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Context. Globular clusters are considered key objects for understanding the formation and evolution of the Milky Way. In this sense, the characterisation of their chemical and orbital parameters can provide constraints on chemical evolution models of the Galaxy. Aims. We use the heavy element abundances of globular clusters to trace their overall behaviour in the Galaxy, with the aim to analyse potential relations between the hot H-burning and s-process elements. Methods. We measured the content of Cu I and s- and r-process elements (Y II, Ba II, La II, and Eu II) in a sample of 210 giant stars in 18 galactic globular clusters from high-quality UVES spectra. These clusters span a broad metallicity range and the sample is the largest that has been uniformly analysed to date, with respect to heavy elements in globular clusters. Results. The Cu abundances did not show a considerable spread in the sample, nor any correlation with Na, indicating that the Na nucleosynthesis process does not affect the Cu abundance. Most GCs closely follow the Cu, Y, Ba, La, and Eu field stars' distribution, revealing a similar chemical evolution. The Y abundances in mid-metallicity regime GCs (−1.10 dex < [Fe/H] < −1.80 dex) display a mildly significant correlation with the Na abundance, which ought to be further investigated. Finally, we do not find any significant difference between the n-capture abundances among GCs with either Galactic and extragalactic origins. [ABSTRACT FROM AUTHOR]

Globular clusters have been widely studied in terms of light element variations present in their different stellar populations. However, the nature of the polluter(s) responsible for this phenomenon is still debated. The study of heavy elements and their relation to light ones can provide further constraints. In particular, we aim to explore the possible contribution of asymptotic giant branch stars of different stellar masses to the internal pollution in the cluster. We derive abundances of elements from different nucleosynthetic chains, such as Na, Mg, Ca, Sc, Cu, Y, and Ba. We did not find clear relations between the light s-process elements (represented by Y ii) or heavy ones (represented by Ba ii) with light elements (Li, Na, or Al). This indicates that the polluter(s) responsible for the Na (Al) or Li production does not produce large amounts of Y ii and Ba ii. Furthermore, the comparison with models discards a possible significant contribution to the cluster pollution from AGB stars with masses lower than 5 M⊙. In addition, we found a potential CH star in our sample. [ABSTRACT FROM AUTHOR]

We present the result of a survey of Monte Carlo simulations of globular clusters hosting two generations of stars including a large (⁠|$f_{b}=50{{\ \rm per\ cent}}$|⁠) fraction of primordial binaries in both populations. The dynamical evolution of the two stellar populations is followed for a Hubble time taking into account the effect of the tidal field, two-body relaxation, stellar evolution, and three/four-body interactions. The fraction of surviving binaries, once accounted for the observational bias and uncertainties, is compared with the available radial velocity time-series performed in real globular clusters, and it is used to constrain the initial spatial concentration of the second generation. The fraction of second generation binaries appears to depend only on the ratio between the total cluster mass and the initial size of the second generation that determines the average velocity dispersion across the extent of this stellar population. In spite of the various uncertainties, we find that the observed fraction can be obtained only assuming a strong initial concentration of the second generation (⁠|$r_{h,S}\sim 0.1~(M/10^{6} \, \mathrm{M}_{\odot })\, \mathrm{pc}$|⁠). The evolution of the first generation binary fraction is more sensitive to the tidal field strength (with a non-negligible effect of the cluster orbital eccentricity) since the tidal field has a direct impact on the first generation structural properties. [ABSTRACT FROM AUTHOR]

To understand globular clusters (GCs) we need to comprehend how their formation process was able to produce their abundance distribution of light elements. In particular, we seek to figure out which stars imprinted the peculiar chemical signature of GCs. One of the best ways is to study the light-element anti-correlations in a large sample of GCs that are analysed homogeneously. As part of our spectroscopic survey of GCs with FLAMES, we present here the results of our study of about 30 red giant member stars in the low-mass, low-metallicity Milky Way cluster NGC 6535. We measured the metallicity (finding [Fe/H] = −1.95, rms = 0.04 dex in our homogeneous scale) and other elements of the cluster and, in particular, we concentrate here on O and Na abundances. These elements define the normal Na-O anti-correlation of classical GCs, making NGC 6535 perhaps the lowest mass cluster with a confirmed presence of multiple populations. We updated the census of Galactic and extragalactic GCs for which a statement on the presence or absence of multiple populations can be made on the basis of high-resolution spectroscopy preferentially, or photometry and low-resolution spectroscopy otherwise; we also discuss the importance of mass and age of the clusters as factors for multiple populations. [ABSTRACT FROM AUTHOR]

