Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....96..207c&link_type=abstract
Astronomy and Astrophysics, vol. 96, no. 1-2, Mar. 1981, p. 207-214.
Astronomy and Astrophysics
Astrophysics
22
B Stars, Globular Clusters, Lyra Constellation, Stellar Evolution, Variable Stars, Abundance, Dichotomies, Helium, Metallic Stars, Pulsed Radiation, Stellar Luminosity, Stellar Mass Ejection, Stellar Radiation
Scientific paper
Synthetic Horizontal Branches have been generated under wide assumptions on the original helium (Y) and metal content (Z). Theoretical period-frequency histograms of RR Lyrae pulsators in metal poor (Z ∼ 10-4) clusters have been compared with the three RR-Lyrae rich Oosterhoff II clusters M 15, M 53, and M 62. To reach agreement between observed and computed quantities we were compelled to increase the evolutionary mass size of the He-core at the beginning of HB evolution by ∼6%. Under this assumption properties of Oo II type clusters can be fitted, giving Y ∼ 0.22. The same value of Y is obtained, within an indeterminacy ΔY ∼ 0.02, for Oo I type clusters when the evolutionary He-cores are increased by the same amount in the case Z = 10-3. In both cases, all the main pulsational parameters are well reproduced by computations. On this basis we simulated RR Lyrae-rich globular clusters by holding fixed =0.22) and by varying Z between the above reported limits. Taking into account the "hysteresis mechanism" and the "blue nose" effect, we find that a dichotomy in the mean value of ab-type pulsators occurs when Z ∼ 2 10-4, as observed in our Galaxy.
As a conclusion, we find that theory can be matched with observations if actual HBs are populated by stars evolving along typical low-He (Y ∼ 0.20) evolutionary tracks but overluminous with respect to presently available theoretical prediction, the increase in Mc being a possible way to increase star luminosities.
If one assumes, among the various possibilities, that MS rotation is causing the quoted HB morphology, we derive that galactic globular clusters are fitted by the values Y ∼ 0.22, t ∼12 109 yr, ω0 = 3.6 10-4, provided that the efficiency of mass loss is supposed to sensitively increase in increasing the metal content Z.
Castellani Vittorio
Tornambe' Amedeo
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