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1990Aj 100. .445V the Astronomical Journal .445V THE ASTRONOMICAL JOURNAL VOLUME 100, NUMBER 2 AUGUST 1990 100. MEASURING AGE DIFFERENCES AMONG GLOBULAR CLUSTERS HAVING SIMILAR METALLICITIES: A NEW METHOD AND FIRST RESULTS Don A. VandenBerg 1990AJ Department of Physics and Astronomy, University of Victoria, P.O. Box 1700, Victoria, British Columbia V8W 2Y2, Canada Michael BoLTEa) and Peter B. STETSONa),b) Dominion Astrophysical Observatory, National Research Council of Canada, 5071 West Saanich Road, Victoria, British Columbia V8X 4M6, Canada Received 2 April 1990; revised 4 May 1990 ABSTRACT A new method is described for comparing observed color-magnitude diagrams to obtain accurate relative ages for star clusters having similar chemical compositions. It is exceedingly simple and straightforward: the principal sequence for one system is superimposed on that for another by applying whatever vertical and horizontal shifts are needed to make their main-sequence turnoff segments coin- cide in both V magnitude and B — V color. When this has been done, any apparent separation of the two lower giant branch loci can be interpreted in terms of an age disparity since, as is well known from basic theory, the color difference between the turnoff and the giant branch is a monotonie and inverse function of age. This diagnostic has the distinct advantage that it is strictly independent of distance, reddening, and the zero-point of color calibrations; and theoretical isochrones show it to be nearly independent of metallicity—particularly for [m/H] < — 1.2. (In fact, if the cluster photometry is secure and the metal abundance is accurately known, our technique provides an excellent way to determine relative reddenings. ) Furthermore, computed models need be used only in a differential sense to calibrate the observed variations in the turnoff-to-giant-branch color difference in terms of relative age. We apply this technique to the principal sequences of 22 globular clusters divided into three abundance bins—[m/H] ^ — 2.1, — 1.6, and — 1.3—and take the fiducials for M92, NGC 6752, and NGC 362 to be the standard sequences in each bin, respectively. More detailed star-by-star statistical analyses are undertaken for three groups of clusters of particular interest: M68/M92/NGC 6397, M3/ Ml3, and NGC 288/NGC 362. We find that the most metal-poor systems are extremely uniform in age, with no convincing evidence for differences as great as 0.5 Gyr. The [m/H] ^ — 1.6 clusters also seem to be nearly coeval, though the data are not yet of sufficiently high quality to rule out some age spread (at about the 1.5cr confidence level). However, the most metal-rich globulars that we have considered do appear to encompass a significant range in age. In particular, our analysis supports the conclusion reached by several others that NGC 288 is older than NGC 362 and M5 by ^2 Gyr. We also find that most of the distant objects in our sample (e.g., Palomar 5 and NGC 7492) are not distinctly different in age from nearer globulars having the same metal content, suggesting that there is not a significant age- Galactocentric-distance relation in the outer halo. Palomar 12 still seems to be an anomaly in that its C- M diagram differs from the adopted fiducial for its metallicity—that of NGC 362—by more than any other cluster in our sample. It seems younger than NGC 362 by about 4 Gyr, in agreement with previous findings. The main result of this work, then, is that the dispersion in globular cluster ages as a function of metal abundance appears to increase from near zero at [m/H] — — 2.1 to —2 Gyr at [m/ H] — — 1.3. This suggests that the collapse of the Galaxy was of prolonged rather than of brief (i.e., < 1 Gyr) duration. A further implication of our analysis is that age is unlikely to be the “second parameter” in the globulars unless it can be demonstrated that age differences of ^ 2 Gyr can cause the observed wide variation in horizontal-branch morphology among clusters of the same [m/H]. I. INTRODUCTION of the universe is a fundamental cosmological constraint (e.g., see Tayler 1986; VandenBerg 1988). Second, their There are many reasons why globular cluster (GC) re- spreads in age as a function of metallicity and Galactocentric search continues to be one of the most active areas in stellar distance, and from cluster to cluster at 2l given [m/H] and astrophysics, but two stand out above the others. First, be- location in the halo, provide vital clues about the formation cause the globulars are the oldest objects for which reliable of galaxies—our own, in particular. An accurate assessment ages can be derived, the lower limit which they set to the age of relative GC ages should, for instance, be able to distin- guish between the Eggen, Lynden-Bell, and Sandage ( 1962) a) scenario for the