Angular Momentum Transport by Internal Gravity Waves II- Pop II stars from the Li plateau to the horizontal branch

Abstract

This paper is the second in a series where we examine the generation and filtering of internal gravity waves in stars and the consequences of wave induced transport of angular momentum at various stages of the stellar evolution. Here we concentrate on Pop II dwarf stars and we focus in particular on the differential properties of internal gravity waves as a function of the stellar mass. We show that, for the range of masses corresponding to the lithium plateau, gravity waves are fully efficient and should thus lead to a quasi-solid rotation state, similar to that of the Sun. In the slightly more massive progenitors of currently observed horizontal branch star however, internal wave generation is not efficient on the main sequence, and large internal differential rotation can thus be maintained. This leads to a natural explanation of the large rotational velocities measured on the horizontal branch in some globular clusters.