A Comparison Between the Patterns of CN, O, and Na Inhomogeneities on the Red Giant Branch of 47 Tucanae

Transcription

1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 127: , 215 September 215. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. A Comparison Between the Patterns of CN, O, and Na Inhomogeneities on the Red Giant Branch of 47 Tucanae GRAEME H. SMITH University of California Observatories and Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 9564; graeme@ucolick.org Received 215 May 24; accepted 215 June 26; published 215 August 21 ABSTRACT. Data from the literature are drawn together to compare the λ4215 CN band strength of red giant stars in the globular cluster 47 Tucanae with their sodium and oxygen abundances. Most cluster giants included in this study have absolute magnitudes brighter than M V ¼þ:5, i.e., they are brighter than the horizontal branch. Among the majority of these cluster giants, it is possible to identify an anticorrelation between the CN band strength and the [O/Fe] abundance, along with a correlation between CN band strength and [Na/Fe], although a small number of stars seem to be exceptions to these trends. As regards the CN-weak and CN-strong populations of red giants in 47 Tuc, the former has a higher mean oxygen abundance and a lower mean sodium abundance than the later. Both populations may contain a small fraction of stars whose sodium or oxygen abundances differ by :2 dex from the mean of each population. A scenario is discussed in which 47 Tuc was the result of an early merger between a CNpoor and a CN-strong subcluster of stars that formed at different locations within the interior of a chemically evolving massive gas complex. 1. INTRODUCTION The DDO photometric system was developed by McClure & van den Bergh (1968) at the David Dunlap Observatory for the purpose of providing photometric measurements of the basic properties of late-type stars. In the 197s, it played a key role in the discovery and study of abundance inhomogeneities within globular clusters. Of particular utility in this regard was the Cð41 42Þ color of the DDO system, which is sensitive to the strength of the λ4215 CN band in the photospheric spectra of red giants (McClure 1973). One of the first clusters in which DDO photometry was used to reveal the presence of star-to-star differences in the λ4215 CN band absorption was 47 Tucanae (Hesser et al. 1976, 1977). Norris & Freeman (1979) thereafter found that the CN band strength on the red giant branch (RGB) of 47 Tuc is bimodal, making this the first globular cluster in which a CN bimodality was discovered. Norris & Freeman (1979) found that the red giants of 47 Tuc could be categorized as either CN-weak or CN-strong. Through spectroscopy it was found soon thereafter that the CNstrong giants in 47 Tuc tended to have stronger NaD lines than their CN-weak counterparts (Cottrell & Da Costa 1981; Da Costa 1981; Lloyd Evans et al. 1982). More recent investigations by Carretta et al. (24, 29, 213), Gratton et al. (213), Cordero et al. (214), Thygesen et al. (214), Dobrovolskas et al. (214), and Johnson et al. (215) have shown from high-resolution spectroscopy that there is a significant spread in the O, Na, and Al abundances among stars of 47 Tuc in various evolutionary states, with there being an anticorrelation between [O/Fe] and [Na/Fe]. In several previous papers, we have used literature data to explore correlations between λ3883 CN band strengths and [Na/Fe] and [O/Fe] abundances among red giant branch stars in the globular clusters M13, M4, NGC 6752, and M5 (Smith & Briley 25, 26; Smith 27; Smith et al. 213, 214). The present paper constitutes an extension of this work to the higher metallicity cluster 47 Tuc for which the λ4215 CN band becomes a useful tracer of CN inhomogeneities. Selected [Fe/H] measurements for 47 Tuc include :76 (Koch & McWilliam 28; Gratton et al. 213), :78 (Thygesen et al. 214), and :79 (Cordero et al. 214). It is a combination of the CN surveys of Norris & Freeman (1979) and Norris et al. (1984), with the measurement of [Na/Fe] and [O/Fe] abundances for large samples of RGB stars in 47 Tuc by Carretta et al. (29) and Cordero et al. (214), that make this literature-based investigation possible. 