Cluster Horizontal Branch Research Articles

A Search for Variable Stars in the Globular Cluster M4 with K2

Abstract We extract light curves for 4554 objects with 9 < G < 19 in the K2 superstamp observations of the globular cluster M4, including 3784 cluster members, and search for variability. Among cluster-member objects, we detect 66 variables, of which 52 are new discoveries. Among objects not belonging to the cluster, we detect 24 variables, of which 20 are new discoveries. We additionally discover 57 cluster-member suspected variables, 10 cluster-non-member suspected variables, and four variables with ambiguous cluster membership. Our light curves reach sub-millimagnitude precision for the cluster horizontal branch, permitting us to detect asteroseismic activity in six horizontal branch stars outside the instability strip and one inside the strip but with only ∼1 mmag amplitude variability. Nineteen additional stars along the red giant branch also have detected asteroseismic variability. Several eclipsing binaries are found in the cluster, including a 4.6 day detached eclipsing binary and an EW-class eclipsing binary, as well as an EW with uncertain cluster membership and three other candidate EWs. A 22 day detached eclipsing binary is also found outside the cluster. We identify a candidate X-ray binary that is a cluster member with quiescent and periodic ∼20 mmag optical variability. We also obtain high-precision light curves for 10 of the previously known RR Lyrae variables in the cluster and identify one as a candidate Blazhko variable with a Blazhko period in excess of 78 days. We make our light curves publicly available.

The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope (UVIT)

Abstract We present the UV photometry of the globular cluster NGC 1851 using images acquired with the Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. Point-spread function fitting photometric data derived from images in two far-UV (FUV) filters and one near-UV (NUV) filter are used to construct color–magnitude diagrams (CMDs), in combination with HST and ground-based optical photometry. In the FUV, we detect only the bluest part of the cluster horizontal branch (HB); in the NUV, we detect the full extent of the HB, including the red HB, blue HB, and a small number of RR Lyrae stars. UV variability was detected in 18 RR Lyrae stars, and three new variables were also detected in the central region. The UV/optical CMDs are then compared with isochrones of different age and metallicity (generated using Padova and BaSTI models) and synthetic HB (using helium-enhanced Y 2 models). We are able to identify two populations among the HB stars, which are found to have either an age range of 10–12 Gyr, or a range in Y ini of 0.23–0.28, for a metallicity of [Fe/H] = −1.2 to −1.3. These estimations from the UV CMDs are consistent with those from optical studies. The almost-complete sample of the HB stars tends to show a marginal difference in spatial/azimuthal distribution among the blue and red HB stars. Thus, this study showcases the capability of UVIT, with its excellent resolution and large field of view, to study the hot stellar population in Galactic globular clusters.

Multiple populations along the asymptotic giant branch of the globular cluster M4

Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and horizontal branch stellar evolution models. Here we have investigated this issue by employing the CUBI= (U-B)-(B-I) index, that previous studies have demonstrated to be very effective in separating multiple populations along both the red giant and asymptotic giant branch sequences. We confirm previous results that the RGB is intrinsically broad in the V-CUBI diagram, with the presence of two components which nicely correspond to the two populations identified by high-resolution spectroscopy. We find that AGB stars are distributed over a wide range of CUBI values, in close analogy with what is observed for the RGB, demonstrating that the AGB of M4 also hosts multiple stellar populations.

A DOUBLE WHITE-DWARF COOLING SEQUENCE IN ω CENTAURI

We have applied our empirical-PSF-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {\omega} Cen, and found a well-defined split in the right part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of ~2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component (~0.55 Msun CO-core DA objects) while the red WDCS hosts the end-products of the He-rich population (~0.46 Msun objects, ~10% CO-core and ~90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He-ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

DOUBLE HORIZONTAL BRANCHES IN NGC 6440 AND NGC 6569 UNVEILED BY THE VVV SURVEY

We report the discovery of a peculiar horizontal branch (HB) in NGC 6440 and NGC 6569, two massive and metal-rich Galactic globular clusters (GGCs) located in the Galactic bulge, within 4 kpc from the Galactic Center. In both clusters, two distinct clumps are detected at the level of the cluster HB, separated by only ~ 0.1 magnitudes in the Ks band. They were detected with IR photometric data collected with the "VISTA Variables in the Via Lactea" (VVV) Survey, and confirmed in independent IR catalogs available in the literature, and HST optical photometry. Our analysis demonstrates that these clumps are real cluster features, not a product of field contamination or interstellar reddening. The observed split HBs could be a signature of two stellar sub-populations with different chemical composition and/or age, as recently found in Terzan 5, but it cannot be excluded that they are caused by evolutionary effects, in particular for NGC 6440. This interpretation, however, requires an anomalously high helium content (Y > 0.30). Our discovery suggests that such a peculiar HB morphology could be a common feature of massive, metal-rich bulge GGCs.

