Placing the Fornax and Sagittarius Dwarf Spheroidal Globular

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We plot the globular clusters of the Fornax galaxy and those associated with the Sagittarius dwarf spheroidal galaxy in the horizontal-branch type versus ...
THE ASTRONOMICAL JOURNAL, 115 : 2369È2373, 1998 June ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

PLACING THE FORNAX AND SAGITTARIUS DWARF SPHEROIDAL GLOBULAR CLUSTERS IN THE HORIZONTAL-BRANCH TYPE VERSUS METALLICITY DIAGRAM EDGAR O. SMITH, R. MICHAEL RICH, AND JAMES D. NEILL Department of Astronomy, Columbia University, Mail Code 5242, 538 West 120th Street, New York, NY 10027 ; edgar=astro.columbia.edu, rmr=astro.columbia.edu, neill=astro.columbia.edu Received 1997 December 29 ; revised 1998 February 18

ABSTRACT We plot the globular clusters of the Fornax galaxy and those associated with the Sagittarius dwarf spheroidal galaxy in the horizontal-branch type versus metallicity diagram. The horizontal-branch types for the Fornax clusters include corrections for red horizontal-branch stars from the Ðeld and are based on our recent work and new results in the literature. Fornax globular clusters continue to stand out as having red horizontal branches for their low ([Fe/H] D [2) metallicities, with no counterparts in either the outer Galactic halo or the Magellanic Clouds. The clusters associated with Sagittarius lie to the blue of the Fornax clusters, except for the metal-rich cluster Ter 7. Although the metallicities of the three metal-poor Sagittarius globular clusters are similar to those of the Fornax clusters, their horizontal branches are bluer and they lie in a region also populated by the old LMC and old halo clusters. Neither cluster system resembles the younger Galactic halo globular clusters, often suggested to have been accreted from disrupted dwarf spheroidal galaxies. Except for Ter 7, both the Fornax and Sagittarius globular clusters are metal-poor compared with their Galactic counterparts of the same horizontal-branch type. We Ðnd no correlation between HB type and other cluster properties such as central concentration, luminosity, central surface brightness, and estimated collision rate. Key words : galaxies : dwarf È galaxies : halos È galaxies : individual (Fornax, Sagittarius) È galaxies : star clusters È globular clusters : general È stars : horizontal-branch 1.

The Fornax globular clusters have, based on their kinematics and spatial location, a clear association with their parent galaxy (see Mateo et al. 1991). While tidal e†ects have spread Ter 7, Ter 8, Arp 2, and M54 across the entire photometric extent of the Sgr dwarf, their association with Sgr is secure (Da Costa & Armandro† 1995 ; Ibata et al. 1997). It has also been noted that the four globular clusters Pal 12, Arp 2, Ter 7, and Ter 8 lie on a great circle and, possibly, on common orbits consistent with accretion by the Galaxy (Buonanno et al. 1994 ; Salaris & Weiss 1997). Horizontal-branch morphologies are an important way to compare globular clusters. It has long been known that the color distribution of stars on the horizontal branch (the HB morphology) has a strong correlation with metallicity. However, globular clusters of the same metallicity may have very di†erent HB morphologies, the explanation of which requires at least a second parameter. While a number of causes may be responsible for this e†ect, it has been widely suggested that age might be among the best explanations (Lee, Demarque, & Zinn 1994). On the other hand, Stetson, VandenBerg, & Bolte (1996) argue that the evidence does not favor age as the dominant second parameter. The ability to conÐdently interpret HB morphology in terms of relatively Ðne age di†erences would be of great value in the study of distant stellar populations where the turno† is fainter than the detection limit. The HB type parameter, described below, has been calculated for almost all Galactic globular clusters, and a plot of HB type versus [Fe/H] shows that the Fornax globular clusters di†er from the old halo, young halo, and Magellanic Cloud clusters (Zinn 1993). We are now in a position to reconsider the Fornax clusters and to plot the new associated clusters of Sgr in the HB typeÈmetallicity diagram. Smith et al. (1996), Smith, Neill, & Rich (1997), and Beauchamp et al. (1995) present new CCD photometry

INTRODUCTION

A detailed comparison of the globular cluster systems of the Galaxy and its satellites may yield insights into the formation history of the Galactic halo, particularly as to whether the outer halo globular clusters were formed in now disrupted dwarf spheroidal galaxies. Fornax and Sagittarius (Sgr) are the most massive and luminous of the nine Galactic dwarf spheroidals and are the only two to have globular clusters (Hodge 1961 ; Ibata, Gilmore, & Irwin 1994). Zinn (1993) reÐned the Searle & Zinn (1978) picture of an inhomogenous outer halo, proposing that the Galactic globular clusters are a mixture of two categories, the old halo clusters formed in the Galaxy and the young halo clusters formed in destroyed satellites of relatively large mass. Subsequent observations suggest even greater complexity. Harris et al. (1997) Ðnd that the low-metallicity Galactic globular clusters from 7 to 90 kpc are all of similar age, suggesting a common origin but not a simple cloud collapse. Chaboyer, Demarque, & Sarajedini (1996) conclude that the 50% of the outer halo globular clusters that exhibit no strong second-parameter e†ect are members of the old halo, formed at the same time but perhaps accreted later. Unavane, Wyse, & Gilmore (1996) argue that there is little evidence that disruption of satellite galaxies has contributed substantially to the halo Ðeld population. There is growing interest in the idea that dwarf galaxies such as Fornax and Sgr may have contributed many of the clusters in the outer halo, including some of the old halo clusters. This has gained recent impetus from the Ðnding that the Sgr dwarf is close to being tidally disrupted and may be contributing its associated clusters to the halo (Ibata et al. 1997). One test of this hypothesis is to compare the properties of the dwarf spheroidal clusters with those in the outer halo. 2369

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and HB types (including statistical Ðeld star subtraction for clusters 2 and 3) for Fornax clusters 1, 2, 3, and 5. In this paper, we use these data and data from the literature to derive a new HB type versus [Fe/H] diagram and compare the clusters of Fornax and Sagittarius with the clusters of the Galaxy and the Magellanic Clouds. 2.

COMPARISON OF HORIZONTAL-BRANCH TYPES

The HB type parameter (Lee et al. 1994), (B[R)/ (B]V ]R), where B, V , and R refer to blue HB, variable, and red HB stars, is a scale-independent measure of the HB color distribution. We plot in Figure 1 the HB types and metallicities of Fornax and Sagittarius clusters, the old and young halo clusters, and the Magellanic clusters. Zinn (1993) deÐnes the category of young halo clusters, which consists of 20 of the 44 clusters in the range 6 kpc \ R \ GC 40 kpc and Ðve of the six clusters in the remote halo (R [ GC 40 kpc). We also plot the theoretical isochrones calculated by Lee et al. (1994). The HB types of Fornax clusters 1, 3, and 5, including error analysis, were presented by Smith et al. (1996, 1997) and that of cluster 2 is from Beauchamp et al. (1995). The HB types are for cluster 1 a red ([0.50 ^ 0.10), for cluster 2 a blue (0.21), for cluster 3 an intermediate ([0.11 ^ 0.10), and for cluster 5 an intermediate ([0.09 ^ 0.12) color. The calculation of HB type for cluster 3 did not include core helium burning stars from an apparent red clump, also visible in Buonanno et al. (1996), which would have given an even redder HB type. For those clusters that have red clumps, a more inclusive deÐnition of

FIG. 1.ÈPlot of HB type vs. metallicity for globular clusters in the Fornax and Sagittarius dwarf galaxies, the Milky Way (divided into young and old halo), and the Magellanic Clouds. The Fornax and Sgr cluster numbers are indicated. We also identify some outer halo globular clusters of interest. Note that the Fornax clusters lie below the ““ young halo ÏÏ clusters because of their red horizontal branches, while the Sgr clusters fall in the region occupied by the old LMC and old halo clusters. M68 and NGC 4147, while plotted as young halo clusters, have ages as great as old halo clusters (see text). NGC 4147 is the halo cluster next to Arp 2. The isochrones shown are ]2, 0, and [2 Gyr (top to bottom), as in Lee, Demarque, & Zinn (1994).

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the HB type parameter might provide a more accurate way of distinguishing between all clusters. The data for the halo and Magellanic Cloud clusters are from Harris (1996)1 and R. Zinn (1997, private communication), respectively. The Harris data were updated as follows : Rup 106 has been studied at high resolution (FrancÓois et al. 1997 ; Brown, Wallerstein, & Zucker 1997), and we adopt the average [Fe/H] of [1.52. A Keck high-resolution study of NGC 7006 (Kraft et al. 1998) Ðnds [Fe/H] of [1.55 for this prototypical secondparameter cluster. M68 has an [Fe/H] of [2.1 and HB type of 0.44 from Walker (1994). The data for the Sgr clusters come from various sources. For Ter 8 we count 31 B, seven V , and no R, and calculate an HB type of 0.82 ^ 0.08 and use the value for [Fe/H] of [2.00 ^ 0.10 from Montegri†o et al. (1998). For the sparsely populated Ter 7, we count one B, no V , and at most 13 R, and calculate an HB type of [0.86 ^ 0.20 and use the value for [Fe/H] of [0.82 ^ 0.15 (Buonanno et al. 1995 ; Sarajedini & Layden 1997). Arp 2 has an [Fe/H] of [1.79 ^ 0.09 and an HB type of 0.53 ^ 0.17, where the Ðeld contamination has been subtracted (Sarajedini & Layden 1997 ; Sarajedini, Chaboyer, & Demarque 1997). 2.1. T he Horizontal-Branch T ype of M54 M54 has been studied by Sarajedini & Layden (1995), who measure [Fe/H] to be [1.79 ^ 0.08. The colormagnitude diagram of M54 is complicated by the foreground disk population and by stars in the Sgr dwarf lying at the distance of the cluster. Measurement of the HB type requires Ðeld subtraction. Sarajedini & Layden point out that the population of the Sgr dwarf galaxy varies on scales of tens of arcminutes, complicating a subtraction using adjacent Ðelds. We have taken two approaches to the Ðeld subtraction, using the published data of Sarajedini & Layden. First, we subtract Ðeld stars 12@ north of the cluster (their Fig. 4) from cluster members within 200A of the cluster (their Fig. 3). Their frame of M54 covers a large enough area that one may designate stars farther than 200A from M54 as Ðeld stars, producing a subtraction based on the population closest to the cluster. We give the results of our subtractions in Figure 2. The Sgr Ðeld stars adjacent to M54 appear to have a blue HB that is not present in the Ðeld population 12@ north of the cluster. We have used the test that any successful subtraction must preserve the character of the M54 red giant branch (RGB), and while the blue HB is subtracted, some red HB stars remain. Sarajedini points out that the morphology of this red HB is peculiar compared with that of M68, which has comparable metallicity, but many factors can a†ect detailed HB morphology. M54 might have captured stars from the Sgr dwarf, and the shadow RGB and some of the blue horizontal-branch stars could have been acquired in this way. We note the large mass of M54 (106 M ; Illingworth 1976) and its membership in the Sgr dwarf,_which is experiencing tidal disruption by the Milky Way. (Note that the central velocity dispersion of the Sgr dwarf of 11.4 km s~1 [Ibata et al. 1997] is less than that of M54, 14 km s~1.) Bica et al. (1997), seeking to explain a metal-rich RGB in the metal-poor ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 A revised version is available at http ://www.physics.mcmaster.ca/ Globular.html.

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FIG. 2.ÈTwo approaches to Ðeld subtraction of M54 using the photometry of Sarajedini & Layden (1995). The full data set is illustrated in the top panel. We use the statistical subtraction algorithm described in Smith, Neill, & Rich (1997) to subtract the population in a Ðeld 12@ north of M54 from the cluster data (far-Ðeld subtraction ; middle). We also subtract the Ðeld stars with R [ 200A from those with R \ 200A from the center of M54 (near-Ðeld subtraction ; bottom). In both cases, the metal-poor RGB is clearly present in the subtraction. The blue HB may belong to the Ðeld population of Sgr, but the red HB adjacent to the instability strip persists, leading us to conclude that it is intrinsic to the cluster. A sparsely populated metal-rich giant branch and clump also remain ; we speculate that these stars may have been captured by M54.

cluster HP 1, show that globular clusters in the bulge could capture stars. Alternatively, if Sgr experienced multiple bursts of star formation, it is possible that some stars formed bound to M54. Now returning to the HB type of M54, using the adjacent-Ðeld subtraction, we count 18 R, 17 B, and eight V and get an HB type of [0.27, and using the distant-Ðeld subtraction, we count 30 R, 77 B, and six V and get an HB type of 0.42. We average these two values and use in our Figure 1 an HB type of 0.20 and consider the uncertainty in the HB type to span this entire range. Hubble Space Telescope imaging of the core of M54 will be needed to properly disentangle this clusterÏs complications, and a larger Ðeld must be obtained to improve the Ðeld subtraction. However, despite the large mass and high central surface brightness of M54, and our expectation of dynamical e†ects on the HB (perhaps producing a blue tail), we Ðnd evidence of some red HB stars and a surprisingly red HB classiÐcation. While we Ðnd a large collision rate for M54, Djorgovski et al. (1991) found no color gradient, unlike for the postÈcore-collapse clusters in their study. 3.

DISCUSSION

Figure 1 shows that, for low-metallicity clusters, the Fornax clusters have redder HB types than the clusters of the Magellanic Clouds, and in turn the Magellanic Cloud clusters are redder than the Galactic clusters. Old halo clusters with these HB types are nearly an order of magnitude more metal-rich. We conÐrm ZinnÏs (1993) conclusion that

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the Fornax clusters stand apart, but our conclusion is based on our newly determined redder HB types. The Sgr clusters lie between the Fornax clusters and the majority of old halo globular clusters with blue HBs. Recent photometry by Brocato et al. (1996) and Mighell et al. (1996) conÐrms that old, metal-poor LMC globular clusters are probably as old as the oldest Milky Way globular clusters. Two Galactic globular clusters (NGC 4147 and M68) plotted as young halo clusters lie near the Sgr locus. However, NGC 4147 is found by Friel, Heasley, & Christian (1987) to have *V HB \ 3.6 mag, and M68 is found by TO HB \ 3.42 mag, qualifying both as Walker (1994) to have *V TO old halo clusters. We conclude that the Sgr clusters with blue HBs most closely resemble the old LMC and old halo clusters. For metal-rich clusters, the HB type is insensitive to age (being red even for very old clusters), which explains why Ter 7 lies close to the old halo locus despite its demonstrated younger age (Buonnano et al. 1994). We now discuss those Galactic clusters with HB types and metallicities similar to those of the Fornax clusters. There are three low-metallicity Galactic clusters (AM 1, Pal 3, and Rup 106) that are even redder than the Fornax clusters. AM 1 and Pal 3 are in the remote halo at distances of 119 and 93 kpc, respectively. Zinn identiÐes AM 1 as a young halo cluster, but an accurate age determination is needed ; Madore & Freedman (1989) suggest it may have been captured from a satellite galaxy. The cluster Rup 106 is known to be young (Buonanno et al. 1993) and, at a distance of 18 kpc, possibly has been captured from the Magellanic Clouds (Lin & Richer 1992). Other young halo clusters of similar low metallicity and HB type include NGC 7006 at a distance of 38 kpc and Pal 13 at a distance of 27 kpc, having a sparse red HB. Various authors have noted that some of the (apparently youngest) outer halo globular clusters appear to lie on common great circles with similar radial velocities (Lynden-Bell & Lynden-Bell 1995). While we acknowledge that some outer halo clusters may have had a common origin, the issue we address is whether the Fornax and Sgr cluster systems are similar to those halo clusters considered to be young. It is interesting that the Fornax clusters and the metalpoor Sgr clusters illustrate the second-parameter e†ect (red vs. blue HB color at similar metallicity). If age is the second parameter, then the Fornax clusters are younger than the low-metallicity Galactic and Magellanic Cloud clusters (Sarajedini et al. 1997). This would be consistent with the apparent discovery of carbon stars in Fornax 3 (JÔrgensen & Jimenez 1997). If age is not the second parameter, then the extreme position of the Fornax clusters demands another explanation, perhaps anomalous composition, contribution of binaries, or intrinsic stellar angular momentum. Also, the stars in the cores of the clusters may be a†ected by dynamical processes that can alter the color distribution of the HB populations for reasons not yet understood (Djorgovski & Piotto 1993). We now consider dynamics and mass loss as candidate explanations for the wide dispersion of HB types. 3.1. Does HB T ype Depend on Core Dynamics ? We check whether any intrinsic cluster properties could correlate with HB type (cf. Rich et al. 1997). None of our clusters are classiÐed as having collapsed cores by Trager, Djorgovski, & King (1993). The HB type is not correlated with three indicators for strong dynamical e†ects : central

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FIG. 3.ÈPlot of central concentration (top) and the concentration parameter of Fusi Pecci et al. (1993 ; bottom) as a function of HB type for Fornax and Sgr globular clusters. Fusi Pecci et al. (1993) Ðnd a correlation between their concentration parameter and the blue extension of the HB. We Ðnd no correlation between these parameters and HB type. We also Ðnd no dependence of HB type on luminosity or collision rate (see Table 1).

concentration [c \ log (r /r )], c [ 2M , and relative collit Fusi c v al. (1993) Ðnd that sion rates (Table 1 ; Fig. 3). Pecci et c [ 2M is the parameter that most strongly correlates with v the appearance of extended blue HBs. King (1998) Ðnds that the total collision rate of a globular cluster is proportional to (&3 r )1@2, where & is the central surface brightness in 0pc~2 c and r is the 0 core radius in parsecs. M54 has a L V,_ c large velocity dispersion (14 km s~1) and high central surface brightness ; it is a good candidate for interesting dynamical phenomena in its core and worthy of future study, but we Ðnd no evidence that these properties have endowed it with the bluest HB. While the relative collision rates range over 4 orders of magnitude, the metal-poor cluster with the reddest HB (Fornax 1) has the same collision rate as Ter 8, with the bluest HB. 3.2. Sensitivity of HB T ype to Mass L oss at L ow Metallicity Given that dynamics does not appear to be responsible for the dispersion in HB type, could stochastic mass loss be the cause ? In Figure 4 we have updated Fusi Pecci et al.Ïs

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FIG. 4.ÈTotal mass loss required for a red giant to become a zero-age horizontal-branch star with a given e†ective temperature. The hatched region indicates the instability strip. This plot updates Fig. 1 of Fusi Pecci et al. using the HB models of Lee & Demarque (1990). The upper curve for [Fe/H] \ [1.96 is for a 1 Gyr younger population. The curve above the [Fe/H] \ [1.66 locus is derived from A. SweigartÏs (1997, private communication) models ; we Ðnd no signiÐcant di†erence using the newer models. Note that at [Fe/H] \ [1.96 a total mass loss of 0.07 M is _ enough to turn an intrinsically red HB stars into a blue HB star.

(1993, their Fig. 1) consideration of the mass loss required to produce an HB star of given e†ective temperature, using the horizontal-branch models of Lee & Demarque (1990). We also include A. SweigartÏs (1997, private communication) HB models at [Fe/H] \ [1.6 ; these show that adoption of newer opacities has little e†ect on the predicted mass loss. An increase in e†ective temperature in this plot corresponds to changing the HB type from red to blue. At [Fe/H] B [2 and age 15 Gyr, a total mass loss of 0.07 M is sufficient to move a star across the instability strip from_red to blue. At no other metallicity is HB type so sensitive to slight variations in total mass loss. At even lower metallicities, the HB type is always blue, even if no mass is lost, while at higher metallicity increasingly large amounts of total mass must be lost just to reach the red boundary of the instability strip. At [Fe/H] \ [2, a small (1 Gyr) decrease in age can also shift the HB type from blue to red. We modify RoodÏs formula for log M (as given in Renzini 1977, eq. [2.5]) to RG

TABLE 1 RELEVANT PHYSICAL PARAMETERS OF THE CLUSTERS IN THIS STUDY Cluster

R _ (kpc)

[Fe/H]

HB

Fornax 1 . . . . . . . . . . . . . . . . Fornax 2 . . . . . . . . . . . . . . . . Fornax 3 . . . . . . . . . . . . . . . . Fornax 5 . . . . . . . . . . . . . . . . M54 \ NGC 6715 . . . . . . Terzan 7 . . . . . . . . . . . . . . . . . Arp 2 . . . . . . . . . . . . . . . . . . . . Terzan 8 . . . . . . . . . . . . . . . . .

132 132 132 132 26.2 23.0 27.6 25.4

[2.0 ^ 0.2 [1.8 ^ 0.1 [1.93 ^ 0.15 [1.89 ^ 0.15 [1.79 ^ 0.08 [0.82 ^ 0.15 [1.79 ^ 0.09 [2.20 ^ 0.10

[0.50 ^ 0.10 0.21 ^ 0.20 [0.11 ^ 0.10 [0.09 ^ 0.12 0.20 ^ 0.08 [0.86 ^ 0.20 0.53 ^ 0.17 0.82 ^ 0.08

M

v [5.2 [7.3 [8.2 [7.4 [10.0 [6.0 [5.3 [5.0

r c (pc) 11.3 ... ... 1.3 0.84 4.07 12.8 7.4

k

0,V 22.75 ... ... 17.35 14.36 20.46 23.87 22.39

c 0.46 ... ... 1.27 1.84 1.08 0.90 0.60

c [ 2M v 10.86 ... ... 16.07 21.84 13.08 11.50 10.60

N

col 1 ... ... 590 28000 14.2 120 1.3

NOTES.ÈAll data from Harris 1996 except for the data on Fornax clusters, which are from Smith et al. 1996, and the collision rates in the last column, which are calculated using the formula (King 1998) given in the text. The collision rates are all given relative to Fornax 1, which has the reddest HB in the sample. Note that Ter 8 has the same collision rate as Fornax 1 but has the bluest HB in the sample.

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obtain M t RG \ [0.28 log 9 , M 15 RG(15) where M is the mass at the RGB tip for the age in quesRG tion, M is the mass at the RGB tip for a 15 GyrÈold RG(15) star, and t is the age in Gyr. The e†ect of decreasing the age 9 by 1 Gyr is equivalent to adding a constant (0.018 M ) to _ the total mass loss. We applied Lee & DemarqueÏs sequences to a population 1 Gyr younger. Only at [Fe/H] \ [2 is the additional 0.018 M a large fraction of _ the total mass loss required to transform a red to a blue HB star. In considering the placement of the low-metallicity Fornax and Sgr clusters, we see that small changes in one or more of several parameters (e.g., age, metallicity, and total mass loss) can produce a large change in HB type. Considering the ease with which mass loss can transform a red into a blue HB star at low metallicity, it is difficult to understand why there is no correlation between HB type and the cluster physical properties related to collision rates in the core. log

4.

CONCLUSIONS

The Fornax and Sgr cluster systems are separated in metallicity and/or HB type from the Galactic clusters and, therefore, are unlikely examples of the possible protogalac-

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tic fragments that Zinn (1993) proposes are the source of the young halo globular cluster population. The Fornax clusters have horizontal branches so red for their low metallicities that no counterparts are known, either in the young halo of the Galaxy or in the Magellanic Clouds. The cluster members of Sgr resemble the old LMC and those old halo clusters that have low metallicity and blue HB types. Even the suggestion of Da Costa & Armandro† (1995) that the two older Sagittarius clusters, M54 and Ter 8, might be prototypical old halo clusters seems problematic given the revised position of M54 in Figure 1. Our conclusions depend upon accurate measurement of [Fe/H] and ultimately on the detailed composition of the clusters. While we believe the giant branch loci securely support their low [Fe/H], it will be important to obtain high-resolution spectroscopy (especially for the Fornax clusters) when 8 mÈclass telescopes in the Southern Hemisphere become available. We are grateful to R. Zinn for providing the HB types and metallicities for the Magellanic globular clusters and A. Sarajedini and Y. Lee for providing the isochrones shown in Figure 1. We thank M. Shara for suggesting that we correlate HB type with cluster structure. We are also grateful to A. Sweigart for providing his zero-age horizontal-branch models in advance of publication.

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