2. THE CYANOGEN DISTRIBUTION OF 47 TUCANAE A compilation of Cð41 42Þ measurements for red giant branch stars in 47 Tucanae has been obtained from the spectroscopic studies of Norris & Freeman (1979; NF79) and Norris, Freeman, & Da Costa (1984; NFD84). Many of these measurements, although derived from spectroscopic data, have been transformed to the usual Cð41 42Þ color of the DDO photometric system (McClure & van den Bergh 1968) by the original authors. In the case of stars for which both NF79 and NFD84 provide a value for Cð41 42Þ these have been averaged. There are some stars for which Norris & Freeman (1979) do not 825

2 826 SMITH provide a value for Cð41 42Þ, but list an index that they denote Sð41 42Þ. TheseSð41 42Þ values have been transformed onto the DDO color system according to the NF79 equation Cð41 42Þ ¼:742Sð41 42Þþ:236. A transformed value of Cð41 42Þ acquired in this way is used only in cases where neither NF79 or NFD84 list a value of Cð41 42Þ. Stars listed as either field or asymptotic giant branch stars by NF79 or NFD84 are not considered here. The papers of Lee (1977) and Chun & Freeman (1978) were adopted as sources for V and B V photometry. The resulting database of DDO and Johnson colors for 47 Tuc RGB stars was corrected for distance and reddening using EðB V Þ¼:4 and ðm MÞ V ¼ 13:37 (Percival et al. 22). The distribution of CN band strengths on the RGB of 47 Tuc is shown as a plot of Cð41 42Þ versus absolute magnitude M V in Figure 1. Typical uncertainties in the Cð41 42Þ values are quoted as.3 mag by Norris & Freeman (1979). The great majority of stars in Figure 1 have absolute magnitudes of M V < þ:5, and so are brighter than the horizontal branch in a color-magnitude diagram. Two inclined straight lines are shown in Figure 1. The lower (solid) line corresponds to the equation C NF ð41 42Þ ¼:67 :79 M V : (1) It forms a reasonable lower bound to the data points for 47 Tuc, and was obtained by rewriting an analogous equation from Norris & Freeman (1979) in terms of absolute magnitude (upon applying the Percival et al. [22] distance modulus). Norris & Freeman (1979) defined a CN-excess index δcð41 42Þ as the vertical displacement of a data point from the baseline C NF ð41 42Þ. As such, δcð41 42Þ is a parameter that can be used to quantify the dispersion in CN band strengths among stars at a given absolute magnitude on the red giant branch. Norris & Freeman (1979) found the distribution of δcð41 42Þ on the 47 Tuc RGB to be bimodal. An additional line plotted in Figure 1 corresponds to a CN residual of δcð41 42Þ ¼:1 (dotted line). In the discussion below, we will categorize CN-weak giants as those having δcð41 42Þ :1, while CN-strong giants are taken to be those with δcð41 42Þ > :1. The basis for this classification can be seen from the generalized histogram of δcð41 42Þ plotted by Norris & Freeman (1979) for stars more than 6 from cluster center. The histogram is bimodal with local maxima at δcð41 42Þ :4 and.19, and a local minimum at δcð41 42Þ :1. Also shown in Figure 1 are two models for the 47 Tuc RGB by Briley (1997). They are computed from synthetic spectra for a run of effective temperature and surface gravity appropriate to the RGB of 47 Tuc (see Table 1 of Briley [1997]). The lower locus of models (long-dash line) was computed for an abundance combination of ½C=FeŠ ¼; ½N=FeŠ ¼:4, and ½O=FeŠ ¼ :45, while the upper locus (short-dash line) has ½C=FeŠ ¼ :3, ½N=FeŠ ¼1:4, ½O=FeŠ ¼:45. There is an anticorrelation FIG. 1. A C ð41 42Þ index (corrected for interstellar reddening) vs. M V color-magnitude diagram for red giants in 47 Tucanae. The solid line corresponds to equation (1), while the dotted line is offset from this equation by.1 mag. The long-dash and short-dash lines correspond to model RGB sequences for 47 Tuc computed by Briley (1997). Each of these two curves is labeled with the ([C/Fe], [N/Fe], [O/Fe]) abundance combinations for which they were calculated. between C and N abundance between these two model sequences, but they have identical oxygen abundance. These model RGBs are roughly parallel, and their abundance combinations were chosen by Briley (1997) so as to replicate the mean run of the data points for CN-weak and CN-strong giants. A reasonable match to the C-normal, lower-n model sequence for 1: <M V < þ:4 is C B97 ð41 42Þ ¼:113 :52 M V : (2) This equation can be used as the reference for another CNresidual that will be designated δ B97 Cð41 42Þ, the subscript B97 denoting that it is measured relative to the lower-nitrogen RGB locus of Briley (1997). 