THE QUANTITATIVE ASSESSMENT OF UV EXTINCTION DERIVED FROM IUE DATA OF GIANTS AND SUPERGIANTS

In this paper we show that the ultraviolet interstellar extinction towards hot, luminous stars (spectral types BO-B2; MK luminosity classes III-Ia) can be determined as accurately as for hot main-sequence stars. These luminous stars are reasonably numerous and can be observed over large distances and through substantial reddenings. We present an atlas of International Ultraviolet Explorer (IUE) dereddened fluxes for 1160 less-than-or-equal-to lambda less-than-or-equal-to 3125 angstrom of 13 lightly-reddened stars within this temperature/luminosity range, taken from the IUE Spectral Atlas of Wu et al. [IUE, NASA Newsletter, No. 22 (1983)]. These stars form a well-sampled grid (albeit not necessarily uniform) in spectral type and luminosity class and are all suitable comparison stars for the "pair method" of extinction determination. Their fluxes show absorption line strengths that allow a rather accurate IUE-based determination of relative temperatures and luminosities which is more suitable for the determination of UV extinction via the pair method than choosing a comparison star based only on quoted optical MK classifications. We discuss and tabulate the data used to deredden these standard stars along with dereddening error estimates, as well as the photometric and mismatch errors associated with using these comparison stars to derive extinction curves. We illustrate the accuracy of the method by considering a sample of four reddened luminous stars and conclude that the tabulated and plotted luminous standard star data can be used to produce UV extinction curves as accurate as those found for main-sequence stars.