Testing the opacity and equation of state of LTE and non-LTE model atmospheres with OPAL and OP data for early-type stars

摘要

Context. Complex investigations of stars are made by studying their atmospheres, evolutionary states and oscillation properties, but the opacities and equation of state are still uncertain. Aims. Compatibility of model atmospheres with internal structure of early-type stars are investigated at photospheric and sub-photospheric regions. Methods. The problem was studied quantitatively by means of diagrams that involve the Rosseland-mean opacities, temperatures, densities, and gradients of radiation pressure as functions of depth using stellar atmosphere and envelope models. Results. Two new grids of radiative-equilibrium models of atmospheres were calculated assuming the local thermodynamic equilibrium (LTE). The first one is based on Kurucz’s ATLAS12 computer code implemented with the occupation probability formalism, which accounts for the destruction of loosely bound states of atoms by interactions with particles in the plasma. The second grid of models is based on monochromatic opacities and equations of state taken from the well-known international opacity project (OP). Non-LTE model atmospheres are also calculated and some aspects of the modeling are pointed out to improve agreement with ATLAS12, OP data, as well as with results of the Rogers & Iglesias OPAL opacity computing code (OPAL). The model atmospheres are also discussed by analyzing emerging fluxes of radiation. Conclusions. Commonly used LTE and non-LTE models of stellar atmospheres of early-type stars differ markedly from each other and do not fit OPAL or OP stellar envelope models at great optical depths. The results of the OP project distributed as the OPCD v?3.3 base are useful for calculating not only stellar envelopes but also fully line-blanketed LTE model atmospheres. These models have diagnostic values for studying the atomic physics used for modeling of photospheric and sub-photospheric regions of stars. The OPAL and OP opacities are markedly underestimated in comparison with the Rosseland-mean opacities taken from the Castelli & Kurucz (2003, IAU Symp., 210, 10) atmosphere models in which a new opacity bump appears at?lgT?≈?5.06. This additional opacity bump affects the OPAL- and OP-driving zone for stellar pulsations and therefore the new envelope models may markedly change predicted spectra of unstable oscillation modes. Differences between current non-LTE and LTE models based on the TLUSTY200 and ATLAS codes, respectively, cannot be assigned entirely to non-LTE effects, but for B stars they reflect mainly opacity effects.