3. COMPARISON WITH FIELD GIANTS AND M67 That it is the CN-strong giants of 47 Tuc that are atypical can be demonstrated by comparing their Cð41 42Þ colors with those of field giants of similar metallicity. Photometry in the DDO and BV bandpasses for field giants having a similar metallicity to 47 Tuc was sought from the General Catalog of Photometric Data (Mermilliod et al. 1997), and the compilation of Mermilliod & Nitschelm (1989). A listing of such giants in

3 47 TUC RED GIANT BRANCH INHOMOGENEITIES 827 TABLE 1 PHOTOMETRY OF FIELD GIANTS WITH :8 Fe=H : Tuc (circles), M67 (crosses) Star [Fe/H] Ref a Eðb yþ Ref b C ð41 42Þ ðb V Þ HD HD HD HD HD HD HD HD HD HD HD HD CD BD C (41 42) (B V) a References to metallicity: (1) Gratton et al. (2); (2) Claria et al. (1994); (3) Anthony-Twarog & Twarog (1994); (4) Cayrel de Strobel et al. (21). b References to reddening: (1) Anthony-Twarog & Twarog (1998); (2) Anthony- Twarog & Twarog (1994); (3) Claria et al. (1994); (4) Beers et al. (2); (5) Based on Schlegel et al. (1998) reddening distribution. FIG. 2. Comparison between RGB stars in 47 Tuc (filled circles) and the solar-metallicity open cluster M67 (crosses) in a plot of C ð41 42Þ vs. ðb V Þ. Table 1 contains the [Fe/H] abundance, a reference to the source of this abundance, a reddening Eðb yþ and its source, and dereddened Cð41 42Þ and ðb V Þ colors. Reddening ratios of EðB V Þ¼1:37Eðb yþ (Crawford 1975) and E½Cð41 42ÞŠ ¼ :66EðB V Þ (McClure 1973) have been applied. Field giants in Table 1 were selected to have metallicities in the range :8 ½Fe=HŠ :7. These stars are compared with the red giants of 47 Tuc in Figure 2 in a C ð41 42Þ versus ðb V Þ diagram. Most of the field giants fall in a region of this plot close to the locus of the CN-weak RGB stars of 47 Tucanae. Thus it is the CN-weak giants of the cluster that are most analogous to the general Galactic field population. Curiously, there are two field giants, HD 443 and HD , that do fall close to an extension of the CN-strong 47 Tuc locus. Gratton & Ortolani (1984) obtained a spectroscopic metallicity of ½Fe=HŠ ¼ :7 for HD 443, while listing the reddening as uncertain. They did not draw any specific attention to the abundances of this star. Also instructive is the comparison between the CN band strengths of 47 Tuc RGB stars and red giants in the open cluster M67, which has a near-solar metallicity (e.g., Friel et al. 21; Pancino et al. 21). Janes & Smith (1984) have published DDO and BV photometry for red giants in M67 that can be used for this comparison. For M67, we adopt EðB V Þ¼:4 on the basis of the extensive review by Taylor (27). A plot of C ð41 42Þ versus ðb V Þ for 47 Tuc and M67 RGB stars is shown in Figure 3, where giants in these two clusters are depicted by filled circles and crosses, respectively. The CN-strong giants in 47 Tuc have comparable Cð41 42Þ colors to the RGB stars of M67, despite the later being higher in overall metallicity by :8 dex. This is an alternative way of demonstrating that the CN-strong giants of 47 Tuc have λ4215 CN bands that are much stronger than would be expected of stars having the same metallicity as 47 Tuc and a solar-like C:N:Fe abundance ratio. Thus the CN-weak giants in 47 Tuc can be thought of as having normal CN abundances, whereas the CN-strong stars are enhanced in CN. Synthetic spectrum analyses of the λ4215 CN and λ43 CH bands by Dickens et al. (1979), Norris & Cottrell (1979), and Briley (1997) indicate that the enhanced CN bands of the CN-strong giants in 47 Tuc are driven largely by nitrogen abundance enhancements. C (41 42) Tuc (circles), field giants (crosses) (B V) FIG. 3. A plot of C ð41 42Þ vs. ðb V Þ showing comparison between RGB stars in 47 Tuc (filled circles) and field giants (crosses) with metallicities in the range :8 ½Fe=HŠ :7.

