CCO Photometry of Globular Clusters G. ALCAfNO, Instituto Isaac Newton, Santiago, Chile The Subject different regions of the diagram, corre­ second, much fainter image of each spond to the various phases of their star. Thus, in a boot-strap manner, the In a similar way that humans inhabit lives. relatively bright photoelectric sequence our planet, stars are distributed in galax­ The theoretical tracks of different could be extended to fainter mag­ ies. There are regions where their densi­ masses for identical ages provide the nitudes. Then came CCOs. Suddenly it ty is low, such being the case where our isochrones, in whose extension the vari­ became possible to carry out photome­ sun exists, or places where the popula­ ous masses are placed on the hy­ try with a substantially higher precision tion is large and highly concentrated, pothesis of a common origin. The calcu­ owing to the stability and the linearity of the counterparts of the Tokios, New lation of these models provides the this marvellous detector, especially at Yorks, denominated in the astronomical evolution in time of the stellar tempera­ low light levels. Also, its sensitivity range world as globular clusters, because of ture and luminosity for a homogeneous extended beyond 1 micron making it their globular appearance rendered by chemical composition. From the com­ possible to work with ease in more col­ their gravitational field. Their stellar parison of the observed colour-mag­ ours than Band V wh ich had been the population ranges in the order of the nitude diagrams with the isochrones we workhorse wavelength bands for many tens of thousands up to beyond a mil­ can deduce the age of the cluster. In years. lion. About 140 of these objects are order to be able to make these compari­ In 1981, following two decades of seen in our Galaxy and a few others may sons we need to know the chemical professorship at Harvard, the American be hiding, obscured by the dust in the composition, distance and reddening of astronomer William Liller moved perma­ plane of the Galaxy. The majority are the cluster, and the transformation of nently to Chile at a time when CCOs placed towards the direction of the the physical parameters of luminosity were still not user-available at ESO. We galactic centre, and hence ideally situ­ and temperature ought to be weil de­ had known each other since 1968, and ated to be observed from the southern fined. because of his interest in globular clus­ hemisphere. ters, we began to collaborate, initially The study of globular clusters pro­ using the now old-fashioned tech­ History vides most relevant information, such as niques, but then changing over to CCOs the minimum age of the Universe, as In the mid-sixties, when I began to when they became available. We shall they are the oldest known objects, study astronomy, I gave careful thought describe herewith the results of our re­ supposedly formed in the early stages to the kind of research that might be cent work. of the contraction of the galaxy, per­ carried out from Chile. At that time, the centagewise very soon following the European Southern Observatory was Current Status of Research birth of the Universe. lt explains further­ just being started. I was primarily in­ more the process of stellar evolution, terested in photometry, and the beauti­ Colour-magnitude diagrams reaching due to the fact that we can analyse a ful work on globular clusters that Allan down to the main sequence have been large sampie of coeval stars all placed at Sandage had recently done at the obtained for about 40 globular clusters the same distance; provides knowledge Mount Wilson and Palomar Obser­ in the Galaxy. The primary motivation for of the chemical composition of the origi­ vatories had impressed me greatly, so I this research has been the determina­ nal material which prevailed during the decided to stop off in Pasadena and talk tion of the ages of the clusters found by "first epoch", as weil as their mass, and with the expert. lt was then and there matching the observed V, (B-V) diagram location within our Galaxy, and neigh­ that the decision was made: do photo­ to the theoretically derived luminosity­ bouring galaxies, such as the metry on the many globular clusters that temperature relationship converted to Magellanic Clouds, close enough to lie at negative declinations (65 per cent the observed parameters. lnitially, it was scrutinize these objects. are south of -20°). At the time, globular not clear if globular clusters had a sub­ The observations of globular clusters cluster photometry in the Southern stantial spread in ages wh ich would im­ are performed with high-precision Hemisphere was almost a virgin fjeld, ply a gradual formation of the halo of the photometry, measuring the luminosity and almost any cluster that one worked Galaxy, or if the ages were all similar and the colour of a sampie of stars, on was