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Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........7f&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL, 1995.Source: Dissertation Abstracts International, Volume: 57
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1
Scientific paper
This work addresses the problem of the age and formation history of the inner regions of the Milky Way. I have undertaken a photometric and spectroscopic study of a small sample of globular clusters within 5 kpc of the Galactic center: NGC 6723, NGC 6352 and NGC 5927. The ages and chemical compositions of these clusters provide clues to the formation and chemical enrichment timescales of the Galactic (thick) disk and inner halo. Deep color -magnitude diagrams (CMDs) are constructed for the clusters using data obtained both from the ground and from the Hubble Space Telescope. A flux-conserving deconvolution procedure is described, which was developed to allow accurate photometry of pre-servicing aberrated HST images. Cluster ages are inferred from the luminosities of the main-sequence turnoffs (MSTOs) in the CMDs, once isochrones are chosen for the correct elemental mixes. NGC 6723 was originally chosen for study because its estimated metallicity ((Fe/H) = -1.09, Zinn & West 1984) and RR Lyrae richness index (W RR = 0.330, Carney, Storm, & Jones 1992), made it a good "template" cluster for the Baade's Window RR Lyraes. Here, the chemical compositions of three red giants in NGC 6723 are determined from high resolution echelle spectra. This analysis shows that NGC 6723 is more metal-poor than previously thought ((Fe/H) = -1.3), in keeping with its extremely blue horizontal branch morphology. Neither anomalously old nor anomalously young, NGC 6723 is comparable in age to other (old halo) globular clusters with similar metallicities. The ages presented here for the metal-rich (thick) disk globular clusters NGC 5927 and NGC 6352 increase by half again the number of disk globulars for which accurate ages are known. NGC 6352 is found to be comparable in age to 47 Tuc, implying that the thick disk had begun to form not too long after the halo. NGC 5927 is 2-3 Gyr younger than the other disk globulars, the rest of which appear to be coeval. The halo clusters may exhibit a trend of age with abundance such that the most metal-poor clusters are on average older than those more metal-rich. The young age derived for NGC 5927 implies that the disk clusters may follow this same general trend. The age of the bulge is estimated consistently with the ages of the globulars from the magnitude of the Baade's Window MSTO (Holtzman et al. 1993). That "mean" bulge age thus derived is ~4 Gyr younger than the age of the majority of the disk globulars. The bulge appears to lie on the extension of the age-metallicity relation defined by the halo globulars.
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