Atmospheric Heating and Wind Acceleration: Results for Cool Evolved Stars based on Proposed Processes

A chromosphere is a universal attribute of stars of spectral type later than ~F5. Evolved (K and M) giants and supergiants (including the zeta Aurigae binaries) show extended and highly turbulent chromospheres, which develop into slow massive winds. The associated continuous mass loss has a significant impact on stellar evolution, and thence on the chemical evolution of galaxies. Yet despite the fundamental importance of those winds in astrophysics, the question of their origin(s) remains unsolved. What sources heat a chromosphere? What is the role of the chromosphere in the formation of stellar winds? This chapter provides a review of the observational requirements and theoretical approaches for modeling chromospheric heating and the acceleration of winds in single cool, evolved stars and in eclipsing binary stars, including physical models that have recently been proposed. It describes the successes that have been achieved so far by invoking acoustic and MHD waves to provide a physical description of plasma heating and wind acceleration, and discusses the challenges that still remain.Comment: 46 pages, 9 figures, 1 table; modified and unedited manuscript; accepted version to appear in: Giants of Eclipse, eds. E. Griffin and T. Ake (Berlin: Springer

Radiative transfer in stellar chromospheres

SIGLEAvailable from British Library Document Supply Centre-DSC:D194175 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Fluorescence in Astrophysical Plasmas

Following the initial detection by Bowen in 1934 of the strong O III linesbeing due to accidental resonance with strong He II radiation, many strong spectralemission lines are explained as produced by fluorescence. Many of these areFe II lines pumped by H Lyα, as a consequence of strong radiation from hydrogenand a favorable energy level structure for Fe II. The lines are observed in manytypes of objects with low density plasma components. The Weigelt condensationsin the vicinity of the massive star Eta Carinae is one location where these lines areobserved and can be studied in detail, as well as been used for diagnostics.These gas condensations do not only show a spectrum indicating a nonequilibriumexcitation but also non-equilibrium ionization, where the strong hydrogenradiation plays a key role. Early studies identified certain strong lines beingthe result of Resonance Enhanced Two-Photon Ionization (RETPI). Further investigationssuggest that RETPI can be the responsible mechanism for the ionizationstructure of gas condensation.We will review the resonance processes, with emphasis on the Eta Carinae spectrum.Large spectral, spatial and temporal coverage is available for this fascinatingobject, allowing for detailed analysis