4 828 SMITH TABLE 2 COMPARISON BETWEEN O AND NA ABUNDANCES Star a [O/Fe] (Cor14) b [Na/Fe] (Cor14) b [O/Fe] (Car9) c [Na/Fe] (Car9) c L L L L L L L L L L L L D D D D NOTE. From Cordero et al. (214) and Carretta et al. (29). a L, D = star designation from Lee (1977) and Chun & Freeman (1978), respectively. b Abundances from Cordero et al. (214). c Abundances from Carretta et al. (29). FIG. 4. Oxygen abundances for 47 Tuc giants measured by Carretta et al. (29) vs. measurements from Cordero et al. (214). Equality between the two data sets would lead to points falling along the solid line. 4. THE RELATION BETWEEN CN BAND STRENGTH AND SODIUM AND OXYGEN ABUNDANCES IN 47 TUC Sodium and oxygen abundances for giant stars in 47 Tuc are available from both Carretta et al. (29; Car9) and Cordero et al. (214; Cor14). The uncertainties quoted by Cor14 in their [O/Fe] and [Na/Fe] abundances are.15 dex and.14 dex, respectively, while Car9 gives.14 dex and.8 dex, respectively, as their typical star-to-star errors. Sixteen stars are in common to these two programs. They are listed in Table 2, together with both the [Na/Fe] and [O/Fe] abundances derived by each investigation. Star designations are taken from Lee (1977) and Chun & Freeman (1978). Comparison between the two sets of oxygen abundances is illustrated in Figure 4. Among stars with ½O=FeŠ > :2, there is a reasonable correlation between the two sets of oxygen abundances, with little systematic offset. However, for the four stars with lower oxygen abundances, there is substantial scatter between the two data sets. Sodium abundances from these two programs are contrasted in Figure 5, and for ½Na=FeŠ > :4, there is reasonable agreement. Among stars for which Cordero et al. (214) find [Na/Fe] to be less than.4 dex, however, there may be a systematic offset between the two abundance programs, with those derived by Carretta et al. (29) being some.1.15 dex greater on average than those of Cordero et al. (214). Rather than attempt to combine the O and Na from the Car9 and Cor14 sources into a single homogenized sample, these two sets of abundance data are considered FIG. 5. Sodium abundances for 47 Tuc giants measured by Carretta et al. (29) vs. measurements from Cordero et al. (214). The solid line would correspond to equality between the two data sets.

5 47 TUC RED GIANT BRANCH INHOMOGENEITIES 829 separately in the following search for (anti)correlations between λ4215 CN band strength, on one hand, and [O/Fe] and [Na/Fe] on the other Comparisons Based on the Abundances of Cordero et al. (214) Stars common to the compiled database of Cð41 42Þ colors and the abundance program of Cordero et al. (214) are listed in Table 3. The Cor14 measurements are based on spectra from either the Hydra spectrograph on the CTIO 4 m Blanco telescope or the FLAMES-GIRAFFE multiobject spectrograph on the VLT-UT2 telescope, and are sorted according to these instruments in Table 3. Plots of CN-excess indices versus the [O/Fe] and [Na/Fe] abundances derived by Cordero et al. (214) are shown in Figures 6 and 7, respectively, with abundances being plotted against the δcð41 42Þ index in the lower panel, while in the upper panel, δ B97 Cð41 42Þ is used as the abscissa. In cases where both Hydra and FLAMES-based abundance measurements are available for a star, they have been averaged. A distinction can be seen in Figure 6 between the oxygen abundances of the CN-strong and CN-weak giants, albeit with some scatter. All of the CN-weak giants have oxygen abundances of ½O=FeŠ > :2, whereas lower oxygen abundances are only encountered among CN-strong giants in Figure 6. Nonetheless, there are several CN-strong giants with δcð41 42Þ > :1 whose [O/Fe] abundances of :3 are comparable to those of the CN-weak giants. By contrast, the lowest oxygen abundances of ½O=FeŠ < : are confined exclusively in Figure 6 TABLE 3 CN-EXCESS AND O AND NA ABUNDANCES FOR 47 TUCANAE RGB STARS Star a M V ðb V Þ Cð41 42Þ δcð41 42Þ [O/Fe] (Hydra) [Na/Fe] (Hydra) [O/Fe] (FLAMES) [Na/Fe] (FLAMES) L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L D NOTE. From Cordero et al. (214). a L, D = star designation from Lee (1977) and Chun & Freeman (1978), respectively.

