Distances and ages of galactic globular clusters via infrared photometry
Buckley, David R. V.
One of the most promising routes to the formation of a quantitative model of the processes and timescales involved in transforming primordial, pre-galactic material into the galaxy as we know it today is the study of the ‘fossil record’, one important component of which is the galactic globular-cluster system. It is therefore vital to have information of the basic parameters - reddening, chemical composition, distance, age and internal and orbital dynamics - of many globular clusters.This thesis is concerned with the derivation of the distances and ages of the four galactic globular clusters M13 (NGC6205), M30 (NGC7099), NGC6752 and 47 Tuc (NGC104). To this end we have obtained infrared photometry of respectively 21, 344, 123 and 688 stars in these clusters. We have matched this photometry with existing optical photometry to form the optical-infrared colour magnitude diagrams which are the basic tool for this study. These colour-magnitude diagrams are to the best of our knowledge the deepest of their kind in existence, and in NGC6752 and 47 Tuc reach to some five V magnitudes below the main-sequence turn-off.The distances are derived using a sample of six metal-poor field subdwarf stars with accurate trigonometric parallax measurements, and a discussion is given of the sensitivity of these distances to various sources of error. We find that distances derived using the optical-infrared colour-magnitude planes are less sensitive to some systematic errors in the photometry than purely optical colour-magnitude diagrams, while distances determined in K, (B-V) are least sensitive to errors in the adopted ratio of total-to- selective extinction. Other sources of error considered are small photometric errors and errors in the adopted values of E[B-V] and [Fe/H], For M13, M30, NGC6752 and 47 Tuc we derive distances of 7.1, 8.1, 4.0 and 4.4 kpc without Lutz-K elker corrections. Distances derived applying full Lutz-K elker corrections to the subdwarf magnitudes are also given.Ages are derived for each cluster both directly from the Bergbusch & VandenBerg isochrones and also from the distance and reddening independent (V-K) and (B-V) colour differences between the main-sequence turn-off and the subgiant branch, calibrated against age using the isochrones. The results from both of these methods are compared, and ages of respectively 14.1±1.4, 14.9+2.3, 13.6±1.1 and 14.6± 1.5 Gyr for M13, M30, NGC6752 and 47 Tuc are adopted. Since the foreground reddening to one of our clusters - M30 - is controversial, the colours of the subgiant branches are also used to check for internal consistency among the adopted cluster reddenings. We find good internal consistency among all the reddenings, although there is marginal evidence that the value adopted for M30 (E[B-V]=0.07) is too high by 0.01 to 0.02 magnitudes.New Kurucz ATLAS9 model atmosphere results are used to derive colour to effective temperature relations and bolometric corrections, and these are applied to transform the Bergbusch & VandenBerg theoretical isochrones to the V, (B-V) and V, (V-K) planes for comparison with the data. The rationale behind this transformation was to use the very- latest model atmospheres to produce isochrones in the observational planes which are independent of colour-shift problems found particularly in V, (V-K).The cluster distances and ages are re-examined in the light of these ‘new’ models. The adopted ages are found to agree within the errors with those derived from the ‘original’ modelsFinally, we examine the consequences of these cluster ages in the context of the currently favoured galactic formation scenario. We find that recognisable structure must have existed within the inner halo at least as long ago as (14.1 ±1.4) Gyr. From the age and chemical composition of 47 Tuc, we conclude that protostellar material significantly enriched in heavy elements either co-existed with metal-poor protostellar material, or that the enrichment occurred over a period of ~2 Gyr. Since 47 Tuc has disk kinematics, we also conclude that the galactic disk began to form within ~2 Gyr of the inner halo.We also conclude that if, as has been suggested, such very m etal-poor clusters as M30 formed in outlying proto-galactic fragments that subsequently merged into the main body of the galaxy, then recognisable structure had formed within these ‘ancestral objects’ within ~2 Gyr of the formation of structure within the halo and disk.