The Transition of Supergiant CS Matter fom Cool Winds to Coronae — New Insights with X Aur Binary Systems

Summary

The age of astronomical satellites gave us access to X-rays and UV spectroscopy. New challenging questions for astronomy have been raised, one of which is the transition from hot, corona-like matter to cool winds and chromospheres. Such a transition occurs in a region of the HR diagram which extends as a band from G super giants to K giants.

After an introduction to the topic from the point of view of single star observations, the author then focusses on recent research of ζ Aur binary systems. These stars, with primaries situated in or near the region of the HR diagram in question, offer two unique advantages: — the main sequence B6 — A2 secondary star offers a reliable way of determining the system’s distance and thereby the absolute magnitude of the primary (much more accurately than for single supergiants): — moving through the wind and passing behind the supergiant’s chromosphere before and after eclipse, the secondary is an ideal light probe which provides us with a unique opportunity to monitor chromospheric densities and to achieve spatial resolution of the structure of the CS matter of the primary. But sophisticated diagnostic tools are required, such as two-dimensional radiative transport in the wind.

From high-resolution IUE spectra and ground based Coudé-spectroscopy (for λ ≲ 4500 Å) we can separate the light of the hot companion from the composite spectrum of the spatially unresolvable binary. While detailed optical observations of several chromospheres and their interpretation are still in preparation, some recent results from lUE-based studies are presented. The most important one may be the direct detection of 2 ‘cool coronae’ (HR 6902 and HR 2554) which coincide with depleted chromospheres. HR 6902 is also a crucial star for fulfilling the v < r criterion of MgII h₂ and k₂ emission but definitely not showing cool outflow at all. Earlier interpretations of this criterion are therefore questioned.