Two Lines of Sight with Exceedingly Anomalous Ultraviolet Interstellar Extinction

Abstract

Low-resolution IUE data obtained toward HD 62542, a B5 V star in a line of sight associated with a dark cloud in the Gum nebula region near ζ Pup and γ Vel, yield an extinction curve with an extremely broad and weak UV extinction bump and the highest far-UV extinction yet observed in the Milky Way, reaching a value of E(1200-V)/E(B-V) ≈ 15. The weak bump is very similar to the bump observed toward HD 29647, a highly reddened late B HgMn star in the Taurus dark cloud. The authors have derived a new extinction curve for HD 29647. The far-UV extinction toward HD 29647 is larger than normal but not as extreme as that observed for HD 62542. An extensive discussion of the nature of the UV-absorbing matter in these two lines-of-sight is given. The role of environmental factors in producing an anomalous, extreme UV extinction is investigated.