being studied for the first time. It wh ich would lead one to conclude that ideally placed not so near the cluster's was possible to reach below the hori­ there was a rather abrupt galactic col­ centre, as to avoid the high contamina­ zontal branch, home of the RR Lyrae lapse. However, improved work of re­ tion; and not so far from the nucleus, as stars, thereby allowing one to derive cent years, both observational and to elude intruders from the field. The plot reasonably reliable distances and inter­ theoretical, has made it c1ear that there derived from such observations is called stellar reddening for these objects. The is a small spread in the ages of globular the colour-magnitude diagram, wh ich is approximate metallicities could also be clusters, perhaps no more than 2 billion a fundamental tool in understanding the determined. years, an average age somewhere be­ evolution of stars in these objects. It The early photometry was carried out tween 16 and 18 Gyrs. As we have said, informs us on the relation magnitude as in two steps: first, a dozen or so stars one of the most important aspects of a function of the stellar colour, which were measured photoelectrically (UBV these results is that it bears directly on can be transformed with good approxi­ usually), covering as wide a range of the age of the Universe and the value of mation to those theoretically derived for colour and magnitude as possible. Then the Hubble Constant since globular their luminosity and temperatures. As it photographs were taken and star im­ clusters are among the oldest known is assumed that all stars in a cluster ages measured with a suitable photo­ objects in existence. Therefore, estab­ shared a common origin, their location meter. Later, when it became available lishing firmly and accurately the age of in the colour-magnitude diagram is due at the ESO 3.6-m telescope, a small the oldest clusters would set an upper to the fact that the more massive the thin-wedge prism could be placed be­ limit on the Hubble Constant, the exact star is, the faster it evolves, hence the fore the objective, thereby producing a value of which continues to be very 32 .. .. '" ". : .... , " ." . '.: .. ~ ' .. '. " ," " '" .. " '';' . ,', .' " ..; "':~ .' .... " . ~ :. .. .' . '. ... :'. : ,. ': .: . ", ." • .. .... " . .... '.. ~ .. ", , ....: .. , .. " , ••• 1.... " . ..~. .. ' .. ',' ". .. .. .,.. .' " ,. ..: .' .' . .. ", ' .. .. N ,.; .. ' .. I , , . 47. 'Tut 5' ', .... .. "" •• ,04 ! ", .' . Figure 1: The northwest sector of the globular cluster 47 Tucanae. The reproduction is from a 14-minute yellow plate (lIa-O + GG 495) obtained with the ESO 3.6-m telescope. Not/ce /he secondary images produced by the Pickering-Racine wedge, displaced 14 arcsec towards the northeast of /he brighter primary images. The general view shows /he typical field that can be analysed with ph%graphic techniques. The rec/angle of 4 x 2.5 arcmin shows the region actually s/udied with the eeo camera. much under discussion at the present Bell have now carried out the difficult our baseline wh ich enhances effects time. calculations needed to predict the loca­ seen less c1early with a smaller range in One of the weaknesses of the ex­ tion of isochrones in longer wavelength colour. An additional benefit of this mul­ tremely popular and often-used BV bands where metallic absorption is ti-colour approach is that observational photometric system is that metallic line much less than in the blue. This work uncertainties are reduced by having absorption in the blue and violet can be came largely as a response to the several independently derived CMOs of considerable. Thus, the interpretation of appearance of the modern generation of the same cluster. With separate evalua­ observations with stellar evolution electronic detectors, especially the tions of the age of a single cluster, one theory rests heavily on model stellar at­ charge-coupled device (CCO), which not only can assess more reliably the mospheres wh ich are needed to predict has made it possible to carry out im­ accuracy of the final result, but also can the effects of this extinction in stars proved photometry at magnitudes faint­ derive ages with a higher precision than where often the metallicity is not weil er than photographic limits and at attained previously or with only two col­ known. The metallicity of globular clus­ wavelengths extending into the near in­ ours. Consequently, for the past several ters, usually expressed by the parame­ frared. Besides the obvious advantage years we have had underway a pro­ ter [Fe/H], has a range of over two or­ of reducing uncertainties arising from gramme of BVRI photometry of globular ders of magnitudes, making a precise poorly known metallicities, use of the clusters using CCOs and new powerful knowledge of absorption line effects im­ red and infrared wavelengths (R and I reduction software.