The maximum effect of convection in stellar atmospheres on the observed properties of stellar spectra

Abstract

In spite of many attempts to solve the problem of convection in stellar atmospheres our present knowledge of its effects on the structure of the outer layers of stars is still inadequate. The observations of solar granulation and theoretical investigations show clearly that there is an unstable zone in stellar atmospheres, in which the transfer of energy is partly due to radiation and partly to convection and in which therefore the actual temperature gradient has a value intermediate between the two limiting values of the radiative and adiabatic gradients, but we have no sure basis on which to investigate theoretically the influence of convection on the temperature distribution in the convectively unstable zone. Since however the energy distribution in the stellar spectrum is a function of the temperature distribution in the outer layers of stellar atmospheres, it can be hoped that the analysis of stellar spectrum may lead to an approximate evaluation of the influence of convection on the structure of the convective zone, if there are any properties of stellar spectra, which are sensibly affected by convection and which can be easily measured. The present investigation shows that the stellar colour-gradient and the equivalent widths of the calcium lines satisfy both the above conditions and therefore may be used for that purpose. The present investigations consist in the computation of these properties for a number of pairs of model stellar atmospheres, the models of every pair having the same surface temperature and the same value of surface gravity, but differing by disregarding convection in one model altogether, and by taking it at its highest amount, i.e the adiabatic value, in the convective zone in the other; the colour gradients of real stellar atmospheres must then lie between the two values found in the way. Such pairs of model stellar atmospheres were computed ten years ago by B. Stromgron and his collaborators, but they soon became obsolete, when the investigation of S. Chandrasekhar and F.H. Breen showed that the old value of absorption coefficient for negative hydrogen ion used by B. Stromgren was only a rough approximation. Since they seemed to be inadequate for the study of the effect of convection in stellar atmospheres on the observable properties of stellar spectra, a new series of models based on recent values of absorption coefficients was computed.