2,839 research outputs found
Morphological classification of post-AGB stars
We present a complete study of the morphology of post-Asymptotic Giant Branch
(AGB) stars. Post-AGB is a very short evolutionary phase between the end of the
AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and
10,000 yrs). We have defined the end of the post-AGB phase and the beginning of
the PN phase when the star is hot enough to fully ionize the hydrogen envelope.
Post-AGB stars have a circumstellar shell that is illuminated by the central
stars or partially ionized. However, this circumstellar shell is too small to
be resolved from ground-based observations. Thus, we have used data from the
Hubble Space Telescope (HST) database to resolve these shells. About 150
post-AGB were found in this database. Here we present the preliminary results
on their morphological classification and the correlation with several
parameters such as galactic latitude and IRAS fluxes. Our preliminary results
show that 40% of the sample are stellar-like (S), 33 % bipolar (B), 12 %
multi-polar (M) and 15 % elliptical (E).Comment: proceedings of the conference "Why Galaxies Care About AGB Stars II",
Vienna 2010, eds. Franz Kerschbaum, Thomas Lebzelter and Bob Wing, ASP
Conf.Ser (in press
Circumstellar effects on the Rb abundances in O-rich AGB stars
For the first time we explore the circumstellar effects on the Rb (and Zr)
abundance determination in O-rich asymptotic giant branch (AGB) stars by
considering the presence of a gaseous circumstellar envelope with a radial
wind. A modified version of the spectral synthesis code Turbospectrum was used
to deal with extended atmosphere models and velocity fields. The Rb and Zr
abundances were determined from the resonant 7800A Rb I line and the 6474A ZrO
bandhead, respectively, in five representative O-rich AGB stars with different
expansion velocity and metallicity. By using our new dynamical models, the Rb I
line profile (photospheric and circumstellar components) is very well
reproduced. Interestingly, the derived Rb abundances are much lower (by 1-2
dex) in those O-rich AGB stars showing the higher circumstellar expansion
velocities. The Zr abundances, however, remain close to the solar values. The
Rb abundances and Rb/Zr ratios derived here significantly alleviate the problem
of the present mismatch between the observations of intermediate-mass (4-8
solar masses) Rb-rich AGB stars and the AGB nucleosynthesis theoretical
predictions.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures, and 2 tables); final version (language corrected
- …