As part of our on-going project on the homogeneous chemical characterisation of multiple stellar populations in globular clusters (GCs), we studied NGC 5634, associated to the Sagittarius dwarf spheroidal galaxy, using high-resolution spectroscopy of red giant stars collected with VLT/FLAMES. We present here the radial velocity distribution of the 45 observed stars, 43 of which are cluster members, the detailed chemical abundance of 22 species for the seven stars observed with UVES-FLAMES, and the abundance of six elements for stars observed with GIRAFFE. On our homogeneous UVES metallicity scale, we derived a low-metallicity [Fe/H] =-1.867 ± 0.019 ± 0.065 dex (±statistical ±systematic error) with s = 0.050 dex (7 stars). We found the normal anticorrelations between light elements (Na and O, Mg and Al), a signature of multiple populations typical of massive and old GCs. We confirm the associations of NGC 5634 to the Sgr dSph, from which the cluster was lost a few Gyr ago, on the basis of its velocity and position, and the abundance ratios of α and neutron capture elements. [ABSTRACT FROM AUTHOR]

We present the abundance analysis of 82 red giant branch stars in the dense, metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars analysed in this way for this cluster. From high-resolution UVES spectra of 14 stars and intermediate resolution GIRAFFE spectra for the other stars we derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean metal abundance of M 80 is [Fe/H] = -1.791 ± 0.006 ± 0.076 (±statistical ±systematic error) with σ = 0.023 (14 stars). M 80 shows star-to-star variations in proton-capture elements, and the extension of the Na-O anti-correlation perfectly fit the relations with (i) total cluster mass; (ii) horizontal branch morphology; and (iii) cluster concentration previously found by our group. The chemistry of multiple stellar populations in M 80 does not look extreme. The cluster is also a representative of halo globular clusters concerning the pattern of α-capture and Fe-group elements. However we found that a significant contribution from the s-process is required to account for the distribution of neutron-capture elements. A minority of stars in M 80 seem to exhibit slightly enhanced abundances of s-process species, compatible with those observed in M 22 and NGC 1851, although further confirmation from larger samples is required. [ABSTRACT FROM AUTHOR]

We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae variables, and 17 blue horizontal branch (BHB) stars in the low-concentration, moderately metal-rich globular cluster NGC 6723 ([Fe/H] = -1.22 ± 0.08 from our present sample). The spectra were optimized to derive O and Na abundances. In addition, we obtained abundances for other elements, including N, Fe, Mg, Ca, Ni, and Ba. We used these data to discuss the evidence of a connection between the distribution of stars along the horizontal branch (HB) and the multiple populations that are typically present in globular clusters. We found that all RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these stars probably belong to the first stellar generation in this cluster. Only the four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and they probably belong to the second generation. While our sample is not fully representative of the whole HB population in NGC 6723, our data suggest that in this cluster only HB stars warmer than ~9000 K, that is one fourth of the total, belong to the second generation, if at all. Since in many other clusters this fraction is about two thirds, we conclude that the fraction of first/second generation in globular clusters may be strongly variable. In addition, the wide range in colour of chemically homogeneous first-generation HB stars requires a considerable spread in mass loss (>0.10 M⊙). The reason for this spread is yet to be understood. Finally, we found a high Ba abundance, with a statistically significant radial abundance gradient. [ABSTRACT FROM AUTHOR]

We present the results of a spectroscopic survey of red giants in the globular cluster Terzan 8 with the aim of studying its kinematics. We derived accurate radial velocities for 82 stars located in the innermost 7 arcmin from the cluster centre identifying 48 bona fide cluster members. The kinematics of the cluster have been compared with a set of dynamical models accounting for the effect of mass segregation and a variable fraction of binaries. The derived velocity dispersion appears to be larger than that predicted for mass-segregated stellar systems without binaries, indicating that either the cluster is dynamically young or it contains a large fraction of binaries (>30 per cent). We detected seven stars with a radial velocity compatible with the cluster systemic velocity but with chemical patterns which stray from those of both the cluster and the Galactic field. These stars are likely members of the Sagittarius stream surrounding this stellar system. [ABSTRACT FROM PUBLISHER]

Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, and Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is [Fe/H] = -2.015±0.004±0.084 dex (rms = 0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme [O/Na] ratios. More striking is the finding that large star-to-star variations are also seen for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. These extreme changes are also seen in giants of the much more massive GCs M 54 and ω Cen, and our conclusion is that NGC 4833 has probably lost a conspicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit. [ABSTRACT FROM AUTHOR]

We obtained high-resolution spectra for 94 candidate stars belonging to the HB ofM22 with FLAMES. Previous works have indicated that this cluster has split subgiant (SGB) and red giant branches (RGB) and hosts two different stellar populations, differing in overall metal abundance and both exhibiting a Na-O anti-correlation. The HB stars we observed span a restricted temperature range (7800 < Teff < 11 000 K), where about 60% of the HB stars of M 22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor, and O-rich: our abundances for this (cooler) group match those determined for the primordial group of RGB stars (a third of the total) from previous studies very well. Group 2 (23 stars) is still metal-poor, but it is N- and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation and include about 10% of the RGB stars. The third group (20 stars) is metal-rich, Na-rich, and O-rich. This hotter group most likely corresponds to the most O-rich component of the previously found metal-rich RGB population (a quarter of the total). We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. Furthermore, we found that the metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. We do not have C abundances for our stars. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y = 0.338 ± 0.014 ± 0.05); the error bar due to systematics is large, but a consistent analysis of data for several GCs confirms that stars in these groups within M 22 are probably overabundant in He. We conclude that on the whole, our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M 22. Fast rotating stars might be slightly less massive and bluer than slowly rotating ones, but other interpretations are possible. [ABSTRACT FROM AUTHOR]

We obtained FLAMES GIRAFFE+UVES spectra for both first- and second-generation red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them to derive abundances of 21 atomic species for a sample of 92 stars. The surveyed elements include proton-capture (O, Na, Mg, Al, Si), α-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be clustered into two discrete groups along the Na-O anti-correlation with a gap at [O/Na] ~ 0 dex. Na-rich, second generation stars show a trend to be more centrally concentrated, although the level of confidence is not very high. When compared to the classical second-parameter twin NGC 288 with similar metallicity but different horizontal branch type and a much lower total mass, the proton-capture processing in stars of NGC 362 seems to be more extreme, confirming previous analysis. We discovered the presence of a secondary RGB sequence, which is redder than the bulk of the RGB. A preliminary estimate shows that this sequence comprises about 6% of RGB stars. Our spectroscopic data and literature photometry indicate that this sequence is populated almost exclusively by giants rich in Ba and is probably rich in all s-process elements, as found in other clusters. In this regard, NGC 362 joins previously studied GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2). [ABSTRACT FROM AUTHOR]

Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and ω Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K–Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds for both giants and less-evolved stars. At present, NGC 2419 seems unique among GCs. [ABSTRACT FROM AUTHOR]

We present aluminium abundances for a sample of about 100 red giant stars in each of the Galactic globular clusters 47 Tuc (NGC 104) and M 4 (NGC 6121). We have derived homogeneous abundances from intermediate-resolution FLAMES/GIRAFFE spectra. Aluminium abundances are from the strong doublet Al i 8772-8773 Å, as in previous works done for giants in NGC 6752 and NGC 1851, and nitrogen abundances are extracted from a large number of features of the CN molecules by assuming a suitable carbon abundance. We added previous homogeneous abundances of O and Na and newly derived abundances of Mg and Si for our samples of 83 stars in M 4 and 116 stars in 47 Tuc to obtain the full set of elements from proton-capture reactions produced by different stellar generations in these clusters. By simultaneously studying the Ne-Na and Mg-Al cycles of H-burning at high temperature, our main aims are to understand the nature of the polluters at work in the first generation and to ascertain whether the second generation of cluster stars was formed in one or, rather, several episodes of star formation. Our data confirm that in M 4 only two stellar populations are visible. On the other hand, for 47 Tuc a cluster analysis performed on our full dataset suggests that at least three distinct groups of stars are present on the giant branch. The abundances of O, Na, Mg, and Al in the intermediate group can be produced within a pollution scenario; results for N are ambiguous, depending on the C abundance we adopt for the three groups. [ABSTRACT FROM AUTHOR]

