Sensitivity of the Hα line to expansion, geometrical extent and temperature gradient in the G and K supergiant chromospheres

Abstract

We report on preliminary results of new radiative transfer calculations of Hα in differentially expanding nonisothermal spherical chromospheres, including explicitly the effects of ionization.

The Hα line in late G and K supergiants is a very strong absorption feature with a blue displaced core. Small emission components are often seen on either or both sides of the absorption profile. In an earlier paper (Mallik 1986), detailed Hα transfer calculations were reported for an isothermal chromosphere and line profiles were computed using a comoving frame transfer code developed by Peraiah (1980, 1981). Because Hα is mainly Doppler broadened, its width is dependent on the temperature structure of the atmosphere. In addition the n = 2 level population is controlled by the temperature-density structure of the atmosphere through excitation and ionization. Although the earlier work was based on a non-LTE approach for the transfer problem, the hydrogen ionization was not accounted for and the n = 2 level population was directly related to the total hydrogen density via the Boltzmann factor.