FUSE Measurements of Far Ultraviolet Extinction. II. Magellanic Cloud Sight Lines

Abstract

We present an extinction analysis of 9 paths through the LMC and SMC based on FUSE observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Clouds using spectra including wavelengths as short as 950 A. We supplement these with results from 4 regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar and SMC wing. Despite the distinct characters of Milky Way and Magellanic Cloud extinction, our results are generally analogous to those found for Galactic curves in that the FUSE portions of each extinction curve are described reasonably well by FM curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the CCM formalism continue into FUV wavelengths. An MEM analysis of these curves suggests that LMCAvg and SMC wing sight lines require more silicon and/or carbon in dust than current abundance measurements would allow, while the requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs. An intermediate product of this analysis is the measurement of new H_2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H_2 column densities, E(B-V)/N(HI) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B-V)/N(H_2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H_2)-values are lower than those common in the Milky Way for paths with similar degrees of reddening.