A study of the H-alpha line in late G and K supergiants

Abstract

H-alpha is investigated spectroscopically in order to gain information on the properties of expanding chromospheres of late G and K supergiants. Spectra of 23 stars brighter than V equals 6.0 are obtained at dispersions of 4-10 A/mm using the coude and the coude-echelle spectrographs at the 102-cm telescope of Kavalur Observatory. H-alpha profiles are computed theoretically using radiative transfer in spherically symmetric expanding atmospheres covering a large range of integrated optical depths. These are then compared with the characteristics of the observed line in the program stars. The analysis reveals that the H-alpha line is formed in a region with velocity increasing outward. The computer equivalent widths and line core displacements are matched with those observed to obtain hydrogen column densities and expansion velocities.