To check the impact of the multiple population scenario for globular clusters on their horizontal branch (HB), we present an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M 5. In 47 Tuc we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p-capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small (Δ?Y < 0.03) in 47 Tuc to reproduce our observations; this agrees with previous analyses. There is possibly a correlation between the abundances of p- and n-capture elements in 47 Tuc. If confirmed, this might suggest that asymptotic giant branch stars of moderate mass contributed to the gas from which second-generation stars formed. Considering the selection effects in our sample (which does not include stars warmer than 11 000 K and RR Lyrae variables, which were excluded because we could not obtain accurate abundances with the adopted observing procedure) is important to understand our results for M 5. In this case, we find that, as expected, RHB stars are Na-poor and O-rich, and likely belong to the primordial population. There is a clear correlation of the [Na/O] ratio and N abundance with colour along the BHB. A derivation of the He abundance for these stars yields a low value of Y = 0.22 ± 0.03. This is expected because HB stars of a putative He-rich population in this cluster should be warmer than 11 000 K, and would accordingly not have been sampled by our analysis. However, we need some additional source of scatter in the total mass loss of stars climbing up the red giant branch to reproduce our results for M 5. Finally, we found a C-star on the HB of 47 Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M 5. These stars are among the brightest and coolest HB stars. [ABSTRACT FROM AUTHOR]

It has been found that globular clusters host multiple stellar populations. In particular, in NGC 1851 the subgiant branch (SGB) can be divided into two components and the distribution of stars along the horizontal branch (HB) is multimodal. Various authors have found that NGC 1851 possibly has a spread in [Fe/H], but the relation between this spread and the division in the SGB is unknown. We obtained blue (3950-4600 Å) intermediate resolution (R ∼ 8000) spectra for 47 stars on the bright and 30 on the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The spectra were analysed by comparing with synthetic spectra. The determination of the atmospheric parameters to extremely high internal accuracy allows small errors to be recovered when comparing different stars in the cluster, in spite of their faintness (V ∼ 19). Abundances were obtained for Fe, C, Ca, Cr, Sr, and Ba. We found that the b-SGB is slightly more metal-poor than the f-SGB, with [Fe/H] = -1.227 ± 0.009 and [Fe/H] = -1.162 ± 0.012, respectively. This implies that the f-SGB is only slightly older by ∼0.6 Gyr than the b-SGB if the total CNO abundance is constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is consistent with what is found for HB stars, if b-SGB are the progenitors of red HB stars, and f-SGB those of blue HB ones. As previously found, the abundances of the n-capture elements Sr and Ba have a bimodal distribution, reflecting the separation between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting that there is a real spread in the abundances of n-capture elements. By looking at the distribution of SGB stars in the [C/H] versus (vs.) Teff diagram and in the [Ba/H] vs. [Sr/H] diagram, not a one-to-one relation is found among these quantities. There is some correlation between C and Ba abundances, while the same correlation for Sr is much more dubious. We identified six C-rich stars, which have a moderate overabundance of Sr and Ba and rather low N abundances. This group of stars might be the progenitors of those on the anomalous RGB in the (v, v - y) diagram. These results are discussed within different scenarios for the formation of NGC 1851. It is possible that the two populations originated in different regions of a inhomogeneous parent object. However, the striking similarity with M 22 calls for a similar evolution for these two clusters. Deriving reliable CNO abundances for the two sequences would be crucial. [ABSTRACT FROM AUTHOR]

We studied the distribution of aluminum abundances among red giants in the peculiar globular cluster NGC 1851. Aluminum abundances were derived from the strong doublet Al i 8772-8773 Å measured on intermediate-resolution FLAMES spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We coupled these abundances with previously derived abundances of O, Na, and Mg to fully characterize the interplay of the NeNa and MgAl cycles of H-burning at high temperature in the early stellar generation in NGC 1851. The stars in our sample show well-defined correlations between Al, Na and Si; Al is anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily increases from the first generation stars to the second generation populations with intermediate and extremely modified composition. We confirm on a larger database the results recently obtained by us: the pattern of abundances of proton-capture elements implies a moderate production of Al in NGC 1851. We find evidence of a statistically significant positive correlation between Al and Ba abundances in the more metal-rich component of red giants in NGC 1851. [ABSTRACT FROM AUTHOR]

