The variation of interstellar extinction in the ultraviolet

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Abstract
One hundred and fifty-four reddened stars that are apparently normal in the visible were selected from the S2/68 Ultraviolet Sky Survey. The ultraviolet data for 92 of these cannot be explained in terms of a fixed interstellar extinction law. Between 1400 and 2740 Å, the extinction curve for each star can be well represented by two parts; a straight-line scattering component and a Lorentzian 2200 Å absorption feature. Independent variations are found in both parts and these cannot be explained by photometric or spectral classification errors. Both parts vary smoothly, implying that there is no fixed extinction law, and one star in three is found to depart from the mean law by more than 1 mag at either 1500 or 2200 Å. The two variations allow not only all 154 stars to be explained but also anomalous stars reported by other authors. These are not special but merely situated towards the limits of the variations. A variation in the relative proportions of graphite and silicate grains goes some way towards explaining the observations. The profile of the 2200 Å feature is determined, the symmetrical shape is confirmed, and the profile fits a Lorentzian very closely.

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