1996PASP..108..545F

In an effort to bring more data to bear on the problem of the age and formation history of the inner regions of the Milky Way, I undertook 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 timescale of the Galactic (thick) disk and inner halo.

Deep color-magnitude diagrams (CMDs) were constructed for the clusters from data obtained both from the ground and from the Hubble Space Telescope (HST). A total of 104 UBV frames of two fields in NGC∼6723 were obtained during three separate runs at CTIO in 1989 and 1993. Twenty-eight images of NGC∼5927 in filters F555W and F814W (similar to Johnson VI) were obtained during Cycle 4 with HST. All photometry of these images was done using DAOPHOT II and ALLSTAR (Stetson 1987, PASP, 99, 191 and updates) following standard reduction procedures. A self-consistent photometric reduction of all 110 NGC 6723 data frames was performed using ALLFRAME (Stetson 1994, PASP, 106, 260). NGC 6352 was observed during Cycle∼2 with HST. Four exposures in each of two filters (F555W and F785LP) were obtained. Because these images suffered from the effects of spherical aberration in the telescope's primary mirror, a new flux-conserving deconvolution procedure was developed to allow accurate photometry for this data set.

Ages were inferred from the magnitude differences between the clusters' horizontal branches (HBs) and main-sequence turnoffs (MSTOs) in the CMDs. A comparison between the age thus derived for NGC 6723 and that derived using an independent age estimator, the color difference between the MSTO and the base of the subgiant branch, indicated that the two methods produced similar relative ages for NGC∼6723 with respect to other clusters.

Three red giants in NGC∼6723 were observed with the CTIO 4m echelle spectrograph in 1994. Usable wavelength coverage was from 5000-7800A. The spectra were extracted in the normal manner using standard IRAF routines. Using photometrically derived stellar parameters and preliminary metallicity estimates for the cluster, model atmospheres were computed using ATLAS9 and abundances were derived using WIDTH9 (Kurucz 1993, Model Atmospheres CD-ROM). Mean derived [Fe/H] and [alpha/Fe] ratios from all three stars were [Fe/H] = -1.26 ± 0.09∼dex, [Si/Fe] = +0.68 ± 0.13∼dex, [Ca/Fe] = +0.33 ± 0.13 dex, and [Ti/Fe] = +0.24 ± 0.15∼dex.

The inner halo cluster NGC 6723 was originally chosen for study because its estimated metallicity ([Fe/H] = - 1.09, Zinn & West 1984ApJS...55...45Z) and richness in RR Lyraes made it a good "template" cluster for the Baade's Window (BW) RR Lyraes. However, this analysis showed that NGC 6723 is more metal-poor than previously thought, in keeping with its blue horizontal branch morphology. The lower metallicity weakens the association of NGC 6723 with the bulge RR Lyraes. The latter were used by Lee (1992, AJ, 104, 1780) to infer that bulge formation began 1-2 Gyr before formation of the outer halo. My age derivation demonstrated that NGC 6723 is comparable in age to other (old halo) globular clusters that have similar metallicities. The age differences between NGC∼6723 and the oldest, most metal-poor globulars suggest that if NGC∼6723 is representative, formation of the inner halo began between 1-3 Gyr later than the onset of halo formation, but occurred more or less simultaneously with the bulk of halo star formation.

The ages determined 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 ([Fe/H] = -0.63 ± 0.04) was found to be comparable in age to other disk globulars with known ages. If these clusters are representative, then it appears that the thick disk began to form not long after the halo, in agreement with previous findings of Carney et al. (1990, AJ, 99, 572) and Marquez & Schuster (1994A&AS..108..341M).

The derived age of NGC 5927 ([Fe/H] = -0.24 ± 0.06) depends critically on the adopted helium abundance, and the helium to hydrogen ratio in metal-rich clusters may be higher than in metal-poor clusters. Buzzoni et al. (1983A&A...128...94B) derived formulae that predict values of the helium fraction, Y, given R, the ratio of the number of horizontal branch (HB) stars to the number of red giant branch (RGB) stars. From the CMD, I derived R = 2.0 ± 0.5, which corresponds to Y = 0.29 ± 0.04 using equation 11 of Buzzoni et al. This helium abundance was used with appropriate stellar isochrones to derive an age 2–3 Gyr younger than the other disk globulars.

Finally, the age of the bulge was derived using values from the literature for the Baade's Window MSTO magnitude, metallicity, helium abundance and distance. As far as possible, the age was derived in a manner fully consistent with those of the globular clusters. The derived "mean" bulge age was ∼4 Gyr younger than the age of the typical disk globular. However, the large uncertainty in the magnitude of the bulge MSTO translated directly into a 5 Gyr uncertainty in the age difference, rendering its significance dubious.