ULTRAVIOLET EXTINCTION BY INTERSTELLAR DUST IN EXTERNAL GALAXIES - M31

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra o f stars in OB associations of M31 are used to derive the UV extinction by interstellar dust in M31 by three different methods: (1) comparing spectra of M31 star pairs, (2) comparing spectra of M31 stars to thos e of Galactic standard stars, and (3) comparing M31 star spectra to at mosphere models. The derived intrinsic M31 extinction curve has an ove rall wavelength dependence very similar to that of the average Galacti c extinction curve but possibly has a weaker 2175 Angstrom bump, howev er, with a significance of only 1 sigma. This result is different from the LMC (30 Dor)-like curves published earlier, which contained both intrinsic M31 extinction and ''foreground'' extinction, and were based either on low-signal IUE spectra, or on FOS data affected by inaccura cy in the preliminary flux calibration, and were not computed with the pair method used in this work. In this work, the foreground extinctio n component from the Galactic halo is also investigated. The foregroun d curve is consistent with the standard curve. While there is a slight indication for a steeper foreground curve than the standard one, the difference is not significant considering the data uncertainties.