Microturbulence in A-Type Supergiants

A survey has been undertaken of microturbulence in high-luminosity stars. In this paper, details are given of an analysis of the small-scale velocity fields in ten A-type Ia supergiants. The results suggest that (1) in the A supergiants, the microturbulence initially decreases with increasing height in the atmosphere, reaches a minimum, and then increases again at small optical depths, and (2) time-dependent phenomena are important in the atmospheres of these stars. In addition, the combination of the present observations with published data indicates that in stars of luminosity class Ia, the micwturbulence increases from middle B through late A, reaches its maximum somewhere in the spectral range F2-F5, and then decreases suddenly at about spectral type GO-G2. The behavior appears to be similar in the range A-M in the less luminous supergiants and bright giants.