A search for magnetic fields in cool sdB stars

AbstractHot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (Teff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with Teff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Horizontal branch evolution, metallicity, and sdB stars

Context. Abundance anomalies have been observed in field sdB stars and in nearly all Horizontal Branch (HB) stars of globular clusters with Teff > 11 000K whatever be the cluster metallicity. Aims. The aim is to determine the abundance variations to be expected in sdB stars and in HB stars of metallicities Z \geq 0.0001 and what observed abundances teach us about hydrodynamical processes competing with atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative acceleration, have been computed from the zero age main-sequence for metallicities of Z0 = 0.0001, 0.001, 0.004 and 0.02. On the HB the masses were selected to cover the Teff interval from 7000 to 37000K. Some 60 evolutionary HB models were calculated. The calculations of surface abundance anomalies during the horizontal branch depend on one parameter, the surface mixed mass. Results. For sdB stars with Teff < 37000K and for HB stars with Teff > 11 000K in all observed clusters, independent of metallicity, it was found that most observed abundance anomalies (even up to ~ x 200) were compatible, within error bars, with expected abundances. A mixed mass of ~1.E-7 M\odot was determined by comparison with observations. Conclusions. Observations of globular cluster HB stars with Teff > 11 000K and of sdB stars with Teff < 37 000K suggest that most observed abundance anomalies can be explained by element separation driven by radiative acceleration occuring at a mass fraction of ~1.E-7 M\odot. Mass loss or turbulence appear to limit the separation between 1.E-7 M\odot and the surface.

A NEW CENSUS OF THE VARIABLE STAR POPULATION IN THE GLOBULAR CLUSTER NGC 2419

We present B, V, and I CCD light curves for 101 variable stars belonging to the globular cluster NGC 2419, 60 of which are new discoveries, based on data sets obtained at the Telescopio Nazionale Galileo, the Subaru telescope, and the Hubble Space Telescope. The sample includes 75 RR Lyrae stars (38 RRab, 36 RRc, and one RRd), one Population II Cepheid, 12 SX Phoenicis variables, two δ Scuti stars, three binary systems, five long-period variables, and three variables of uncertain classification. The pulsation properties of the RR Lyrae variables are close to those of Oosterhoff type II clusters, consistent with the low metal abundance and the cluster horizontal branch morphology, disfavoring (but not totally ruling out) an extragalactic hypothesis for the origin of NGC 2419. The observed properties of RR Lyrae and SX Phoenicis stars are used to estimate the cluster reddening and distance, using a number of different methods. Our final value is μ0 (NGC 2419) = 19.71 ± 0.08 mag (D = 87.5 ± 3.3 kpc), with E(B − V) = 0.08 ± 0.01 mag, [Fe/H] = −2.1 dex on the Zinn & West metallicity scale, and a value of MV that sets μ0 (LMC) = 18.52 mag. This value is in good agreement with the most recent literature estimates of the distance to NGC 2419.

THE VARIABLE STAR POPULATION OF THE GLOBULAR CLUSTER B514 IN THE ANDROMEDA GALAXY

A rich harvest of RR Lyrae stars has been identified for the first time in B514, a metal-poor ([Fe/H] = 1.95 +/- 0.10 dex) globular cluster of the Andromeda galaxy (M31), based on Hubble Space Telescope Wide Field Planetary Camera 2 and Advanced Camera for Surveys time-series observations. We have detected and derived periods for 89 RR Lyrae stars (82 fundamental-mode -RRab- and 7 first-overtone -RRc- pulsators, respectively) among 161 candidate variables identified in the cluster. The average period of the RR Lyrae variables (<Pab> = 0.58 days and <Pc> = 0.35 days, for RRab and RRc pulsators, respectively) and the position in the period-amplitude diagram both suggest that B514 is likely an Oosterhoff type I cluster. This appears to be in disagreement with the general behaviour of the metal-poor globular clusters in the Milky Way, which show instead Oosterhoff type II pulsation properties. The average apparent magnitude of the RR Lyrae stars sets the mean level of the cluster horizontal branch at <V(RR)> = 25.18 +/- 0.02 (sigma=0.16 mag, on 81 stars). By adopting a reddening E(B-V) = 0.07 +/- 0.02 mag, the above metallicity and M_V=0.44 +/- 0.05 mag for the RR Lyrae variables of this metallicity, we derive a distance modulus of mu_0=24.52 +/- 0.08 mag, corresponding to a distance of about 800 +/- 30 kpc, based on a value of M_V that sets mu_0(LMC)=18.52.

Environmental effects on the globular cluster blue straggler population: a statistical approach

Context. Blue stragglers stars (BSS) constitute an ubiquitous population of objects whose origin involves both dynamical and stellar evolution. Aims. In this paper we study the properties of a catalogue of BSS extracted from an homogeneous sample of 56 Galactic globular clusters (GC) observed with Wide-Field Planetary Camera 2 on board of Hubble Space Telescope (WFPC2/HST). Methods. With the purpose of investigating the environmental dependence of the BSS formation mechanisms, we explore possible monovariate relations between the frequency of BSS (divided in different subsamples according to their location with respect to the parent cluster core radius and half-mass radius) and the main parameters of their host GC. We also performed a principal component analysis (PCA) to extract the main parent cluster parameters, which characterise the BSS family. Results. We find that any subpopulation of BSS strongly depends on the luminosity of the cluster, on the extension of the cluster horizontal branch, and on the central velocity dispersion: more luminous clusters and clusters with a smaller central velocity dispersion have a higher BSS frequency. Moreover, we find that clusters having higher mass, higher central densities, and smaller core relaxation timescales have, on average, more luminous BSS. Finally, different dependencies seem to hold for clusters with different integrated luminosity: brighter clusters show a BSS population that depends on the collisional parameter, while BSS in fainter clusters are mostly influenced by the cluster luminosity and the dynamical timescales.

A Triple Main Sequence in the Globular Cluster NGC 2808

Accurate photometry with HST/ACS shows that the main sequence of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with complexities of the cluster's horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter

Aims. The interpretation of globular cluster horizontal branch (HB) morphology is a classical problem that can significantly blur our understanding of stellar populations. Methods. We present a new multivariate analysis connecting the effective temperature extent of the HB with other cluster parameters. The work is based on Hubble Space Telescope photometry of 54 Galactic globular clusters. Results. The present study reveals the important role of the total mass of the globular cluster on its HB morphology. More massive clusters tend to have HBs more extended to higher temperatures. For a set of three input variables including the temperature extension of the HB, [Fe/H] and M_V, the first two eigenvectors account for 90% of the total sample variance. Conclusions. Possible effects of cluster self-pollution on HB morphology, stronger in more massive clusters, could explain the results derived here.

Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808

the date of receipt and acceptance should be inserted later Abstract. We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808. Our measurements are based on moderate-resolution spectra taken with the multi-object fiber facility FLAMES- UVES, mounted on Kueyen at the Very Large Telescope. We confirm that stars hotter than a threshold temperature have super-solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. (Fe/H)≃-1.1. The threshold temperature is estimated to be about 12 000 K, corresponding to the so called u-jump, and coincides with the sudden departure of the cluster horizontal branch from the models. The metallicity increases with temperature for star hotter than the ju mp, confirming the hypothesis that the process responsible for t his abrupt metallic enhancement is the levitation due to the strong radiation field in absence of a significative convective enve lope. A metallicity dependence of the abundance enhancement is also suggested, with more metal poor clusters having a higher increase in metal content. The slope in the temperature vs. abundance diagram is higher than the errors involved, and the metal content of the cluster plays possibly a role in determining the a mplitude of the jump (more metal poor clusters show more enhancement after the jump), although other parameters, such as clusters ' characteristics and even the atomic species involved, may also someway contribute.

Discovery of More than 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue Horizontal Branch

We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which places it among the top three most RR Lyrae-rich globular clusters known. Likely members of the cluster in our studied field, from our preliminary number counts, include ≈130 fundamental-mode (RRab) pulsators, with Pab = 0.548 days, and ≈75 first-overtone (RRc) pulsators, with Pc = 0.300 days. The average periods and the position of the RRab variables with well-defined light curves in the Bailey diagram both suggest that the cluster is of Oosterhoff type I. However, the morphology of the cluster's horizontal branch (HB) is strikingly similar to that of the Oosterhoff type II globular cluster M15 (NGC 7078), with a dominant blue HB component and a very extended blue tail. Since M15 and M62 differ in metallicity by about 1 dex, we conclude that metallicity, at a fixed HB type, is a key parameter determining the Oosterhoff status of a globular cluster and the position of its variables in the Bailey diagram.

