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We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employs stellaratmosphere models in lieu of unreddened "standard" stars. This technique iscapable of virtually eliminating spectral mismatch errors in the curves. Italso allows a quantitative assessment of the errors and enables a rigoroustesting of the significance of relationships between various curve parameters,regardless of whether their uncertainties are correlated. Analysis of thecurves gives the following results: (1) In accord with our previous findings,the central position of the 2175 A extinction bump is mildly variable, itswidth is highly variable, and the two variations are unrelated. (2) Strongcorrelations are found among some extinction properties within the UV region,and within the IR region. (3) With the exception of a few curves with extreme(i.e., large) values of R(V), the UV and IR portions of Galactic extinctioncurves are not correlated with each other. (4) The large sightline-to-sightlinevariation seen in our sample implies that any average Galactic extinction curvewill always reflect the biases of its parent sample. (5) The use of an averagecurve to deredden a spectral energy distribution (SED) will result insignificant errors, and a realistic error budget for the dereddened SED mustinclude the observed variance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfully characterizeaverage extinction properties, they demonstrate that extinction curves respondsensitively to local conditions. Thus, each curve contains potentially uniqueinformation about the grains along its sightline.

An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology