Ages Of Globular Clusters Research Articles

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

New improved distances and absolute ages for the Galactic globular clusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the Main Sequence Fitting Method. We derived accurate estimates of reddening and metal abundance for these three clusters using a strictly dierential procedure, where the Johnson B V and Stromgren b y colours and UVES high resolution spectra of turn-o stars and early subgiants belonging to the clusters were compared to similar data for field subdwarfs with accurate parallaxes measured by Hipparcos. The use of a reddening free temperature indicator (the profile of H) allowed us to reduce the error bars in reddening determinations to about 0.005 mag, and in metal abundances to 0.04 dex, in the scales defined by the local subdwarfs. Error bars in distances are then reduced to about 0.07 mag for each cluster, yielding ages with typical random errors of about 1 Gyr. We find that NGC 6397 and NGC 6752 have ages of 13:9 1:1 and 13:8 1: 1G yr respectively, when standard isochrones without microscopic diusion are used, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by other techniques sensitive to relative ages. If we use models that include the eects of sedimentation due to microscopic diusion in agreement with our observations of NGC 6397, and take into account various sources of possible systematic errors with a statistical approach, we conclude that the age of the oldest globular clusters in the Galaxy is 13:4 0:8 0:6 Gyr, where the first error bar accounts for random eects, and the second one for systematic errors. This age estimate is fully compatible with the very recent results from WMAP, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z 2:5, in a standardCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z 1:3. On the other hand, our new age estimate once combined with values of H0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of M < 0:57, independently of type Ia SNe, and strongly supports the need for a dark energy. The new cluster distances lead to new estimates of the horizontal branch luminosity, that may be used to derive the zero point of the relation between the horizontal branch absolute magnitude and metallicity: we obtain MV (HB)= (0:22 0:05)((Fe=H)+ 1:5)+ (0:56 0:07). This zero point is 0.03 mag shorter than obtained by Carretta et al. (2000) and within the error bar it agrees with, but it is more precise than most of the previous individual determinations of the RR Lyrae absolute magnitude. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m M)0= 18:50 0:09.

Oxygen Abundance in the Template Halo Giant HD 122563

HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ((Fe/H ��� 2:7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200- 400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric effective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity (Fe/H �¼� 2:8 and micro- turbulence velocity vt ¼ 2: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +1.1, and +0.7, based on the (O i) 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The different oxygen abundance indicators give different oxygen abundances, illustrating the problem of oxy- gen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for the metal-poor stars. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable table

Is the age problem resolved?

The cosmological, astrophysical, and nucleocosmochronological methods for estimating the age of the universe and the corresponding uncertainties are comparatively studied in the present paper. We are led to the conclusion that the new measurements of cosmological parameters, and the recent estimates of the age of globular clusters have led to the gradual disappearance of the age problem from the arena of modern cosmology.

The Young, the Old, and the Globular

They hover in the haloes and bulges of galaxies: globular clusters, gravitationally bound collections of 100,000 to 1 million stars following highly eccentric, elliptical orbits that drift across the galactic plane with very long periods. Among the first to recognize that these enigmatic objects were made of stars was the German-born astronomer William Herschel, who coined the term “globular cluster” in 1789. For the next three centuries, globular clusters remained fuzzy in detail but bright enough to be used as standard candles in cosmology. In the past three decades, major advances in observing tools and computing prowess have led to significant improvements in our understanding of their evolution, structure, dynamics, and perhaps most critically, ages. Age is central to globulars' fascination. Thought to be among the oldest stellar systems, they can provide valuable information about the first population of stars and the age of the universe. That status caused some vexation in the past, when astronomers pegged cluster ages between 15 billion and 20 billion years at a time when cosmologists calculated that the universe itself was only 10 billion to 15 billion years old. Nowadays, however, harmony reigns. In their review of age estimates for globulars in the Milky Way, Krauss and Chaboyer (p. 65) suggest that astronomers now have the lower limit about right, making the universe at least 11.2 billion years old. That cosmic youthfulness, coupled with evidence that the universe is geometrically flat and with current estimates of the Hubble constant, implies that the universe must be dominated by the mysterious antigravity-producing stuff called dark energy. In a new twist, astronomers have recently started estimating the ages of globular clusters from the decay of long-lived radioactive elements such as thorium (Th) and uranium (U) inside their stars. They have calculated stellar ages at 11 billion to 15 billion years. Although those numbers are consistent with estimates of the lower age limit of the universe, the “old” end of the range makes theorists uncomfortable, and more work is needed. To get more reliable ages, astronomers need to know how much U and Th the stars started with. That requires modeling the sites of nucleosynthesis and the masses of the neutron-rich nuclei that create the heavier elements. Sneden and Cowan (p. [70][1]) review these issues as well as other discoveries about the nucleosynthesis of heavy elements in the Milky Way. Globular clusters are also interesting for the light they shed on stellar processes and dynamics and, of course, as objects in their own right. Recent work has turned up some surprises: for example, evidence that hitherto unexpected star formation in old clusters may be confusing age estimates, and that a black hole or a concentration of neutron stars lurks in the core of some clusters. Irion (p. 60) summarizes these observations and describes how interactions between binary stars can turn placid-seeming clusters into stellar combat zones. Globulars also flourish in other galaxies. Schilling (p. 63) describes evidence that some extragalactic clusters, unlike those in the Milky Way, harbor two different generations of stars, one younger, redder, and more metal-rich than the other. Some theorists suggest that the younger clusters form in starbursts that ignite when galaxies collide—a challenge to the standard view that all clusters form in the collapse of molecular clouds during galaxy formation. We've come a long way since Herschel's insight that globulars consist of stars. A little more time should resolve what kinds of stars they are: a bit too old, a bit too young, or just right. [1]: /lookup/doi/10.1126/science.1077506

Deflationary cosmology: Constraints from angular size and ages of globular clusters

Observational constraints to a large class of decaying vacuum cosmologies are derived using the angular size data of compact radio sources and the latest age estimates of globular clusters. For this class of deflationary $\ensuremath{\Lambda}(t)$ models, the present value of the vacuum energy density is quantified by a positive \ensuremath{\beta} parameter smaller than unity. In the case of milliarcsecond compact radio-sources, we find that the allowed intervals for \ensuremath{\beta} and the matter density parameter ${\ensuremath{\Omega}}_{m}$ are heavily dependent on the value of the mean projected linear size l. For $l\ensuremath{\simeq}{20h}^{\ensuremath{-}1}\ensuremath{-}{30h}^{\ensuremath{-}1} \mathrm{pc},$ the best fit occurs for $\ensuremath{\beta}\ensuremath{\sim}0.58,$ ${\ensuremath{\Omega}}_{\mathrm{m}}\ensuremath{\sim}0.58,$ and $\ensuremath{\beta}\ensuremath{\sim}0.76,$ ${\ensuremath{\Omega}}_{\mathrm{m}}\ensuremath{\sim}0.28,$ respectively. This analysis shows that if one minimizes ${\ensuremath{\chi}}^{2}$ for the free parameters l, ${\ensuremath{\Omega}}_{\mathrm{m}}$ and \ensuremath{\beta}, the best fit for these angular size data corresponds to a decaying $\ensuremath{\Lambda}(t)$ with ${\ensuremath{\Omega}}_{\mathrm{m}}=0.54,$ $\ensuremath{\beta}=0.6,$ and ${l=22.64h}^{\ensuremath{-}1}$ pc. Constraints from age estimates of globular clusters and old high redshift galaxies are not so restrictive, thereby suggesting that there is no age crisis for this kind of $\ensuremath{\Lambda}(t)$ cosmologies.

