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Ultraviolet photometry from the Orbiting Astronomical Observatory. XX - The ultraviolet extinction bump
Ultraviolet extinction bumps are investigated in the interstellarextinction curves between 1800 and 3600 A for 36 stars which have (B-V)excesses ranging from 0.03 to 0.55 and are mostly confined to thebrighter OB associations distributed along the galactic plane. Eachextinction curve is found to have a broad bump which peaks near 2175 Aand whose position and profile appear to be constant among all thestars. It is shown that the bump is probably interstellar in origin andthat the constancy of its position and shape places such severerestrictions on grain geometrical parameters that classical scatteringtheory cannot be used to explain the feature unless the dust grains inwidely separated regions of space and with very different physicalconditions are assumed to have nearly identical size and shapedistributions. Three extinction curves which extend to 1100 A areexamined and found to have the same general characteristics as theothers. Several extinction curves are analyzed for fine structure, butno convincing evidence is found in the present interval. Some processesare discussed which may be responsible for the bumps.
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