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

D.H. McNamara, M.B. Rose, P.J. Brown, D.I. Ketcheson, J.E. Maxwell, K.M. Smith, R.C. Wooley
2004 International Astronomical Union Colloquium  
AbstractWe have utilized the latest stellar models of theY2(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
more » ... ] 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, theMV(HB)orMVvalues 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.
doi:10.1017/s025292110001126x fatcat:oiemk2dpvzggbejtoem2tmlrau