The UV high resolution spectrum of A-type supergiants.

Abstract

A study of all the A-supergiants observed with IUE at high resolution shows that they can be divided into two groups: stars which show evidence of wind and mass-loss, and stars which do not show these characteristics. In the first group the resonance lines of Mg II, Fe II, Si II, C II are asymmetric, showing variable shortward shifted components. In the second group, formed by the faintest stars, these lines are symmetric. The velocity of the most shortward shifted component observed in the Fe II lines and the terminal velocity remain constant in most of the stars. However, the intermediate shortward shifted components of Fe II lines vary with time, indicating the ejection of shells from time to time or the existence of CIR (corotating interaction regions). The terminal velocity decreases with increasing escape velocity which is not in agreement with the radiation-driven-wind theory.