Towards abundance determination from dynamic atmospheres
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Transcript of Towards abundance determination from dynamic atmospheres
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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Complex density, temperature,and velocity structure
significantly shallower
density structure
Now
otny et al. (2005)
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Complex line formation
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Effect on equivalent widths
Lebzelter, Aringer, Nowotny (2003)
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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
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Conference invitation
www.univie.ac.at/galagb/Registration deadline: Feb 28, 2006
C/O coreconvective shellatmospheredust formation region
persons with
a sweet tooth