Ultraviolet extinction towards far OB associations.

Abstract

Stars in the Aur OB-2 and Sco OB-1 associations, located in the direction of the galactic anticenter and center, respectively, have been observed for interstellar extinction. No effects due to changes of grain composition across such large distances are seen. A star exhibiting a peculiar extinction is found in each association. The properties of the responsible dust grains are discussed in terms of a two-component dust mixture: graphite and uncoated silicates. The column density both of graphite and silicate atoms are found to be higher than in stars of the same associations exhibiting a regular UV extinction.