Nonradial Oscillations: The Cause of Macroturbulence in Late-Type Stars?

A fundamental problem in contemporary stellar atmospheres research concerns the cause of what the spectroscoplst calls “macroturbulence.” Even in so well studied a star as the Sun, it is unclear as to which of the many resolved velocity fields is most responsible for the broadening of the disk-integrated spectrum. There are several uncertainties attached to the identification of this primary velocity field. To cite one, Beckers (1980) indicates in a recent review that the two principal contributors to macroturbulence, convective granulation and the five-minute nonradial oscilltion pattern, each add only an r.m. s. velocity of 1/2 km s at τ5000 = 0.1. According to him, even when they are put together with related unresolved patterns [e.g. subgranulation and short period (<30) oscilltions] the sum of all known velocities seems to fall short of macroturbulence obtained from line broadening studies [~ 3 km s-1; radial-tangential model (Gray 1977, Smith 1978)]. The most recent models of the solar granulation field (Keil 1980) suggest somewhat higher velocities, e.g. 1.1 km s-1 at τ5000 = 0.1, when revised corrections for terrestrial seeing are taken into account. Nonetheless, such corrections must be added to both the convection and oscillation amplitudes, so it is still not clear whether one of these fields dominates the line formation.