Computer Science
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........70c&link_type=abstract
PhD Dissertation, New South Wales Univ. Kensington, Australia
Computer Science
3
Globular Clusters, Abundance, A Stars, Signal To Noise Ratios, Computerized Simulation, Carbon, Nitrogen, Oxygen, Age Factor, Spectra
Scientific paper
A major source of uncertainty in the derived ages of globular clusters is the assumed CNO abundance. A change of a factor of 3 in the amount of carbon, nitrogen and oxygen in a cluster alters the ages derived from studies of color magnitude diagrams by 2 Gyrs. The clusters NGC 288 and 362 (with a reported age difference of about 3 Gyr) have been central to recent claims of an age spread in the globular cluster system of our galaxy. To investigate the possibility that this apparent age difference may be partially caused by variations in the CNO abundances between these clusters, observations of the (OI) line at 6300A, and the CH and CN bands near 4000A were made using the coude echelle spectrograph on the Anglo-Australian Telescope (with a resolution of approximately 40000, and a signal to noise ratio of more than 50). These spectra have been analyzed using the model atmosphere code MARCS and the synthetic spectrum code SSG. Observations were also made of a selection of stars in NGC 6752, which has a bimodal distribution in the CN band strengths on the giant branch, as well as an extremely blue Horizontal Branch (and hence is a second parameter cluster). The derived abundance (Fe/H) for NGC 288, 362 and 6752 is -1.14, -0.97 and -1.30 respectively. For (CNO/Fe), values for each cluster were 0.29, 0.19 and 0.04 respectively. The difference in abundances for NGC 288 and 362 are not sufficient to account for the claimed age difference. This suggests that if the age difference is 3 Gyr (as claimed) then the formation of the halo lasted longer than a free fall time, ruling out a simple dissipationless collapse. Also, the values of (Fe/H) are 0.2 dex higher than the values of Zinn and West (1984). If the (CNO/Fe) value is 0.2 dex, then the combined effect of these abundance changes is to reduce the derived ages of the clusters by approximately 1.4 Gyr (assuming currently accepted value is 15 Gyr).
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