CCD OBSERVATIONS OF THE H-ALPHA LINE IN LATE G AND K SUPERGIANTS AND THEIR INTERPRETATION

CCD echelle spectra of the Halpha line at 6563 angstrom have been obtained for a sample of 30 G and K supergiants with a spectral resolution of about 0.24 angstrom. The data clearly indicate that the observed Ha 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 Halpha have been carried out in a spherically symmetric atmosphere with outward-positive velocity and temperature gradients, including explicitly the effects of ionization. The Halpha line profiles have been computed for a wide range of parameters in order to reproduce the observed features of the line. In particular, the role of various dynamical processes has been demonstrated in conjunction with opacity in determining the Halpha widths. Reasonably good theoretical fits with the observed profiles yield Halpha optical depths in the range 3-20 and outward linear velocity gradients on the order 20 km s-1. 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-1 to explain the large widths of the observed profiles. The rates of mass outflow have been calculated to lie in the range 10(-8) to 10(-9) M. yr-1.