SED, metallicity and age of halo globular clusters in M 33

In this paper we study the properties of ten halo globular clusters in the nearby spiral galaxy M 33. CCD images of M 33 were obtained as a part of the BATC Colour Survey of the sky in 13 intermediate-band filters from 3800 to 10 000 A. By aperture photometry, we obtain the spectral energy distributions (SEDs) of these globular clusters. We estimate the ages of our sample clusters by comparing the photometry of each object with theoretical stellar population synthesis models for different values of metallicity. Our results suggest that eight of the ten sample halo globular clusters have “intermediate” ages, i.e. between 1 and 8 Gyrs.

The Determination of the Age of Globular Clusters: A Statistical Approach

We present a statistical approach for determining the age of Globular Clusters (GCs) that allows estimating the age derived from CMDs more accurately than the conventional method of isochrone fitting. We measure how closely a set of synthetic CMDs constructed from different evolutionary models resemble the observed ones by determining the likelihood using Saha and χ2 statistics. The model which best matches the observational data, of a set of plausible ones, is the one with the highest value of the estimator. We apply this method to a set of three different evolutionary models presented by three different authors. Each of these sets consists again of many different models of various chemical abundances, ages, input physics and the like. We subsequently derive the age of GCs NGC 6397, M92 and M3. With a confidence level of 99%, we find that the best estimate of the age is 14.0 Gyrs within the range of 13.8 to 14.4 Gyrs for NGC 6397, 14.75 Gyrs within the range of 14.50 to 15.40 Gyrs for M92, and 16.0 Gyrs within the range of 15.9 to 16.3 Gyrs for M3.

M31 Globular Cluster Metallicities and Ages

Globular cluster ages are more than just lower limits to the age of the universe; the distribution of ages constraints the timescales for galaxy formation and chemical evolution. Globular cluster populations with different metallicities have now been detected in many galaxies, and understanding how these populations formed requires knowing their relative ages. We examined the relative ages of the two M31 globular cluster populations using their color and luminosity distributions and found that the metal-rich clusters could be up to 50% younger than the metal-poor clusters. While a small delay in the formation of metal-rich clusters might be imposed by chemical enrichment timescales, a large age gap demands a more detailed explanation. I outline several possibilities and their promises and problems.

Age Dating of Globular Clusters Using UBV(RI) Main-sequence Photometry in the Two-color Diagram: Age of NGC 6397

I propose and apply a method for deriving ages of the metal-poor globular clusters (GCs) in a distance-independent way, which is based on an age indicator related to the main sequence in the two-color diagrams using the U-B index. The age of the metal-poor GC NGC 6397 has been estimated, using Yale isochrones of Demarque et al. (1996), to be close to 16 Gyr provided the cluster metallicity is near [Fe/H] = −2.0.

Formation Scenarios for Globular Clusters and Their Host Galaxies

This review focuses on how galaxies and their globular cluster systems form. I first discuss the now fairly convincing evidence that some globular clusters form in galaxy starbursts/mergers. One way these observations are valuable is they place important constraints on the physics of the formation of globular clusters. Moreover, it is natural to associate the typically metal-rich clusters forming in mergers with the substantial metal-rich population of globulars around ellipticals, thereby implying an important role for galaxy mergers in the evolution of elliptical galaxies. I also highlight some new observational efforts aimed at constraining how and when elliptical galaxies and their globular cluster systems formed. These include systematic studies of the number of globular clusters around galaxies as a function of morphological type, studies of the kinematics of globular cluster populations in elliptical galaxies, and a variety of observational programs aimed at constraining the relative ages of globular clusters within galaxies as a function of cluster metallicity. The understanding of the formation of globular cluster systems and their host galaxies has grown dramatically in recent years, and the future looks equally promising.

