Ultraviolet Extinction Properties in the Milky Way

We have assembled a homogeneous database of 417 ultraviolet (UV) extinction curves for reddened sight lines having International Ultraviolet Explorer spectra. We have combined these with optical and Two Micron All Sky Survey photometry, allowing estimates of the ratio of total-to-selective extinction, R_V, for the entire sample. Fitzpatrick-Massa (FM) parameters have also been found for the entire sample. This is the largest study of parameterized UV extinction curves yet published, and it covers a wide range of environments, from dense molecular clouds to the diffuse interstellar medium, with extinctions A_V ranging from 0.50 to 4.80. It is the first to extend far beyond the solar neighborhood and into the Galaxy at large, with 30 sight lines having distances greater than 5 kpc. Previously, the longest sight lines with FM parameters and R_V extended ~1 kpc. We find that (1) the Cardelli, Clayton, & Mathis (CCM) extinction law applies for 93% of the sight lines, implying that dust processing in the Galaxy is efficient and systematic, (2) the central wavelength of the 2175 Å bump is constant, and (3) the 2175 Å bump width is dependent on environment. Only four sight lines show systematic deviations from CCM: HD 29647, 62542, 204827, and 210121. These sight lines all sample dense, molecule-rich clouds. The new extinction curves and values of R_V allow us to revise the CCM law.