6 83 SMITH FIG. 6. Measurements of [O/Fe] abundance from Cordero et al. (214) vs. the cyanogen excess indices δcð41 42Þ (bottom) and δ B97 Cð41 42Þ (top) for 47 Tuc giants. to giants with very strong CN bands for which δcð41 42Þ > :16. The trend in Figure 6 implies that there is a difference between the average oxygen abundance of CN-strong and CN-weak RGB stars in 47 Tuc. Given that the uncertainties in the oxygen abundances of Cor14 are.15 dex, some of the scatter in [O/Fe] at a fixed CN band strength could be observational FIG. 7. Measurements of [Na/Fe] abundance from Cordero et al. (214) for 47 Tuc red giants plotted against two cyanogen excess indices: δcð41 42Þ (bottom) and δ B97 Cð41 42Þ (top). in nature. In fact, the intrinsic range in [O/Fe] among the CNweak giants could be very small, whereas an intrinsic spread in oxygen may exist within the CN-strong population. Considering the CN-weak group of giants with δcð41 42Þ :1 and the abundances of Cordero et al. (214), the average [O/Fe] for 17 stars is.33, with a sample standard deviation of.7 dex, and a standard deviation in the mean of.2 dex. The standard deviation can be fully accounted for by the observational uncertainty quoted by Cordero et al. (214), and so there is no evidence for an oxygen inhomogeneity within the CN-weak group. With a sample of 15 CNstrong giants in Table 3 having measured oxygen abundances, the average [O/Fe], sample standard deviation, and standard deviation in the mean are.11 dex,.23 dex, and.6 dex, respectively. The mean [O/Fe] for the CN-strong group is therefore.22 dex lower than for the CN-weak group, as compared to the errors in the mean of.6 dex and.2 dex, respectively, for these two populations. The standard deviation in [O/Fe] within the CN-strong sample, although still not much larger than the observational errors given by Cordero et al. (214), is three times that within the CN-weak sample. Based on Figure 7, the CN-weak and CN-strong giants also differ in their pattern of sodium abundances. Turning to the 18 CN-weak stars from Table 3, the average [Na/Fe] is.23, with a sample standard deviation of.1 dex and a standard deviation in the mean of.25 dex. As with the oxygen abundance, the standard deviation of.1 dex in [Na/Fe] for the CN-weak group can be accounted for by observational uncertainties. There are 19 CN-strong giants in Table 3 with measured sodium abundances, and for this sample the mean [Na/Fe] is.43, the standard deviation is.17 dex, and the standard deviation in the mean is.4 dex. Taken as a whole, the giants in the CN-strong sample of Table 3 thus have a mean [Na/Fe] that is.2 dex higher than that of the CN-weak giants. There are three CN-strong giants that fall in an atypical region of Figure 7. All three of these stars, L2734, L471, and L751, have δcð41 42Þ > :1, yet relatively low sodium abundances of ½Na=FeŠ < :2. Thus, there may be a small subset of 47 Tuc giants that have strong λ4215 CN bands yet low sodium abundances. L2734 indeed seems an odd star on the basis of the O and Na data of Cordero et al. (214) because it has both a very low oxygen and a low sodium abundance, such that it does not follow the O-Na anticorrelation established for 47 Tuc by Carretta et al. (29) and Cordero et al. (214). If these three CN-strong giants with anomalously low sodium abundances of ½Na=FeŠ < :2 are momentarily set aside, the remaining sample of 16 CN-strong stars has an average [Na/ Fe] of.49, with a standard deviation of.8 dex, and a standard deviation in the mean of.2 dex. This 16-star subgroup has a mean sodium abundance that is.26 dex higher than the CN-weak sample. Thus, compared to the CN-weak group of giants, the CNstrong giants have a higher mean Cor14 sodium abundance

