6 research outputs found
Chemical composition of A--F type post-AGB candidates
An abundance analysis has been conducted for a sample of nine post-AGB
candidate stars; eight of them have not been explored before. We find four very
promising objects like HD 105262, HD 53300 and CpD among them. We
find strong evidence of dust-gas separation through selective depletion of
refractive elements in HD 105262. The same effect is also observed in HD 53300,
CpD and HD 114855 although abundance peculiarities are relatively
smaller for the last two stars. We find strong enrichment of nitrogen for HD
725, HD 842, HD 1457, HD 9233 and HD 61227 but no further evidence to support
their post-AGB nature. We have compared the observed [N/C] ratios of these
stars with the predictions of evolutionary models which include the rotation
induced mixing.Comment: Accepted in MNRAS, 18 pages, 12 figure
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