early evolution of the Galaxy and that advo- Visiting Astronomer, Cerro-Tololo Inter-American Observatory, Na- cated by Searle and Zinn (1978), since the former argues in tional Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the Na- favor of a uniform, rapid ( 1 Gyr) collapse while the latter tional Science Foundation. asserts that it was more chaotic and lasted for several billion b) Visiting Astronomer, Kitt Peak National Observatory, National Optical years. On the practical side, the recent revolution in photo- Astronomy Observatories, operated by the Association of Universities for metric techniques brought about by the introduction of Research in Astronomy Inc., under contract with the National Science CCDs and the development of automatic data-reduction Foundation. software (e.g., Stetson 1987, 1990a,b) has finally made it 445 Astron. J. 100 (2), August 1990 0004-6256/90/020445-24$00.90 © 1990 Am. Astron. Soc. 445 © American Astronomical Society • Provided by the NASA Astrophysics Data System .445V 446 VANDENBERG ETAL. : GLOBULAR CLUSTER AGE SPREADS 446 100. possible to derive truly trustworthy data for a large sample of of the main-sequence (MS) fitting method for obtaining rel- clusters. ative cluster ages. This involves the detailed comparison of The determination of age differences is, in principle, a high-precision, high-accuracy CMDs for carefully chosen much more tractable problem than the derivation of abso- systems (e.g., Stetson et al. 1989; Boite 1989; Green and 1990AJ lute ages because it requires only differential comparisons of Norris 1990). Specifically, age differences are inferred from cluster data and does not need accurate, independent dis- a match of the unevolved main-sequence loci after appropri- tances and reddenings. Indeed, as we will demonstrate be- ate corrections are made for differences in distance, redden- low, a little experimentation with theoretical isochrones ing, and metallicity. This approach has the important advan- shows that, when observed stellar colors and brightnesses tage that clusters with diverse HB morphologies can be are referred to some natural and well-defined points intrinsic readily compared. However, the results are highly contin- to the cluster sequences, some differential comparisons can gent upon having accurate E(B — V) and [m/H] values for be made with a high degree of reliability. both systems, as well as photometric accuracy at the 1-2% level for stars on the unevolved portion of the main sequence. Current uncertainties in measuring cluster reddenings alone II. A COLOR DIFFERENCE METHOD FOR ESTIMATING RELATIVE CLUSTER AGES probably limit the age discrimination to — + 1.5 Gyr, and it is observationally difficult to apply the method to clusters more distant than (m — M) 15. During the years when most photometric observations of For differential comparisons, we suggest a new method GCs were made with photographic plates and photomulti- that basically measures the changes in the turnoff-through- plier-based photometers, the horizontal branch (HB) pro- subgiant-branch morphology as a cluster ages. It avoids the vided the needed reference point for differential studies. This difficulties mentioned above by making use of intrinsic refer- was in the form of either the magnitude and color of the RR ence points that are related to the cluster main-sequence Lyrae gap, or the intersection of the giant branch with an turnoff (MSTO). As a zero-point of color, the bluest point of extrapolation of the horizontal branch—the so-called the MSTO (where the observed cluster locus on the CMD is (B — F)o,g parameter first generalized by Sandage and vertical) is an obvious choice since it is particularly well Smith (1966). However, these traditional and widely used defined. [In what follows, we will use (B — V)TO to refer to fiducial marks, which are central to, for instance, the A F£b this feature. ] Granted, the intrinsic color of the turnoff is technique for measuring relative cluster ages* (see Sandage well known to be a strong function of age and chemical com- 1982, Iben and Renzini 1984), have several disadvantages: position, but if we assume—for the time being—that all clus- (1) In sonie clusters (e.g., NGC288, NGC 6752, M13), the ters with apparently similar [m/HJ’s also have the same horizontal part of the HB is not occupied—all the core heli- helium content, F, then the color difference between the um-burning stars lie along a nearly vertical sequence far to turnoff and the nearly vertical locus of stars at the base of the the blue of the instability strip. Even today, reliable faint red-giant branch (RGB) is an excellent age indicator. This photometric standards with such blue colors are rare, ren- CMD feature has, in particular, no dependence at all on dis- dering calibrations difficult.
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