Spherically symmetric model atmospheres using approximate lambda operators

V. Static inhomogeneous atmospheres of hot dwarf stars

Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
email: kubat@sunstel.asu.cas.cz

Received: 18 October 2020
Accepted: 7 June 2021

Abstract

Context. Clumping is a common property of stellar winds and is being incorporated to a solution of the radiative transfer equation coupled with kinetic equilibrium equations. However, in static hot model atmospheres, clumping and its influence on the temperature and density structures have not been considered and analysed at all to date. This is in spite of the fact that clumping can influence the interpretation of resulting spectra, as many inhomogeneities can appear there; for example, as a result of turbulent motions.

Aims. We aim to investigate the effects of clumping on atmospheric structure for the special case of a static, spherically symmetric atmosphere assuming microclumping and a 1D geometry.

Methods. Static, spherically symmetric, non-LTE (local thermodynamic equilibrium) model atmospheres were calculated using the recent version of our code, which includes optically thin clumping. The matter is assumed to consist of dense clumps and a void interclump medium. Clumping is considered by means of clumping and volume filling factors, assuming all clumps are optically thin. Enhanced opacity and emissivity in clumps are multiplied by a volume filling factor to obtain their mean values. These mean values are used in the radiative transfer equation. Equations of kinetic equilibrium and the thermal balance equation use clump values of densities. Equations of hydrostatic and radiative equilibrium use mean values of densities.

Results. The atmospheric structure was calculated for selected stellar parameters. Moderate differences were found in temperature structure. However, clumping causes enhanced continuum radiation for the Lyman-line spectral region, while radiation in other parts of the spectrum is lower, depending on the adopted model. The atomic level departure coefficients are influenced by clumping as well.