The Effects of Rotation on the Atmospheres of Early-Type Main-Sequence Stars (Review Paper)

In discussing the status of the theory of rotating stellar atmospheres, it is necessary to draw upon the contributions of many well established aspects of astrophysics and to interconnect them in a cohesive pattern structured so as to provide insight into a rather specific problem – namely, the structure and characteristics of a surface of the star undergoing axial rotation. Many different connections are possible having varying degrees of emphasis and, of necessity, those given here represent only one such presentation. The discussion could be much simplified if it were not necessary to test the efficacy of the theoretical development by referring to observations. Unfortunately, such a comparison is necessary and the results are at the moment somewhat inconclusive. This unhappy situation arises from the retrospectively obvious fact that axial rotation does not play a dominant role in determining the directly observable properties of stars. Indeed, if rotation were a dominant factor, earlier attempts at describing stellar structure and evolution would have met with little success. However, it is becoming increasingly clear that the structure and final evolution of highly evolved stars are greatly influenced by the total angular momentum which they retain from their earlier history. In order to understand the angular momentum distribution present in the final state, it is necessary to understand the effects of stellar evolution on the total angular momentum and its distribution. But even before this step can be taken, one must first successfully describe the rotational structure of the main-sequence phase as it is this state which provides the initial conditions necessary for any further study. It is further appropriate that we attempt to describe this period of a star’s life as the largest body of observational material with which we must test our results exists for these stars. In addition, we should expect a study of the atmospheric structure to be the most fruitful as this is the region of the star which provides the final modification of the radiation we observe.