Atmospheres of the F-type supergiants. II. The line and continuous spectra

Coud spectra of 25 supergiants having spectral types from F2 to G5 and spectrophotometry of four F-type supergiants which are cluster members are used to determine their atmospheric characteristics. Curves of growth obtained from the spectra show that the micro turbulence determined from the Fe ii lines increases with luminosity and is correlated with the oxygen line strengths of Paper I. However, the Fe I microturbulence is insensitive to luminosity at spectral type F5. Both sets of lines have the same radial velocity in Ia and Ib supergiants of this type. Application of standard model-atmosphere techniques to the spectrophotometry produces reasonable effective temperatures, but the gravities of the class Ia supergiants are found to be a factor of 5 less than expected from stellar-interiors theory. The difference was less for the class Ib stars. Clearly, dynamical effects are important. Consideration of the effect of the turbulent motions on the pressure equilibrium indicates that the observed velocities are large enough to reduce the gas pressure and simulate a low gravity. The models also indicate that the observed microturbulence and a decrease in the continuous opacity account for the luminosity effect in the oxygen line. Although non-LTE effects may be present, they are not sensitive to luminosity. The gradient in radiation pressure beneath the photosphere exceeds the surface gravity in F-type supergiants with log g < 2. The resulting instability is the probable cause of mass motions in supergiants. This hypothesis is supported by the observations that the instability first occurs at the gravity where the oxygen line becomes sensitive to luminosity and the observed turbulence begins to increase. Finally, it is shown that a turbulent velocity which increases with height in the atmosphere can account for the different behavior of the Fe I and Fe ii lines at type F5 because the Fe I lines are formed at greater depths in the atmosphere.