Old and New Tools for Understanding the Evolution of Stars in Clusters

The uncertainties in the modelling of some important inputs of stellar evolution must be taken into account for a correct interpretation, both of the HR diagrams of individual stars, and of the integrated colors, of stellar clusters. After a short discussion of Globular Cluster ages, we focus on the problem of convection and discuss the parametrization of convection efficiency and overshooting.Convective efficiency affects the reliability of the giants colors, especially for high metallicity: age - metallicity relations found from integrated colors of clusters must be regarded with caution.Non-instantaneous mixing, both in the formal convective region and in the overshooting region must be taken into account: this is necessary to compute nucleosynthesis in ‘hot bottom burning’ envelopes, but it may also affect the color distribution in clump (core helium burning) stars, as shown here for the case of the LMC cluster NGC 1866.

Toward Better Age Estimates for Stellar Populations: The Y 2 Isochrones for Solar Mixture

We have constructed a new set of isochrones, called the Y^2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of alpha-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main sequence birthline instead of from the zero-age main sequence, and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the alpha-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages. Isochrones are available from http://achee.srl.caltech.edu.

Heavy‐Element Diffusion in Metal‐poor Stars

Stellar evolution models which include the effect of helium and heavy element diffusion have been calculated for initial iron abundances of [Fe/H] = -2.3, -2.1, -1.9 and -1.7. These models were calculated for a large variety of masses and three separate mixing lengths, \alpha = 1.50, 1.75 and 2.00 (with \alpha = 1.75 being the solar calibrated mixing length). The change in the surface iron abundance for stars of different masses was determined for the ages 11, 13 and 15 Gyr. Iron settles out of the surface convection zone on the main sequence; this iron is dredged back up when the convection zone deepens on the giant branch. In all cases, the surface [Fe/H] abundance in the turn-off stars was at least 0.28 dex lower than the surface [Fe/H] abundance in giant-branch stars of the same age. However, recent high dispersion spectra of stars in the globular cluster NGC 6397 found that the turn-off and giant branch stars had identical (within a few percent) iron abundances of [Fe/H] =-2.03 (Gratton et al. 2001). These observations prove that heavy element diffusion must be inhibited in the surface layers of metal-poor stars. When diffusion is inhibited in the outer layers of a stellar model, the predicted temperatures of the models are similar to models evolved without diffusion, while the predicted lifetimes are similar to stars in which diffusion is not inhibited. Isochrones constructed from the models in which diffusion is inhibited fall half-way between isochrones without diffusion, and isochrones with full diffusion. As a result, absolute globular cluster ages which are based upon the absolute magnitude of the turn-off are 4% larger than ages inferred from full diffusion isochrones, and 4% smaller than ages inferred from no diffusion isochrones.

Spectrophotometric Evolution of Old Stellar Systems

Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick system (Hbeta, Mg2, Mg b, Fe5270, Fe5335), broadband color indices (in the UBVRIHK and Washington CMT1 bandpasses), and surface brightness fluctuation magnitudes and colors, ranging from far-UV to near-IR have been consistently computed for simple stellar populations from our evolutionary population synthesis code. The age range of -4 Gyr < Delta_t < +4 Gyr is explored, where Delta_t = 0 Gyr is the currently favored mean age of inner halo Galacitc globular clusters (Galactocentric radius < 8 kpc), ~ 12Gyr. The main aim of this dissertation is to investigate the effects of post-red giant branch stars on the integrated spectrophotometric quantities after employing detailed systematic variation of horizontal-branch morphology with age and metallicity.

The age of the solar neighbourhood

High-quality Hipparcos data for a complete sample of nearly 12 000 main-sequence and subgiant stars, together with Padua isochrones, are used to constrain the star formation history of the solar neigbourhood and the processes that stochastically accelerate disc stars. The velocity dispersion of a coeval group of stars is found to increase with time from ∼8 km s−1 at birth as t0.33. In the fits, the slope of the initial mass function (IMF) near 1 M⊙ proves to be degenerate with the rate at which the star formation rate declines. If the slope of the IMF is to lie near Salpeter's value, −2.35, the star formation rate has to be very nearly constant. The age of the solar neighbourhood is found to be 11.2±0.75 Gyr with remarkably little sensitivity to variations in the assumed metallicity distribution of old disc stars. This age is only a gigayear younger than the age of the oldest globular clusters when the same isochrones and distance scale are employed. It is compatible with current indications of the redshift of luminous galaxy formation only if there is a large cosmological constant. A younger age is formally excluded because it provides a poor fit to the number density of red stars. Since this density is subject to a significantly uncertain selection function, ages as low as 9 Gyr are plausible even though they lie outside our formal error bars.

