Abstract:
CCD echelle spectra of the H-alpha line at 6563 A have been obtained for a sample of 30 G and K supergiants with a spectral resolution of about 0.24 A. The data clearly indicate that the observed H-alpha profile is a deep absorption, implying large optical depths in the chromosphere. Also, the line cores are blueshifted in all the stars which suggests the chromospheres are expanding outward. Detailed radiative transfer calculations of H-alpha have been carried out in a spherically symmetric atmosphere with outward-positive velocity and temperature gradients, including explicitly the effects of ionization. The H-alpha line profiles have been computed for a wide range of parameters in order to reproduce the observed features of the line. Within the framework of our model, the calculations reinforce the idea that the nonthermal velocities have to be as large as 25-30 km/s to explain the large widths of the observed profiles. The rates of mass outflow have been calculated to lie in the range 10 exp -8 to 10 exp -9 solar mass/yr.