A Comparison between Carbon and Nitrogen Abundances of Bright Giants in the Globular Clusters M13, M10, and NGC 7006

Carbon and nitrogen abundances have been derived for red giants in the globular clusters M13, M10, and NGC 7006. These constitute a trio of clusters having very similar metallicities ([Fe/H] ≈ -1.55) but different horizontal-branch (HB) morphologies, with NGC 7006 having a much higher fraction of red HB stars than either M13 or M10. The red giants observed have absolute magnitudes in the range -2.5 < MV < 0.7 in the case of M13, -1.8 < MV < 0.4 for M10, and -2.1 < MV < -1.2 for the more distant NGC 7006. Most of the spectra were obtained with the Lick Observatory 3 m Shane reflector, supplemented by observations of M10 with the McDonald Observatory 2.7 m telescope. These data were reduced in a homogeneous manner and compared with synthetic spectra computed from model atmospheres in order to derive [C/Fe] abundances from the λ4300 G band and [N/Fe] abundances from the λ3883 CN band. The giants in all three clusters appear to exhibit a common relationship between [C/Fe] and MV in which the carbon abundance becomes increasingly diminished with increasing luminosity on the giant branch. Such behavior was previously discovered for M13 by Suntzeff and is known to occur in other low-metallicity globular clusters, such as M3, M92, and M15, as well as among halo field red giants. The phenomenon is widely recognized as indicating a progressive dredge-up of CNO-processed material from regions close to the H-burning shell within Population II giants. The similarities between the [C/Fe]-MV trends in the three second-parameter clusters indicates that the deep-mixing mechanism is relatively uncorrelated with cluster HB type. This result may be inconsistent with scenarios that rely on rotation to both drive deep mixing within metal-poor red giants and to set the eventual zero-age HB position of these stars, unless there is a threshold for rotation-induced mixing that is well below that needed to delay the He-flash. The nitrogen abundance is found to differ by as much as 1.0 dex between giants in the program clusters and to be anticorrelated with the carbon abundance. It is speculated that the behavior of [N/Fe] with respect to MV can be interpreted as a combination of deep mixing and primordial contributions.

Measuring the Slope of the Dust Extinction Law and the Power Spectrum of Dust Clouds Using Differentially Reddened Globular Clusters

We present three methods for measuring the slope of the Galactic dust extinction law, RV, and a method for measuring the fine-scale structure of dust clouds in the direction of differentially reddened globular clusters. We apply these techniques to BVI photometry of stars in the low-latitude Galactic globular cluster NGC 4833, which displays spatially variable extinction/reddening about a mean AV ≈ 1. An extensive suite of Monte Carlo simulations is used to characterize the efficacy of the methods. The essence of the first two methods is to determine, for an assumed value of RV, the relative visual extinction δAV of each cluster horizontal-branch (HB) star with respect to an empirical HB locus; the locus is derived from the color-magnitude diagram (CMD) of a subset of stars in a small region near the cluster center for which differential extinction/reddening are relatively small. A star-by-star comparison of δAV from the (B - V, V) CMD with that from the (V - I, V) CMD is used to find the optimal RV. In the third method RV is determined by minimizing the scatter in the HB in the (B - V, V) CMD after correcting the photometry for extinction and reddening using the dust maps of Schlegel, Finkbeiner, & Davis. The weighted average of the results from the three methods gives RV = 3.0 ± 0.4 for the dust along the line of sight to NGC 4833. The fine-scale structure of the dust is quantified via the difference, (ΔAV)ij ≡ (δAV)i - (δAV)j, between pairs of cluster HB stars (i, j) as a function of their angular separation rij. The variance (mean square scatter) of (ΔAV)ij is found to have a power-law dependence on angular scale: var(r) ∝ rβ, with β = +0.9 ± 0.1. This translates into an angular power spectrum P(κ) ∝ κα, with the index α = -1.9 ± 0.1 for r ~ 1'–5', where κ ≡ 1/r. The dust angular power spectrum on small scales (from optical data) matches smoothly onto the larger scale power spectrum derived from Schlegel et al.'s far-infrared map of the dust thermal emission.

Mass-loss predictions for Subdwarf B stars

We present the results of Monte Carlo mass-loss computations for hot low-mass stars, specifically for subdwarf B (sdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create sdB stars. We argue, however, that mass loss plays a role in the chemical abundance patterns observed both in field sdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manqué, UV-bright) stars, over a range in effective temperatures between ≃10 000 and 50 000 K. Finally, we present preliminary spectral synthesis on the more luminous sdB stars for which emission cores in Hα have been detected (Heber, U., et al.: 2003, in:Stellar Atmosphere Modeling, ASP Conference Proceedings, p. 251). We find that these line profiles can indeed be interpreted as the presence of a stellar wind with mass loss on the order of 10−11 M ⊙ yr −1.

