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Current Problems in Horizontal-Branch Theory: Some Implications for RR Lyrae Variables

Published online by Cambridge University Press:  12 April 2016

Pierre Demarque
Pierre Demarque*
Affiliation:
Yale University Observatory, New Haven, CT 06511 USA

Extract

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There are a number of good reasons for wanting to improve our understanding of the structure and evolutionary status of horizontal-branch (HB) stars. To mention a few:

  1. 1) The HB morphology of globular clusters can be used as a tool for studying the evolution of the galactic halo (Searle and Zinn 1978) and the time scale for halo collapse (Demarque 1980; Zinn 1980).

  2. 2) HB stars offer an opportunity to evaluate the helium abundance of globular clusters. This can be done either by using the R-method (Cole et al. 1983) or the width of the RR Lyrae instability strip [Deupree 1977; but see also the paper by Stellingwerf (1984) in these preceedings which casts doubt on the validity of previous calculations of the dependence of the blue edge on helium content].

  3. 3) HB stars may play an important role in understanding the integrated light of old stellar systems. In particular, blue HB stars seem to make a significant contribution to the ultraviolet light of elliptical galaxies. (Ciardullo and Demarque 1978; Gunn, Tinsley and Stryker 1981). At the same time, HB stars are believed to be the direct progenitors of asymptotic giant branch stars which have proved to be powerful tracers of stellar populations in nearby external systems (Blanco et al. 1980)

  4. 4) The RR Lyrae variables are part of the HB population. One would wish to relate the observable properties of RR Lyrae variables to their chemical compositions and ages so as to use them: a) as distance indicators for galactic globular clusters (Sandage 1982a,b) and in the Magellanic Clouds; b) as tracers of stellar populations. This has been attempted for the galactic halo and disk by Sandage (1982a,b) and for the Magellanic Clouds by Butler et al. (1982).

Type
Part IV. Population II Cepheids
Information
International Astronomical Union Colloquium , Volume 82: Cepheids: Theory and Observations , 1985 , pp. 268 - 271
Copyright
Copyright © Cambridge University Press 1985

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