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D. Luttermoser (ETSU)
The physical processes in the atmospheres of asymptotic giant branch (AGB) stars have many important attributes. Their large mass-loss rates impact their evolution and enrich the metal content in our galaxy. Virtually all AGB stars vary in brightness, which includes the Mira-type variables. The "mira" variability is thought to be caused by pulsations which produce a `shocked' atmosphere. Excitation and ionization collisional rates are small in comparison to radiative rates, due to the low densities, which makes the LTE approximation often invalid. The cool atmospheric temperatures of these stars allow molecules and dust to form which further complicates the picture -- the formation of these species may not be in equilibrium either. Atmospheric modeling of these stars has followed two different approaches: (1) the hydrostatic (HS) method and (2) the hydrodynamic (HD) method. In the HS method, one assumes a hydrostatic atmosphere and models the atmospheric structure either under the assumption of radiative (plus convective) equilibrium (the so-called photospheric models), or semiempirically by assuming a temperature-density atmospheric profile, then adjusting the temperature structure until a synthetic spectrum from the model matches an observed spectrum (the so-called chromospheric models). In these HS models, the radiative transfer and level densities are determined with either the assumption of LTE or non-LTE. In the HD method, the full set of hydrodynamic fluid equations are used (in either a Lagrangian or Eulerian grid) to determine the atmospheric structure with radiative cooling either being approximated with an assumed analytic function, or completely ignored by using isothermal or adiabatic approximations. Each of these techniques has its limitations. This talk will highlight what has been done to date in modeling the atmospheres of these stars and make suggestions as to what should be done in future modeling attempts.
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The author(s) of this abstract have provided an email address for comments about the abstract: lutter@mail.etsu.edu
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Bulletin of the American Astronomical Society, 36 #2
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