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Volume 97, Number 6 THE ASTRONOMICAL JOURNAL VOLUME 97, NUMBER 6 JUNE 1989 97.1688B THE AGE OF THE GLOBULAR CLUSTER NGC 288, THE FORMATION OF THE GALACTIC HALO, AND THE SECOND PARAMETER Michael BoLTEa) 1989AJ Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, British Columbia V8X 4M6, Canada Received 24 January 1989 ABSTRACT A differential comparison of precise CCD photometry in the globular clusters NGC 288, NGC 362, and NGC 1261 shows that differences exist in the positions of the main-sequence turnoff in these clusters that are most naturally explained if NGC 288 is some 3 billion yr older than NGC 362 and — 1 to 2 billion yr older than NGC 1261. This implies that the formation time for the Galactic halo is signifi- cantly longer than a freefall time. Consideration of the inferred ages and horizontal-branch morpholo- gies of the clusters Pal 12, NGC 288, NGC 362, and NGC 1261, all with similar metal abundances, suggests that age may be the parameter that, after overall metal abundance, most determines horizontal- branch morphology. I. INTRODUCTION there was a small dispersion in the age of clusters. This work also includes a summary of the work on cluster ages previous Globular star clusters are the most conspicuous and easily to 1983. With hindsight, it now seems clear that even in the dated component of the Galactic halo. Consequently, stud- best of the photographic studies it was not possible to distin- ies of globular cluster ages provide two key observational guish age differences at the 4 billion yr level. An enormous data points often used to constrain models of the formation improvement in the accuracy of photometry at faint magni- of the halo and models of the universe: ( 1 ) The difference in tudes was made possible by the availability of CCD detectors age between the oldest and youngest globular clusters is and sophisticated reduction software in 1983. The high probably the most reliable indicator of the time over which quantum efficiency and linear response of CCDs made possi- the Galactic halo formed, and provides a fundamental con- ble accurate photometry of the age-sensitive turnoff region straint on any models of the early stages in the collapse of the for many clusters. As important, accurate photometry of the Galaxy. (2) The age of the oldest cluster is arguably the best unevolved main sequence in the nearer clusters is also possi- observational lower limit to the age of the universe, and the ble with CCD detectors. Determining the lower-main-se- ages derived for globulars have played a prominent role in quence position of a cluster allows the estimation of the recent discussions of various cosmological models (e.g., cluster’s distance independent of horizontal-branch consid- Taylor 1987; Sandage 1988). To address the second point erations. This is critical to estimating the ages of clusters requires age determinations of high accuracy, with all sys- with poorly populated or extremely blue horizontal tematic errors in the analyses eliminated. To address the first branches. point requires measuring cluster ages with high precision, The emphasis of the first CCD-based studies has been but systematic errors are not important as long as all clusters measurements in a number of individual clusters, with only a studied are affected identically. few critical intercluster comparisons. Gratton (1985) used In this paper the question of relative cluster ages is ad- published cluster photometry to compile ages based on the dressed by a differential comparison of the clusters NGC difference in V magnitude between the horizontal branch 288, NGC 362, and NGC 1261. The derivation of relative and main-sequence turnoff. He found a trend in cluster ages cluster ages is a difficult task, and one that has an unfortu- with galactocentric distance in the sense that the inner halo nately strong dependence on a number of small details. Con- clusters were coeval while the outer halo clusters showed a sequently, a fair presentation of the data and conclusions range of ages and were generally younger. Peterson ( 1986) requires somewhat involved (and occasionally tedious) dis- carried out a similar study using a larger base of cluster ob- cussions of many aspects of the observations for each of the servations and concluded that the strong galactocentric gra- clusters, as well as a large number of caveats. This introduc- dient in cluster age found by Gratton was not supported. The tion will ( 1 ) give a brief summary of recent cluster age deter- most recent studies favor a significant age spread in the clus- minations, (2) present an overview of the primary difficul- ter system. Pound, Janes, and Heasley ( 1987) compare their ties in making cluster age determinations, and ( 3 ) argue that photometry of NGC 288 with published photometry of a differential