Non-LTE Line Blanketed Model Atmospheres: Application to A-stars

Details Non-LTE Line Blanketed Model Atmospheres: Application to A-stars Non-LTE Line Blanketed Model Atmospheres: Application to A-stars

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.1911h&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #19.11; Bulletin of the American Astronomical Society, Vol. 24, p.1152

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We present a new method for calculating realistic line blanketed model atmospheres without the assumption of local thermodynamic equilibrium (LTE). The method is based on complete linearization, with the following two important modifications with respect to the standard variant. i) Instead of dealing separately with individual energy levels of a complicated metal species (e.g. Fe II), several levels with the same parity and close enough energy are grouped together to form a ``superlevel". Transitions between superlevels, the so-called ``superlines", are treated by resampling the detailed absorption cross-section and forming a partial ``NLTE opacity distribution function", which is representable by a relatively small number of frequency points (typically 15 - 30 points per superline). ii) The radiative rates (together with the heating/cooling rates, and the contributions to the rates in other important transitions - e.g. the hydrogen and carbon continua) are not fully linearized. However, in contrast to the earlier approaches, they are not held fixed. The only fixed quantity here is the approximate lambda operator. The new method is therefore a hybrid combining the complete linearization and the class of modern methods called accelerated lambda iteration (ALI). We have contructed several non-LTE model atmospheres for T_eff = 10000 K, log g = 4, with H, C I, C II, Mg II, and Fe II treated in NLTE. A variable number of Fe II lines, up to 45815 (i.e. all lines originating between the levels with measured energies), have been included, to form 221 superlines. The most interesting preliminary result is that the first 8 Fe II superlevels (about 13000 lines) produce most of the total blanketing effect. The work on the project was supported by the NASA grant No.65 under program NRA 91-OSSA-12.

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