CCD Photometry of the Globular Cluster NGC 4833 and Extinction Near the Galactic Plane

We present CCD photometry for the Galactic globular cluster NGC 4833. Our BVI color-magnitude diagrams extend from above the tip of the red giant branch to several magnitudes below the main-sequence turnoff. The principal sequences of the cluster show the effects of differential reddening. We have created a local extinction map, consistent with IRAS and COBE DIRBE dust maps of the region. We use our map to correct the colors and magnitudes of each star to the value at the cluster center. The cluster horizontal branch (HB) is predominately blueward of the instability strip with 13 confirmed RR Lyrae variables and five additional RR Lyrae candidates. Using the 11 confirmed RR Lyraes measured on our images and the differential reddening corrected photometry, we calculate V(HB) = 15.56 ± 0.063. We have used the simultaneous reddening and metallicity method of Sarajedini to find the mean reddening of the cluster E(B-V) = 0.32 ± 0.03, and the mean metallicity [Fe/H] = -1.83 ± 0.14. As for the age of NGC 4833, we find provisional evidence that NGC 4833 is 2 ± 1 Gyr older than the average of M92 and M5. A more definitive conclusion must await higher resolution imaging of the cluster core where the effects of differential reddening are minimized.

Photometry of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: The Age-Metallicity Relation

We present deep VI-band photometry of the globular cluster M54, a nearby field in the Sagittarius dwarf galaxy, and a control field. The color-magnitude diagrams reach well below the oldest main-sequence turnoffs, thus enabling an analysis of the galaxy's age-metallicity relation with unprecedented clarity. We also study the variable stars in the direction of M54. From 67 RR Lyrae variables, we confirm and improve on our previous estimates of the cluster horizontal branch magnitude, foreground reddening, and horizontal branch morphology. These values are used in determining the ages of M54 and the Sagittarius field populations. We confirm our previous result that M54 is the same age as Galactic globular clusters of similar metallicity. We also derive ages on a self-consistent scale for the other three globular clusters in Sagittarius. We find strong evidence for multiple episodes of star formation (or continuous star formation with a variable rate) in the field of Sagittarius. We characterize the principal episodes with the ages 11, 5, and 0.5 through 3 Gyr and with [Fe/H] values of -1.3, -0.7, and -0.4, respectively. On this scale, M54 has an age of 15 Gyr. Surprisingly, the age-metallicity relation we have derived for the galaxy as a whole is described quite well by a closed-box chemical evolution model. We also find that the populations associated with the Sgr field are clumped spatially around M54, and we consider several explanations for this phenomenon. We again speculate that Sagittarius is a nucleated dwarf elliptical galaxy with M54 as its nucleus.

CLC][ITAL]HUBBLE SPACE TELESCOPE[/ITAL][/CLC][ITAL]Hubble Space Telescope[/ITAL] Photometry of the Fornax Dwarf Spheroidal Galaxy: Cluster 4 and Its Field

Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dwarf spheroidal galaxy and its surrounding field. These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations. The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal-branch (HB) stars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the positions of the HBs of the five globular clusters in Fornax, which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main sequence suggests that star formation began in the field 12 Gyr ago and continued to 0.5 Gyr ago. There are separate subgiant branches in the CMD, which indicates that the star formation was not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H] -2.0. This is considerably lower than estimates from the integrated light of the cluster, and the origins of this discrepancy are discussed. Cluster 4 has a very red HB and is, therefore, a prime example of the second-parameter effect. Comparisons of cluster 4 with the other Fornax clusters and with M68, a very metal-poor globular cluster of the Galactic halo, reveal that cluster 4 is 3 Gyr younger than these other clusters, which have much bluer HBs. This age difference is consistent with the prediction that age is the second parameter to within the uncertainties. The CMD of cluster 4 is virtually identical to that of the unusual globular cluster of the Galactic halo Ruprecht 106, which suggests that they have very similar ages and chemical compositions. We discuss the possibility that cluster 4 also resembles R106 in having a higher [Fe/H] than is indicated by its steep RGB and also a lower [α/Fe] ratio than is usual for a globular cluster, as indicated by some recent observations of R106. The CMDs of the five Fornax clusters indicate that cluster age is a major—but probably not the sole—second parameter. Buonanno et al. recently concluded that cluster density probably influenced the HB morphologies of clusters 1, 2, 3, and 5. Despite a very large difference in central density, the HBs of cluster 4 and R106 are very similar. This suggests that density may act as a second parameter in clusters that have HBs that are on the verge of moving toward the blue or are already blue for another reason, such as very old age.