Models for Old, Metal‐Poor Stars with Enhanced α‐Element Abundances. II. Their Implications for the Ages of the Galaxy’s Globular Clusters and Field Halo Stars

The ages of globular clusters (GCs) and post-turnoff field stars in the Galactic halo are derived using new stellar evolutionary models that explicitly take into account the observed abundances of the α-elements. Whereas the distances of the field subgiants considered in this study are based on Hipparcos parallax measurements, theoretical zero-age horizontal-branch (ZAHB) loci have been used to set the GC distance scale. (As reported in Paper I, the latter imply RR Lyrae luminosities that are within 0.1-0.15 mag, on the high side, of those found for field variables from Baade-Wesselink, trignometic parallax, and statistical parallax studies.) Both the field and cluster observations indicate that the most metal-deficient stars are 14 Gyr old while those with [Fe/H] -1.3 are 2-3 Gyr younger. Unless the O/Fe (or, more generally, the α/Fe) number abundance ratio rises quite steeply with decreasing [Fe/H], it seems unlikely that a significant age-metallicity relation can be avoided. Using what is effectively the ΔV method of determining relative cluster ages, we find that the dispersion in age at any [Fe/H] less than ≈-1.0 is small (10%-15%). Even Arp 2 and IC 4499, which had been previously categorized as young GCs, probably have near-normal ages for their metallicities—though a more definitive conclusion must await improved photometry for these systems. Ruprecht 106 appears no more than 1-1.5 Gyr younger than M3, as opposed to the ~4 Gyr age difference that others have found. However, Palomar 12 and Terzan 7 are undoubtedly young, and they provide strong evidence that, at [Fe/H] -1.0, the ages of GCs differ by as much as 4 Gyr (or ~25%). (A much larger sample of metal-rich clusters must be studied to ascertain whether the distribution of ages at higher metallicities is broad or narrow: Pal 12 and Ter 7 could well be atypical and represent the tail of a distribution that is strongly peaked near 12 Gyr.) Importantly, for the best-observed systems, there is no obvious conflict between the relative age estimates based on the ΔV method, on the one hand, and the Δ(B-V)TO,RGB technique, on the other. However, there are some exceptions, like M68, M53, M13, and NGC 288 for which the two approaches give slightly different results, indicating that something in addition to, or besides, age must be playing a role. From isochrone/ZAHB fits to the C-M diagrams (CMDs) of the outer-halo GCs Pal 3, Pal 4, and Eridanus, we conclude that there is no more than a small dependence of age on Galactocentric distance. Pal 4 seems to have very close to the same age as NGC 362, NGC 1261, and NGC 1851, while Eridanus and Pal 3 appear to be 1 Gyr younger than most inner-halo systems of the same metallicity. Unless our understanding of the HB phase of evolution is seriously in error, cluster-to-cluster age differences (at a given metallicity) are much too small for age to be the most important of the possible second parameters in determining the morphology of the horizontal branch (the first parameter being [Fe/H]). Finally, we suggest that the GC distance scale as inferred from studies of nearby RR Lyraes is not necessarily in conflict with that based on local subdwarfs. A reconciliation of the two approaches is possible simply by adopting a particular metallicity scale for the globular clusters—one that is much closer to the Zinn-West scale than to that recently proposed by Carretta & Gratton. Indeed, such a simple way of achieving consistency between subdwarf-based distances and those inferred from RR Lyraes seems compelling. If this suggestion is correct, then the long distance scale and low estimates of GC ages are not tenable. In fact, the adoption of the short distance scale, implying ages 15 Gyr for the most metal-poor GCs (and 18.30 for the true distance modulus of the Large Magellanic Cloud), leads to much improved agreement between synthetic and observed CMDs in the vicinity of the turnoff. It does cause a mismatch between predicted and observed HB luminosities, in the sense that the models are too bright by ~0.1-0.15 mag, but this may simply be an indication that current conductive opacities or the assumed chemical abundance parameters (Y, [O/Fe], and/or [α/Fe]) are not quite right.

