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...ght within the photosphere. Thus, the photosphere can be well represented by a plane-parallel model atmosphere. Many model atmospheres are available, with those by Holweger and Muller [1] and Kurucz =-=[2]-=- being commonly used. A model atmosphere tabulates temperature, pressure, electron pressure, density, and opacity at 5000 A. With this data, and assuming LTE conditions, then the source function is s...

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...rongly with height within the photosphere. Thus, the photosphere can be well represented by a plane-parallel model atmosphere. Many model atmospheres are available, with those by Holweger and Muller =-=[1]-=- and Kurucz [2] being commonly used. A model atmosphere tabulates temperature, pressure, electron pressure, density, and opacity at 5000 A. With this data, and assuming LTE conditions, then the sourc...

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...Damping due to electrons is 50 times smaller, or even less, and damping due to helium about 30 times smaller. Other sources can be neglected. The line broadening theory developed by Anstee and O'Mara =-=[8]-=- has been shown to be accurate. Collisions with hydrogen atoms in the photosphere are fast; the impact approximation can be used. The line width (HWHM) is coll = N Z 1 0 vf(v)(v)dv (6) where is t...

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...otion 6. Numerical integration of radiative transfer equation (1) 7. Non-plane parallel corrections { large scale mass motion Figure 1. Recipe for spectral synthesis gII and gIII are given by Mihalas =-=[5]-=-. Other sources of opacity which can be considered include the H+2 molecule, for which crosssections have been calculated; a convenient tabulation for interpolation is available in [4]. The abundances...

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...th (HWHM) is coll = N Z 1 0 vf(v)(v)dv (6) where is the line broadening cross section, and f(v) is the normalised Maxwellian velocity distribution. Data calculated using this theory is available =-=[9]-=-, and code to calculate broadening has been developed [10] (available at http://www. astro.uu.se/~barklem) which can be used either for stand-alone calculation, or can be incorporated into a spectral ...

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...the line broadening cross section, and f(v) is the normalised Maxwellian velocity distribution. Data calculated using this theory is available [9], and code to calculate broadening has been developed =-=[10]-=- (available at http://www. astro.uu.se/~barklem) which can be used either for stand-alone calculation, or can be incorporated into a spectral synthesis program. This code calculates the linewidth per ...

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...consider other abundant absorbers. If we are only considering a small spectral region, we can assume that the continuous opacity is uniform. In general, we use approximation formulae and table lookup =-=[3]-=-. The H opacity is due to both bound-free (H + photon $ H + e) and free-free (H + e + photon $ H + e) processes. Approximation formulae for both processes are given by Gingerich [4] Due to its hi...

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...er elements much lower than that of hydrogen, and they contribute far less to the opacity. Only the most abundant species need be considered, Mg and Si being the most important. Calculations by Peach =-=[6]-=- can be used for interpolation. 4. Line opacities The opacity due to a spectral line depends strongly on the wavelength { which is why it's a line { which makes it necessary to calculate the line opac...

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...ler shift due to the most probable speed. We can expect a spectral synthesis code to spend a major part of the execution time repeatedly calculating the Voigt function. Various 3methods are available =-=[7]-=-. Fast methods usually use asymptotic methods where possible (since the Voigt prole approaches the Lorentz and Maxwell proles in the limit), and other approximate (eg power series) methods elsewhere...