Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982apj...252..553s&link_type=abstract
Astrophysical Journal, Part 1, vol. 252, Jan. 15, 1982, p. 553-581.
Astronomy and Astrophysics
Astrophysics
455
Abundance, Globular Clusters, Metallic Stars, Stellar Luminosity, Stellar Radiation, Variable Stars, Blue Stars, Helium, Hubble Constant, Light Curve, Main Sequence Stars, Red Shift
Scientific paper
The Oosterhoff period shifts, determined from the (light curve shape)-period relation in 30 globular clusters, are found to form a continuum rather than to fit into two nearly distinct period bins. The shifts are correlated with metallicity; longer periods occur in clusters with lower metal abundance. Results from Papers I and II require the RR Lyrae absolute magnitudes to be brighter in clusters where the variables have longer periods. A composite color-magnitude diagram, made from eight clusters whose luminosity levels are normalized according to ΔMRRbol=3Δ log P, shows that the main sequence turnoff luminosity varies with metallicity as ∂MTO/∂[Fe/H]=0.29. Lower metallicity clusters have brighter turnoffs. This observed gradient agrees with the models first calculated by Simoda and Iben as the requirement that clusters of different metallicity have the same age. Analysis of the cluster data in the metallicity range 0 ≥ [Fe/H] ≥ -2.3 using the Simoda and Iben condition with the Yale models shows that the ages of all clusters studied here are the same to within the accuracy of the method, which is about ±10%. An independent demonstration that the clusters are coeval to within this limit uses the magnitude difference between the horizontal branch and main sequence turnoff levels. The data show that the metal-rich clusters 47 Tue and NGC 6838 cannot be younger than the metal-poor clusters, again by more than Δt/t ≍ ±10%. The condition for nearly equal age requires that the Oosterhoff period shifts are correlated with metallicity in the way that is observed. It can also be understood why the individual period-amplitude relations for variables in different clusters are ordered by metallicity, provided that amplitude is a unique function of position in the RR Lyrae instability strip. The absolute age of the globular cluster system is t = (17±2) × l09 years if the absolute magnitude of RR Lyrae variables in M3 is MV=+0.80, justified from external data. This age agrees with a global value of the Hubble constant near H0=50 km s-1 Mpc-1 when we add to the age of the clusters the formation time of galaxies from the "beginning," determined from the observed upper redshift limit of Δλ/λ0 ≍ +4 for quasars. The data are, however, in clear violation with the larger values of the Hubble constant occasionally quoted.
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