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Session 63 - The Frontiers of Far Ultraviolet Astrophysics - II.
Topical, Oral session, Wednesday, June 10
Sierra/Padre,
New results from the far ultraviolet are increasing our understanding of the structure and dynamics of cool star atmospheres. The far UV spectrum bridges the gap between the chromospheric ultraviolet and the X-ray corona. The important O VI ion spans the range of activity from a diverse group of systems, and is formed near the peak of the emission measure distribution of less active stars like the Sun, while it is formed at the minimum of the distribution for the highly active RS CVn binaries and rapidly rotating young stars. Density diagnostics from C III, newly accessible with the \lambda977/\lambda1176 line ratio from ORFEUS, indicate a wide spread in transition region pressure among different systems as well.
In active (i.e. closed field) regions, magnetic pressure confinement overcomes fluid domination in the transition region and allows quasi-static structures to support the steep temperature gradients, while in open field line regions (e.g. coronal holes) the wind begins its acceleration through steep gradients. New results from ORFEUS show the first evidence for warm winds in luminous hybrid stars, providing the critical link between solar-like stars with hot coronae and cooler stars with cool winds. Closer to home, the UVCS experiment on SOHO probes the diagnostic O VI doublet throughout the inner coronal region of the Sun. Observed line profiles indicate that ion temperatures in solar coronal holes are hotter than electron temperatures, demonstrating the importance of ion heating and acceleration processes.
