curves, found in the SMC wing. Table 2. The Extinction Pairs E(B−V ) AV N(H I)/AV Reddened Star Comparison Star (mag) RV (mag) (10 21 H atoms/AV ) AzV 18 AzV 462 0.17±0.03 2.90±0.41 0.49±0.11 17.21±4.15 AzV 456 AzV 70 0.26±0.03 2.20±0.23 0.57±0.08 7.00±1.24 3. Modelling the Dust Population The maximum entropy method (MEM) algorithm employed by Clayton et al. (2003) for fitting multiple grain components to an extinction curve was used to derive models of the dust composition and grain size distribution for each SMC pairing; the assumed dust components were spherical grains of astrophysical silicate, amorphous carbon, and graphite. The current work extends the Clayton et al. (2003) analysis into the FUV, where the differences between SMC and Galactic extinction curves are more pronounced. The SMC bar curve (AzV18/AzV462), despite the enhanced FUV rise, is less demanding on SMC carbon and silicon abundances than the mean Galactic curve (RV = 3.1) on the corresponding Milky Way values, requiring about half of the available amounts. In contrast, the Galactic sight line toward HD 210121, whose extinction curve resembles that for the SMC bar, makes abundance demands similar to those of the mean Galactic curve (Sofia et al. 2004; in preparation). The SMC wing curve uses more silicon than is thought available in the SMC, a requirement similar to that which the mean Galactic curve places on Milky Way abundance. SMC wing dust also contains more carbon than is permitted by our abundance estimates, compared to the Galactic demand which allows a significant portion to exist in other forms. FUSE-based dust grain mass-size distributions for the three dust components are less smooth than their counterparts derived using IUE UV data alone, but 420 Cartledge et al. the effect is not as severe as the same trend among Galactic sight lines. Notably, both silicate and amorphous carbon distributions for SMC dust exhibit deficits of large grains relative to their Galactic counterparts.