7 47 TUC RED GIANT BRANCH INHOMOGENEITIES 831 and a lower mean Cor14 oxygen abundance. A different approach to determining whether the data are consistent with a CN [O/Fe] anticorrelation, or a CN [Na/Fe] correlation, is to calculate Spearman rank-order correlation coefficients r s for the stars in Table 3. In the case of δcð41 42Þ versus [Na/Fe], the 37 stars in Figure 7 yield r s ¼ :72, with a two-tailed p value of 1 6, indicative of a correlation. There are five stars in Table 3 for which oxygen was not measured by Cordero et al. (214). The remaining 32 stars give a Spearman coefficient for δcð41 42Þ versus [O/Fe] of r s ¼ :55, with a two-tailed p value of 1 3. The anticorrelation between oxygen abundance and CN strength is less significant, in part because the CN-weak and CN-strong samples in Figure 6 overlap in the oxygen abundance range of :2 < ½O=FeŠ < :35. Interestingly, a Spearman test on the [O/Fe] and [Na/Fe] abundances from Table 3 give an even weaker result of r s ¼ :34. The reason for this can be seen from Figure 8, in which [Na/Fe] from Table 3 is plotted against [O/Fe]. The entire 47 Tuc samples of Cordero et al. (214) do show an anticorrelation between [O/Fe] and [Na/Fe], as does the sample of Carretta et al. (29). However, there is considerable scatter in this anticorrelation, such that in the smaller subset of stars for which values of δcð41 42Þ have been acquired from the literature, it is only weakly discernible. Thus, at least for the sample of 47 Tuc giants in Table 3, the trends of CN versus [O/Fe] and CN versus [Na/Fe] are somewhat more well-defined than the O Na trend. In addition to the comparison between the CN-weak and CNstrong populations, is there any evidence that either of these populations is itself inhomogeneous? As noted earlier, the dispersion in [O/Fe] within the CN-weak population is entirely explicable by observational uncertainties. The dispersion in [O/Fe] within the CN-strong group is greater, although the standard deviation of.23 dex is only about twice the observational uncertainty quoted by Cordero et al. (214). Suppose one excludes the most O-rich CN-strong giant, L6616 with ½O=FeŠ ¼:6, and the two most O-poor, CN-strong giants, L674 with ½O=FeŠ ¼ :39 and L4748 with ½O=FeŠ ¼ :16, this gives a sample of 12 CN-strong stars for which the average [O/Fe] is.13, the sample standard deviation is.13 dex, and the standard deviation in the mean is.4 dex. Among these 12 stars, the dispersion in [O/Fe] could be attributed entirely to observational uncertainties. Thus, any evidence for an intrinsic inhomogeneity in [O/Fe] among the CN-strong giants in Figure 6 hinges upon a small number of stars. Among the CN-weak giants with δcð41 42Þ < :1 in Figure 7, there is a notable range in [Na/Fe] of almost.5 dex, although the bulk of these stars are characterized by :5 < ½Na=FeŠ < :35. Might the CN-weak population of 47 Tuc be itself a chemically inhomogeneous population as regards sodium? There may be some impression of a correlation between [Na/Fe] and δcð41 42Þ for the CN-weak group in the lower panel of Figure 7. However, the correlation within FIG.8. Sodium vs. oxygen abundance from Cordero et al. (214) for the 47 Tuc RGB stars in Table 3. the CN-weak group is not as strongly seen when the δ B97 Cð41 42Þ index is used as the CN-excess parameter (upper panel of Fig. 7). Furthermore, the standard deviation of.1 dex within the CN-weak sample is comparable to the observational uncertainty quoted by Cordero et al. (214). Any evidence for a CN [Na/Fe] correlation among the CN-weak sample would seem to ride upon the two giants with :5 < δcð41 42Þ < :8 (D672 and L8648) and ½Na=FeŠ :4 :5. Such stars could arguably be classified as CN-intermediate and sodium-enhanced, having stronger CN bands than the bulk of the CN-weak population, and sodium abundances typical of the bulk of the CN-strong population. In the case of the dispersion in sodium abundance among the CN-strong giants, the situation is somewhat analogous to that for oxygen. Of the 19 CN-strong giants in Table 3, there is a standard deviation of only.8 dex in [Na/Fe] among those 16 stars having the highest sodium abundances (½Na=FeŠ > :3), which is within the realm of observational uncertainty. Thus, any evidence for an intrinsic inhomogeneity in [Na/Fe] among the CN-strong giants in Table 3 rests upon three stars emphasized above (L2734, L471, and L751) that happen to have sodium abundances comparable to those of the CNweak population. In summary, any evidence for an intrinsic dispersion in either oxygen or sodium abundances within either the CN-weak or CN-strong RGB populations is reliant upon a modest fraction ( 15%) of stars whose values of [O/Fe] or [Na/Fe] stand out from the population means.