study of a carefully chosen group of clusters is NGC 362 and conclude that NGC 288 could be several Gi- required in order to make a strong statement concerning gayears older than NGC 362. King, Demarque, and Green relative cluster ages. ( 1988) derive ages for several clusters (including NGC 288 There have been many studies focused on the age spread in and NGC 362) by comparing published color-magnitude the globular cluster system, and no attempt has been made to diagrams (CMDs) to a single set of isochrones and conclude present a complete history here. VandenBerg (1983) com- there is evidence for age differences between clusters as large piled and compared to isochrones all of the then available as 5 Gyr. The strongest case to date for a cluster-to-cluster main-sequence photometry in globulars and concluded that age variation is the distant cluster Pal 12. Gratton and Orto- a) Visiting Astronomer, Cerro Tololo Inter-American Observatory, Na- lani (1988) and Stetson et al. (1989) present independent tional Optical Astronomy Observatories, operated by AURA, Inc. under photometry of Pal 12 and show that the V magnitude differ- contract with the NSF. ence between the horizontal branch and main-sequence 1688 Astron. J. 97 (6), June 1989 0004-6256/89/061688-11 $00.90 © 1989 Am. Astron. Soc. 1688 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1689 MICHAEL BOLTE: NGC 288 1689 97.1688B turnoff in this cluster is small compared to the average value tances to clusters are the apparent level of the zero-age hori- for clusters. Furthermore, these studies (particularly Stet- zontal branch (ZAHB) and the position of the lower main son et al. ) demonstrate that the turnoff position of Pal 12 is sequence of clusters. The relationship between the level of anomalously bright and blue compared to the well-studied the ZAHB and metal abundance is an unresolved issue at 1989AJ clusters M5 and 47 Tucanae (NGC 104). Both features of this point (see, e.g., Jones et al. 1988; Renzini and Fusi- the Pal 12 CMD indicate that Pal 12 is some 30% younger Pechi 1988; Hesser 1988) which is conveniently bypassed than 47 Tue or M5. when comparing clusters of similar metal abundance. The The primary source of uncertainty in estimating cluster second fiducial mark, the position of the unevolved main ages is the fact that the main-sequence-tumoff position in the sequence, is definitely a function of metal abundance; how- CMD of a cluster depends not only on the cluster age, but ever, the precise relationship has been established only also the cluster distance and reddening, and the metal abun- through model calculations at this time. A definitive empiri- dance of the stars in the cluster. An age difference of 2 billion cal check on the model results has not appeared in the litera- yr produces an effect in the CMD comparable to changing ture to date. The [M/H]-(l? — V) color relation must be the distance modulus by —0.15 mag or changing [M/H] by known quite accurately because the steep slope of the main —0.25 dex (at a metal abundance of [M/H] — — 1.2), sequence in the CMD magnifies (B — V) uncertainties where [M/H] is the logarithmic ratio by number of the ele- when attempting a vertical sliding fit of cluster main-se- ments heavier than helium to hydrogen compared to the so- quence positions to measure relative cluster positions. For lar value. If the distance to a cluster is determined by fitting this last reason it is also important to choose clusters for the lower main sequence of the cluster to a fiducial Pop II analysis that have secure E(B — V) determinations. sequence, errors in the reddening of 0.03 mag translate into distance and age errors of —0.15 mag and —2 Gyr, respec- II. OBSERVATIONS tively. Therefore, to measure age differences on the order of a We present photometry of three clusters, NGC 288, NGC few billion years requires the measurement of relative cluster 362, and NGC 1261. The data were obtained on two separate distances, reddenings, and metallicities to better than the observing runs at the Cerro Tololo Inter-American Observa- values quoted above. tory. The first run was in August 1985 using the 4 m tele- A second major source of uncertainty in determining rela- scope, the second was in September 1986 using the 1.5 m tive cluster ages is the difficulty of typing photometry in telescope. The details of the observations, reductions, and different studies to a single standard photometric system. In calibrations, and photometry of individual stars, are present- a number of clusters for which independent photometric ed in Boite ( 1987a), Boite ( 1988,1989), and Boite and Mar- studies exist, disagreement at the 0.05 mag level in measured leau (1989) for NGC 362, NGC 288, and NGC 1261, re- brightnesses and colors is common (the history of photome- spectively.
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