Ultraviolet extinction by interstellar dust in external galaxies: M31

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of 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 those of Galactic standard stars, and (3) comparing M31 star spectra to atmosphere models. The derived intrinsic M31 extinction curve has an overall wavelength dependence very similar to that of the average Galactic extinction curve but possibly has a weaker 2175 Angstrom bump, however, 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 inaccuracy in the preliminary flux calibration, and were not computed with the pair method used in this work. In this work, the foreground extinction component from the Galactic halo is also investigated. The foreground 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.