The atmospheres of the F supergiants

Citation

Osmer, Patrick Stewart (1970) The atmospheres of the F supergiants. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8N4Y-K187. https://resolver.caltech.edu/CaltechETD:etd-06182008-131615

Abstract

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Coude spectra of 26 supergiants of spectral types F2-G5 were obtained, and spectrophotometric observations of four which occurred in clusters were made. A photoelectric system for measuring the 0 I 7774 line was established, and the line strength was determined in 59 A0-G3 stars of luminosity classes Ia-V, By calibrating the line strength in 10 F supergiants of known luminosity we find that it indicates the absolute magnitude of F stars brighter than [...] = -4 with an accuracy of [...]. The curves of growth obtained from the spectra show that the microturbulence determined from the Fe II lines increases with luminosity and is correlated with the oxygen line strength. However, the Fe I microturbulence exhibits a different behavior near spectral type F5, where it is insensitive to luminosity.

The applicability of model atmospheres to the F supergiants was checked by using the spectrophotometric observations. The derived temperatures were reasonable, but the gravities of the Ia stars were a factor of 5 less than expected from stellar interiors calculations. The difference was less for the Ib stars. It was shown that pulsational effects did not account for the discrepancy. Consideration of how the turbulent motions could affect the pressure equilibrium in the atmosphere indicated that the observed velocities were large enough to reduce the gas pressure and cause the observed low gravities. The models also indicated that a decrease in the continuous absorption combined with the observed microturbulence accounted for the increase in the oxygen line strength with luminosity. Although non-LTE effects may be present in the oxygen line, they do not change significantly with luminosity and do not contribute to the observed increase in strength.

The gradient in radiation pressure beneath the photosphere in F stars with log g <2 is large enough to cause an inversion in the gas pressure. The zone where this occurs is unstable and could be the cause of mass motions in supergiants. This hypothesis is supported by the fact that the zone appears at the gravity where the oxygen lines become sensitive to luminosity and the turbulence begins to increase. Simple calculations show how the existence of the zone depends on gravity and temperature.

Finally, we demonstrated that a turbulent velocity which increases with height in the atmosphere explains the difference in the behavior of the Fe I and II lines at F5 as well as their similarities at other spectral types. At F5Ib the lines are formed at the same depth, but in the F5Ia stars the increased ionization causes the Fe II lines to be formed at greater heights in the atmosphere than the Fe I lines.

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