An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards

We present a new method for deriving UV-through-IR extinction curves, basedon the use of stellar atmosphere models to provide estimates of the intrinsic(i.e., unreddened) stellar spectral energy distributions (SEDs), rather thanunreddened (or lightly reddened) standard stars. We show that this``extinction-without-standards'' technique greatly increases the accuracy ofthe derived extinction curves and allows realistic estimations of theuncertainties. An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves, makingthe procedure valuable for both stellar and interstellar studies. Wedemonstrate how the extinction-without-standards curves make it possible to: 1)study the uniformity of extinction in localized spatial regions withunprecedented precision; 2) determine the relationships between differentaspects of curve morphology; 3) produce high quality extinction curves from lowcolor excess sightlines; and 4) derive reliable extinction curves for mid-lateB stars, thereby increasing spatial coverage and allowing the study ofextinction in open clusters and associations dominated by such stars. Theapplication of this technique to the available database of UV-through-IR SEDs,and to future observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes which modify them, and will enhanceour ability to remove the multi-wavelength effects of extinction fromastronomical energy distributions.Comment: To appear in the Astronomical Journal (September 2005); 28 pages including 9 figure