Towards abundance determination from
dynamic atmospheres
Michael T. Lederer, Thomas Lebzelter, Bernhard Aringer,Walter Nowotny, Josef Hron
Department of Astronomy, University of Vienna
Stefan UttenthalerESO
Susanne HöfnerUppsala University
Setting the scene
• Goal: reliable and accurate element abundances on the AGB
• Relevant spectral features – atomic lines for metallicity
and s-process elements- CO lines (isotopes)- OH/CO lines for C/O ratio
• A diffcult task! Even more difficult in the case of stars with dynamic atmospheres
Lines, lines, everywhere lines!• many strong atomic and
molecular lines• strong blending• calibration very difficult• overlapping weak
molecular lines form a quasi-continuum which affects the appearance of all stronger lines
Model & Observation
Sc line consists oftwo components
Sc linetoo strong
Molecular linesat wrong positions
TiITiI+VI
ScIScI+at FeI
CN OH
problems concerning gf-values of atomic lines
• uncertain positions ofmolecular lines
Beware of metallicity effects
ScI
CaI
CaI
ScI
TiI
H2O
H2O
H2OH2O+OH
H2O+OH
H2O
CN
Water linesdecrease with Z
Metal linesincrease with Z
• Molecular lines show a complex behaviour as a function of abundances
• Changes of the structure and quasi-continuum affect metal lines (may become weaker with higher Z)
Consistency
• Calculation of model atmospheres and subsequent spectral synthesis require consistent treatment of– Metallicity– Sphericity– Opacity
• Inconsistencies lead to wrong spectra making a reliable abundance determination problematic
Example: 47 Tuc AGB variable
Gem
ini S
outh
, P
hoen
ix
Teff ~ 3600 K, M ~ 0.8 M, L ~ 1300 L, [Fe/H] = -0.66
Static models fail here!
Gem
ini S
outh
, P
hoen
ix
Teff ~ 3600 K, M ~ 0.8 M, L ~ 4500 L, [Fe/H] = -0.66• „here“ means: for large-amplitude variables
• Dynamic effects dominate the spectrum
The need for dynamic model atmospheres: observational evidence
observed EW
calculated EW(hydrostatic models)
calculated EW (range)(dynamic models)
Aringer et al. (1999)
ES
O N
TT
, IR
SP
EC
Dynamic Atmospheres
Why do we needdynamic model atmospheres?
• We want to determine abundances along the whole AGB
• Dynamical effects dominate in the upper part of the AGB
• Hydrostatic atmospheres cannotdescribe the spectra of stars with large amplitude pulsations (Miras)
• Dynamic model atmospheres(„Höfner-type“, Höfner et al. 2003)– Time dependent description of radiation hydrodynamics
and dust formation (C-dust, O-dust in progress)– Pulsation introduced as inner boundary– Mass loss, shock waves
Complex density, temperature,and velocity structure
significantly shallower
density structure
Now
otny et al. (2005)
Complex line formation
Effect on equivalent widths
Lebzelter, Aringer, Nowotny (2003)
Outlook
• Matching dynamic modelsto observed objects
• Parameter studies indynamic atmospheres
• Abundance determination in Globular Cluster systems in the Magellanic Clouds and the Galaxy (taking into account the influence of dynamics)
• Abundances as a test forstellar evolutionary and mixing models
Conference invitation
www.univie.ac.at/galagb/Registration deadline: Feb 28, 2006
C/O coreconvective shellatmospheredust formation region
persons with
a sweet tooth
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