The chemical composition of horizontal branch (HB) stars might help to clarify the formation history of individual globular clusters (GCs). We studied the Na-O anti-correlation from moderately high resolution spectra for 91 stars on the bimodal HB of NGC 1851; in addition we observed 13 stars on the lower red giant branch (RGB). In our HB sample, 35 stars are on the blue HB (BHB), one is an RR Lyrae, and 55 stars are on the red HB (RHB). The ratio of BHB to RHB stars is close to the total in the cluster (35 and 54%, respectively), while RR Lyrae variables are under-represented, (they are ~12% of the NGC 1851 stars). We also derived abundances for He and N in BHB stars. For RHB stars we derived Ba abundances and a few interesting upper limits for N. The RHB stars clearly separate into two groups: the vast majority are O-rich and Na-poor, while about 10-15% are Na-rich and moderately O-poor. Most (but not all) Na-rich RHB stars are also Ba-rich and there is an overall correlation between Na and Ba abundances within the RHB. The group of Ba-rich RHB stars resides on the warmer edge and includes ~10% of the RHB stars. We propose that they are the descendant of the stars on the RGB sequence with very red v - y colour. This sequence is known also to consist of Ba and perhaps CNO-rich stars and consistently includes ~5-10% of the RGB stars of NGC 1851. However, the upper limit we obtain for N ([N/Fe] < 1.55) for one of the Ba-rich stars coupled with the low C-abundances for RGB Ba-rich stars from the literature suggests that the total CNO might not be particularly high ([(C+N+O)/Fe] ⩽ 0.15). The other Na-rich RHB stars are also at the warm edge of the RHB and the only RR Lyrae is Na-rich and moderately O-poor.We also find a Na-O anticorrelation among BHB stars, partially overlapping that found among RHB stars, though generally BHB stars are more Na-rich and O-poor. However, there is no clear correlation between temperature and Na and O abundances within the BHB. The average He abundance in BHB stars is Y = 0.29 ± 0.05, which excludes a large population of extremely He-rich stars from our sample. N abundances are quite uniform at [N/Fe] = 1.16 ± 0.14 among BHB stars, with a small trend with temperature. This value is consistent with normal CNO abundance and excludes that BHB stars are very CNO-rich: this leaves an age spread of ~1.5 Gyr as the only viable explanation for the split of the SGB. To help clarifying the formation history of NGC 1851, we computed synthetic HB's trying to identify which HB stars are the descendant of the bright and faint subgiant branch (b-SGB and f-SGB) stars identified by Milone et al. (2008, ApJ, 673, 241), with respectively 2/3 and 1/3 of the stars of NGC 1851. While most BHB stars likely descend from f-SGB stars and are older, and most RHB stars from b-SGB ones and are younger, the correspondence is probably not one-to-one. In particular, the Ba-rich RHB stars should be less massive than the remaining RHB stars, and the location of their progenitors on the SGB is uncertain. If they descend from f-SGB stars, number counts then require that RR Lyrae variables and possibly some mild BHB stars descend from b-SGB stars; this suggestion is supported by a few circumstantial facts. An investigation of the composition of a large enough sample of SGB stars is required to firmly establish these relations. [ABSTRACT FROM AUTHOR]

We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tucanac (47 Tuc), combining high-precision photometry with calculations of synthetic spectra. Using filters covering a wide range of wavelengths, our HST photometry splits the main sequence into two branches, and we find that this duality is repeated in the subgiant and red giant regions, and on the horizontal branch. We calculate theoretical stellar atmospheres for main-sequence stars, assuming different chemical composition mixtures, and we compare their predicted colors through the HST filters with our observed colors. We find that we can match the complex of observed colors with a pair of populations, one with primeval abundance and another with enhanced nitrogen and a small helium enhancement, but with depleted C and O. We confirm that models of red giant and red horizontal branch stars with that pair of compositions also give colors that fit our observations. We suggest that the different strengths of molecular bands of OH, CN, CH, and NH, falling in different photometric bands, are responsible for the color splits of the two populations. Near the cluster center, in each portion of the color-magnitude diagram the population with primeval abundances makes up only ~20% of the stars, a fraction that increases outward, approaching equality in the outskirts of the cluster, with a fraction ~30% averaged over the whole cluster. Thus the second, He/N-enriched population is more concentrated and contributes the majority of the present-day stellar content of the cluster. We present evidence that the color-magnitude diagram of 47 Tuc consists of intertwined sequences of the two populations, whose separate identities can be followed continuously from the main sequence up to the red giant branch, and thence to the horizontal branch. A third population is visible only in the subgiant branch, where it includes ~8% of the stars. [ABSTRACT FROM AUTHOR]