Atomic line shapes in stellar spectra

Abstract

The spectrum emitted by a star is determined by the conditions where the photons last scatter in its atmosphere . It carries a signature of the presence of atomic species, and of the effects of atomic collisions and motions on the radiative process. To the astronomer interested in the star, these spectra offer clues to its composition, temperature, surface gravity, magnetic field, rotation and line‐of‐sight motion. To the atomic physicist, stellar spectra are a probe for collision processes under conditions that are difficult to reproduce in a laboratory. Indeed, an understanding of the underpinning line shape physics is key to exploring a wide range of interesting astronomical phenomena—answering questions about the color and composition of brown dwarfs, the temperature, size and age of white dwarfs, and the detection of extrasolar planets and their atmospheres.The theory of spectral line shapes, especially the unified approach we have developed, makes possible accurate models of stellar spectra that account both for the centers of spectral lines and their extreme wings in one consistent treatment. Under some circumstances it is possible to test the line shape theories in the laboratory, if not under conditions exactly the same as those in stars, at least under closely similar conditions. For application to cool brown dwarf stars, for example, with an atmosphere of molecular hydrogen and the alkali metals, conventional laboratory absorption spectroscopy can be used to examine the line wing, to measure the broadening of the line center, and to determine shifts of lines due to collisions. At the other extreme of temperature, a shock wave in a laser‐produced plasma produces for a few nanoseconds a dense partially ionized atomic hydrogen source at the temperature of a white dwarf star. A comparison of laboratory experimental data with theoretical profiles establishes the accuracy of the interaction potentials, which remain difficult to compute a priori precisely in most cases. In this paper, we will show work now in progress that compares unified line shape calculations with new experiments to determine the wings of the sodium and potassium resonance lines broadened by molecular hydrogen and helium, and that looks at the role of the ion collisions in determining the widths and shift of Balmer α when the electron/proton density is much higher than found in the usual laboratory plasma arc sources.