The age of globular clusters

I review here recent developments which have affected our understanding of both the absolute age of globular clusters and the uncertainties in this age estimate, and comment on the implications for cosmological models. This present estimate is in agreement with the range long advocated by David Schramm. The major uncertainty in determining ages of globular clusers based upon the absolute magnitude of the main sequence turn-off remains the uncertainty in the distance to these clusters. Estimates of these distances have recently been upwardly revised due to Hipparcos parallax measurements, if one calibrates luminosities of main sequence stars. However, it is important to realize that at the present time, different distance measures are in disagreement. A recent estimate is that the oldest clusters are 11.5±1.3 Gyr, implying a one-sided 95% confidence level lower limit of 9.5 Gyr, if statistical parallax distance measures are not incorporated. Incorporating more recent measures, including Hipparcos-based statistical parallax measures, raises the mean predicted age to 12.8±1 Gyr, with a 95% confidence range of 10–17 Gyr. I conclude by discussing possible improvements which may allow a more precise age distribution in the near future.

Distances, Ages, and Epoch of Formation of Globular Clusters

We review the results on distances and absolute ages of galactic globular clusters (GCs) obtained after the release of the Hipparcos catalogue. Several methods for the Population II local distance scale are discussed, exploiting NEW RESULTS for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the so-called Short and Long Distance Scales may be reconciled whether a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Distances and ages for the 9 clusters discussed in Paper I are re-derived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole Hipparcos catalogue. On average, our revised distance moduli are decreased by 0.04 mag with respect to Paper I. The corresponding age of the GCs is t=11.5+-2.6 Gyr (95% confidence range). The relation between Mv(ZAHB) and metallicity for the nine programme clusters turns out to be Mv(ZAHB)=(0.18+-0.09)([Fe/H]+1.5)+(0.53+-0.12).Thanks to Hipparcos the major contribution to the total error budget associated with the subdwarf fitting technique has been moved from parallaxes to photometric calibrations, reddening and metallicity scale. This total uncertainty still amounts to about +-0.12 mag. Comparing the corresponding (true) LMC distance modulus 18.64+-0.12 mag with other existing determinations, we conclude that at present the best estimate for the distance of the LMC is: 18.54+-0.03+-0.06, suggesting that distances from the subdwarf fitting method are 1 sigma too long. Consequently, our best estimate for the age of the GCs is revised to: Age = 12.9+-2.9 Gyr (95% confidence range). The best relation between Mv(ZAHB) and [Fe/H] is: Mv(ZAHB) =(0.18+-0.09)([Fe/H]+1.5)+(0.63+-0.07).

Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way

Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way

Testing Population Synthesis Models with Globular Cluster Colors.

We have measured an extensive set of UBVRIJHK colors for M31 globular clusters. We compare the predicted simple stellar population colors of three population synthesis models to the intrinsic colors of Galactic and M31 globular clusters. The best-fitting models fit the cluster colors very well: the weighted mean color offsets are all smaller than 0.05 mag. The most significant offsets between model and data are in the U and B passbands; these are not unexpected and are likely caused by problems with the spectral libraries used by the models. The metal-rich clusters (&sqbl0;Fe&solm0;H&sqbr0; greater, similar-0.8) are best fit by young (8 Gyr) models, while the metal-poor clusters are best fit by older (12-16 Gyr) models. If this range of globular cluster ages is correct, it implies that conditions for cluster formation must have existed for a substantial fraction of the galaxies' lifetimes.