An Atlas of Theoretical Constraints for Horizontal Branch Stars

Details An Atlas of Theoretical Constraints for Horizontal Branch Stars An Atlas of Theoretical Constraints for Horizontal Branch Stars

Sep 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...276...41c&link_type=abstract

Astronomy and Astrophysics, Vol.276, NO. 1/SEPI, P. 41, 1993

Astronomy and Astrophysics

Astrophysics

27

Stars: Evolution, Stars: Horizontal Branch, Stars: Population Ii, Rr Lyrae Variables, Globular Clusters: General, Galaxy: Halo

Scientific paper

Synthetic horizontal branches (SHB) are constructed from a grid of up-to-date canonical ([α/Fe] =9) horizontal branch (HB) evolutionary tracks with an original helium content Y(MS) =0.23 and metallicity Z in the range 0.0001-0.02. The relevant constraints on the properties of HB and RR Lyrae stars are presented and discussed.
Present results confirm that the mean luminosity logL(RR) and light-to-mass ratio A(RR) of RR Lyraes depend on the metallicity as well as HB morphology. For red to moderately blue HB types, i.e. for RR Lyrae-rich synthetic models, we find log L(RR) = 1.420-0.086 log Z and A (RR) = 1.747-0.026 logZ, for logZ < -2.5, while for logZ > -2.5 we get logL(RR) = 1.133-0.198 logZ and A(RR) = 1.397-0.161 log Z. The comparison with observations of field and Galactic globular cluster RR Lyrae variables shows that all the observed light-to-mass ratios can be fitted with a constant original helium Y(MS) = 0.23.
Present HB models and recent isochrone computations at Y(MS) = 0.23 are used to calibrate in terms of age (tin Gyr) and metal content the difference in bolometric magnitude ΔMbol (TO - HB) between the main- sequence turnoff and the zero-age horizontal branch at the RR Lyrae gap. On this ground, the Buonanno et al. observations of a sample of Galactic globular clusters are analyzed yielding the mean relationships = 13.92-1.34 [Fe/H] at [α/Fe] = 0, and = 13.69-1.26 [Fe/H] at [α/Fe] = 0.48. Concerning the age spread which is present among globular clusters of the same metallicity, present estimates could confirm a relation between age and HB morphology, with the blue HB clusters older by ˜2 Gyr than red HB clusters, provided that no systematic error occurs in the definition of the observed zero-age horizontal branch level.

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