An Unusual Absorption Feature in the Far-Ultraviolet Spectrum of Early-Type Supergiants

The OAO-2 satellite has been used to obtain far ultraviolet scans of six early-type supergiants. The data reveal the presence of a distinct, broad absorption feature centered near 1720 A. This feature is unique in that it remains essentially constant in strength, breadth and central position over the spectral type range B0 I to A2 I. The feature also appears in the spectrum of the B-type shell star beta Tauri, with a strength comparable to that observed for the supergiants. It appears weakly, or not at all, in the B and A main sequence spectra we have examined. The hypothesis that the feature is due to a fortuitous blend of intrinsically strong lines arising primarily from the ground configurations of abundant metallic ions is discussed in detail.