Pre-horizontal branch evolution and the Oosterhoff dichotomy in galactic globular clusters

Details Pre-horizontal branch evolution and the Oosterhoff dichotomy in galactic globular clusters Pre-horizontal branch evolution and the Oosterhoff dichotomy in galactic globular clusters

Jun 1978

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978a%26a....67..107c&link_type=abstract

Astronomy and Astrophysics, vol. 67, no. 1, June 1978, p. 107-113.

Astronomy and Astrophysics

Astrophysics

27

Cepheid Variables, Globular Clusters, Late Stars, Red Giant Stars, Stellar Evolution, Helium, Hertzsprung-Russell Diagram, Histograms, Stellar Luminosity

Scientific paper

The approach of Population II stars to their zero-age horizontal-branch (ZAHB) locations is studied. One finds evidence that after the helium flash, pre-HB evolution runs with gravitational time scale until ZAHB effective temperatures are overtaken. The final approach to the ZAHB location is realized by a decrease of effective temperature with (O-16, p) burning time scale. Under the hypothesis that a hysteresis mechanism is efficient for RR Lyrae pulsators, it is found that pre-HB evolutionary history can control the actual mode of pulsation in ZAHB pulsators. On this basis a discontinuity in the properties of fundamental pulsators is expected, possibly connected with the well-known Oosterhoff dichotomy observed in galactic globular clusters. Observed characteristics in the period-frequency histogram of Oo.II pulsators qualitatively support the previous hypothesis. Quantitative evolutionary estimates agree with this picture, suggesting a value of about 0.22-0.25 for the original helium content in Oo.II-type clusters.

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