Role of nonthermal velocity fields in determining the H-alpha widths in supergiant chromospheres

Abstract

Radiative transfer calculations of the H-alpha line in spherically symmetric, expanding chromospheres explain the large widths of the line observed in late G and K supergiants. The role of various dynamical processes as well as that of opacity in determining the H-alpha width is illustrated. The H-alpha line profile is basically characterized by large optical depths and large Doppler broadening velocity. The calculations show that the existence of large microturbulence has to be invoked in the chromosphere to understand the observed widths.