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