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Deriving a R-Dependent IR-through-UV Extinction Curve

In this Appendix a new estimate of the wavelength dependence of Milky Way extinction from the IR through the UV is derived. The motivation for this is the need for a multiwavelength extinction curve suitable for dereddening IR-through-UV spectrophotometric data now available from, for example, the Hubble Space Telescope's Faint Object Spectrograph (see, e.g., Fitzpatrick & Massa 1998 and Guinan et al. 1998). The specific goals are 1) to produce a curve for the R = 3.1 case which reproduces existing narrow band extinction measurements and the principal Johnson and Strömgren photometric extinction relations and 2) to define the R-dependence of the extinction law, relying principally on spectral regions where this dependence is well-determined. Existing estimates of the average wavelength dependence of optical/near-IR extinction do not accurately portray the true shape of the curve due to bandpass effects arising from the broadband photometry used to derive the curves. This deficiency will be corrected by using synthetic photometry.