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The Luminosities of Horizontal Branches and RR Lyrae Stars in Globular Clusters

Published online by Cambridge University Press:  12 April 2016

D.H. McNamara ,
M.B. Rose ,
P.J. Brown ,
D.I. Ketcheson ,
J.E. Maxwell ,
K.M. Smith  and
R.C. Wooley
D.H. McNamara
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
M.B. Rose
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
P.J. Brown
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
D.I. Ketcheson
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
J.E. Maxwell
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
K.M. Smith
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
R.C. Wooley
Affiliation:
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA

Abstract

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We have utilized the latest stellar models of the Y2 (Yonsei-Yale) collaborators and color-magnitude diagrams of globular clusters to infer ages and absolute magnitudes of their horizontal branches (HB). The intrinsic (B – V) color indices of the turn-offs, of the globular clusters were used to find ages. For 47 clusters that appear to be coeval (within ±0.7 Gyr), we find an average age of 12.5 Gyr. We adopt this age and infer the absolute magnitudes of the turn-offs, from the clusters [Fe/H] values. The absolute magnitude of the horizontal branches or RR Lyrae stars are then determined from the difference between the apparent magnitudes of the horizontal branches (or RR Lyrae stars) and the apparent magnitude of the turn-offs, VTO. We conclude: 1) The slope of the MV(HB), [Fe/H] relation is ~0.3 for clusters with [Fe/H] values between —0.5 to —1.5. The relation has zero slope for [Fe/H] values smaller than −1.5. 2) For [Fe/H] < -1.3, the MV(HB) or MV values of RRLyrae stars are not only a function of [Fe/H], but the horizontal-branch type in the sense that the clusters with the blue horizontal branches have more luminous horizontal branches than clusters with red horizontal branches. The same results are found by inferring the luminosities of the HBs from pulsating blue stragglers.

Type
Part 9. Evolutionary and pulsation properties of variable stars
Information
International Astronomical Union Colloquium , Volume 193: Variable Stars in the Local Group , 2004 , pp. 525 - 529
Copyright
Copyright © Astronomical Society of the Pacific 2004

References

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