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Semiconvection and the RR Lyrae Variables

Published online by Cambridge University Press:  12 April 2016

A. V. Sweigart  and
P. Demarque
A. V. Sweigart
Affiliation:
Yale University Observatory, U.S.A.
P. Demarque
Affiliation:
Yale University Observatory, U.S.A.

Extract

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Theoretical computations (Hoyle and Schwarzschild, 1955; Faulkner, 1966; Iben and Rood, 1970; Demarque and Mengel, 1971a, b) have identified the horizontal-branch stars in globular clusters with the evolution phase in which helium burns within a convective core and hydrogen burns in a shell outside the convective core. Most computations for such double-energy-source models have indicated that the evolution proceeds smoothly on a nuclear time scale during the horizontal-branch phase, leading to small predicted rates of change in the RR Lyrae pulsation period (Iben and Rood, 1970). Sweigart and Demarque (1972) have recently considered the effects of semiconvection on the horizontal-branch evolution of typical Population II stars and have suggested that changes in the composition distribution within the core may occur on a time scale considerably shorter than the nuclear time scale during the phase immediately preceding core-helium exhaustion. It has been found that the composition distribution generated by the growth of a semiconvective zone in the layers surrounding the convective core can become unstable when Yc, the helium abundance within the convective core, decreases below roughly 0.12. The changes in the internal structure caused by this instability result in relatively rapid movement of the models in the HR diagram and consequently produce large predicted rates of change in the RR Lyrae pulsation period. The possibility that RR Lyrae period changes may be associated with the behavior of the semiconvective zone has been previously suggested by Schwarzschild (1970). A similar instability may occur in the late core-hydrogen burning phase for stars around 10 M⊙. Percy (1970) has noted the coincidence of β Cephei stars with stellar models containing semiconvective zones. It is tempting to suggest that such an instability in the semiconvective zone could also be related to the β Cephei phenomenon.

Type
Part IV / Theoretical Considerations of Population II Variables
Information
International Astronomical Union Colloquium , Volume 21: Variable Stars in Globular Clusters and in Related Systems , August 1973 , pp. 221 - 228
Copyright
Copyright © Reidel 1973

References

Castellani, V., Giannone, P., and Renzini, A.: 1971a, Astrophys. Space Sci. 10, 340.CrossRefGoogle Scholar
Castellani, V., Giannone, P., and Renzini, A.: 1971b, Astrophys. Space Sci. 10, 355.CrossRefGoogle Scholar
Cox, A. and Stewart, J.: 1970, Astrophys. J. Suppl. 19, 261.CrossRefGoogle Scholar
Demarque, P. and Mengel, J. G.: 1971a, Astrophys. J. 164, 317.CrossRefGoogle Scholar
Demarque, P. and Mengel, J. G.: 1971b, Astrophys. J. 164, 469.CrossRefGoogle Scholar
Demarque, P. and Mengel, J. G.: 1972, Astrophys. J. 171, 583.Google Scholar
Faulkner, J.: 1966, Astrophys. J. 144, 978.CrossRefGoogle Scholar
Hoyle, F. and Schwarzschild, M.: 1955, Astrophys. J. Suppl. 2, 1.Google Scholar
Iben, I. Jr. and Huchra, J.: 1971, Astron. Astrophys. 14, 293.Google Scholar
Iben, I. Jr. and Rood, R. T.: 1970, Astrophys. J. 161, 587.CrossRefGoogle Scholar
Lauterborn, D., Refsdal, S., and Stabell, R.: 1972, Astron. Astrophys. 17, 113.Google Scholar
Paczyński, B.: 1970, Acta Astron. 20, 195.Google Scholar
Percy, J. R.: 1970, Astrophys. J. 159, 177.CrossRefGoogle Scholar
Robertson, J. W. and Faulkner, D. J.: 1972, Astrophys. J. 171, 309.CrossRefGoogle Scholar
Schwarzschild, M.: 1958, Structure and Evolution of the Stars, Princeton University Press, Princeton.CrossRefGoogle Scholar
Schwarzschild, M.: 1970, Quart. J. Roy. Astron. Soc. 11, 12.Google Scholar
Schwarzschild, M. and Härm, R.: 1969, Bull. Am. Astron. Soc. 1, 99.Google Scholar
Sweigart, A. V. and Demarque, P.: 1972, Astron. Astrophys., 20, 445.Google Scholar
Wesselink, A. J.: 1973, this volume, p. 104.Google Scholar