FUSEMeasurements of Far‐Ultraviolet Extinction. I. Galactic Sight Lines

We present extinction curves that include data down to far ultravioletwavelengths (FUV; 1050 - 1200 A) for nine Galactic sight lines. The FUVextinction was measured using data from the Far Ultraviolet SpectroscopicExplorer. The sight lines were chosen for their unusual extinction propertiesin the infrared through the ultraviolet; that they probe a wide range of dustenvironments is evidenced by the large spread in their measured ratios oftotal-to-selective extinction, R_V = 2.43 - 3.81. We find that extrapolation ofthe Fitzpatrick & Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions of the FUVextinction based upon the Cardelli, Clayton & Mathis (CCM) dependence on R_Vgive mixed results. For the seven extinction curves well represented by CCM inthe infrared through ultraviolet, the FUV extinction is well predicted in threesight lines, over-predicted in two sight lines, and under-predicted in 2 sightlines. A Maximum Entropy Method analysis using a simple three component grainmodel shows that seven of the nine sight lines in the study require a largerfraction of grain materials to be in dust when FUV extinction is included inthe models. Most of the added grain material is in the form of small (radii <200 A) grains.Comment: Accepted for publication in the Astrophysical Journal. 31 pages with 7 figure