Ultraviolet Extinction Properties in the Milky Way

We have assembled a homogeneous database of 417 ultraviolet (UV) extinctioncurves for reddened sightlines having International Ultraviolet Explorer (IUE)spectra. We have combined these with optical and 2MASS photometry allowingestimates of the ratio of total-to-selective extinction, R(V), for the entiresample. Fitzpatrick-Massa (FM) parameters have also been found for the entiresample. This is the largest study of parameterized UV extinction curves yetpublished and it covers a wide range of environments, from dense molecularclouds to the diffuse interstellar medium (ISM), with extinctions A(V) rangingfrom 0.50 to 4.80. It is the first to extend far beyond the solar neighborhoodand into the Galaxy at large, with 30 sightlines having distances > 5 kpc.Previously, the longest sightlines with FM parameters and R(V) extended ~ 1kpc. We find that (1.) the CCM extinction law applies for 93% of thesightlines, implying that dust processing in the Galaxy is efficient andsystematic; (2.) the central wavelength of the 2175 A bump is constant; (3.)the 2175 A bump width is dependent on environment. Only four sightlines showsystematic deviations from CCM, HD 29647, 62542, 204827, and 210121. Thesesightlines all sample dense, molecule-rich clouds. The new extinction curvesand values of R(V) allow us to revise the CCM law.Comment: 32 pages, 12 figure