Line formation in white dwarf atmospheres

Physics

Scientific paper

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Balmer Series, Spectral Line Width, Stark Effect, Stellar Atmospheres, Stellar Spectra, White Dwarf Stars, Thermodynamic Equilibrium, Zeeman Effect

Scientific paper

As a result of high gravity (typically 10 to the 8th cm/sq sec) and low opacities the photospheres of white dwarf stars have very high pressures (higher than 10 to the 11th dyn/sq cm in the coolest objects). Consequently, LTE conditions prevail (except for the cores of some Balmer lines), and the spectral lines are extremely broadened (half-widths up to several hundred Angstroms). For the interpretation of these line profiles, in addition to the usual broadening mechanisms, the following effects have to be taken into account: (1) resonance broadening for the lower members of the Balmer series in cool hydrogen-rich objects; (2) quasi-static van der Waals broadening of some metal lines: and (3) Zeeman splitting for stars with magnetic fields (which may be up to many mega-Gauss). It is shown that the proper inclusion of these effects is necessary for the determination of the abundances of the species causing the line. In some cases this even makes it possible to estimate the concentration of unseen elements (e.g., helium). Some presently unsolved problems are briefly discussed.

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