Evidence for the Hydrodynamic Character of Microturbulence

Abstract

Worrall and Wilson¹ have questioned the validity of determinations of the chemical composition of stellar atmospheres from observations of absorption lines in stellar spectra. Their main argument is directed against the concept of “microturbulence”, which has been used for several decades to explain the high level of the flat part of the curve of growth. It was first introduced by Struve and Elvey² to account for the apparent systematic variation of the abundances with the total amount of absorption (equivalent width) of the spectral lines from which they are derived. Microturbulence is thought of as a statistical velocity field in the stellar atmosphere, with moving gas elements whose optical depths for the photons to be absorbed are smaller than unity. This small scale velocity field widens the frequency interval for the absorption of photons; this has no influence on the strength of the weak lines, but the equivalent width of saturated lines is increased. In the curve of growth, which represents the relation between the effective number of absorbing particles in the stellar atmosphere and the equivalent width of the absorption line, the flat part of the saturated lines is lifted. Thermal motion of the absorbing atoms frequently fails to account for the level of the flat part of the curve of growth; according to Worrall and Wilson this is the only independent evidence for the existence of small scale turbulent motion in stellar atmospheres. Their statements on this point are strong: “ … There are no independent observations even suggesting that such small-scale, non-thermal motion exists, much less defining its amplitude. . . . ”. Worrall and Wilson suggest that the level of the flat part of the curve of growth is determined by non-local thermodynamic equilibrium (LTE) effects. I show here that other observations, supporting the concept of microturbulence, do exist.