8 832 SMITH 4.2. Comparisons Based on the Abundances of Carretta et al. (29) Eleven stars were found in common between the CN survey of Figure 1 and the Na and O abundance program of Carretta et al. (29), which is based on FLAMES/GIRAFFE spectra from the VLT U2 telescope. Relevant data for these stars are listed in Table 4, while plots of the [O/Fe] and [Na/Fe] abundances versus the δcð41 42Þ CN-excess index are shown in the bottom and top panels of Figure 9, respectively. There is an anticorrelation between [O/Fe] and [Na/Fe] that is apparent from the Carretta et al. (29) abundances in Table 4, although the corresponding plot is not included here. However, neither a CN O anticorrelation nor a CN Na correlation is readily seen in Figure 9, possibly due in part to the smaller number of stars than in the Cor14 CN sample. There are two CN-weak giants in the lower panel of Figure 9, but neither of them have higher oxygen abundances than the CN-strong giants. Of the three CN-weak giants in the upper panel, only one has a notably lower [Na/ Fe] abundance than the CN-strong stars plotted therein. Use of the δ B97 Cð41 42Þ index results in a plot that is not reproduced here, but is morphologically very similar to Figure 9. As with the three giants found in Figure 7 which do not follow a CN Na correlation, the Carretta et al. (29) data may also provide evidence for intrinsic scatter in this correlation, as well as in the CN O anticorrelation TABLE 4 CN-EXCESS WITH O AND NA ABUNDANCES FOR 47 TUCANAE RGB STARS Star a δcð41 42Þ [O/Fe] (Car9) [Na/Fe] (Car9) L L L L L L L L L L L NOTE. From Carretta et al. (29). a L = star designation from Lee (1977). 5. THE CN O ANTICORRELATION Based on the larger sample of stars in Figure 6 as compared to Figure 9, we conclude that there is an anticorrelation between CN and [O/Fe] among the majority of stars (but not necessarily all) on the upper RGB of 47 Tuc. In this respect, 47 Tuc is similar to clusters of the Galactic halo such as M13 and M5 (Smith et al. 1996; Ivans et al. 21; Sneden et al. 24; Smith & Briley 26; Smith et al. 213). One interpretation of this anticorrelation is that the material in the CN-strong giants represents a CNO-processed version of the composition mix of a CN-weak star. Suppose starting with material having an initial N:O abundance ratio of 1:16 by number, such as would pertain to an abundance combination of ½N=FeŠ ¼ and ½O=FeŠ ¼:3 dex. If oxygen atoms are converted to nitrogen atoms via the O N cycle of hydrogen burning, and oxygen becomes depleted by.2 dex, such that [O/Fe] drops to a value of.1, as is typical of CN-strong giants in Figure 6, then the N:O number ratio changes to 7:1. Similarly, a depletion by.4 dex in [O/Fe], such as is seen for some giants with Cð41 42Þ > :2 in Figure 6, would change the N:O ratio to 9:6, almost a 1. dex increase in nitrogen abundance. Briley (1997) found that such a nitrogen excess could account for the difference in Cð41 42Þ between the CN-weak and CN-strong giants in 47 Tuc 1 (see Fig. 1). The anticorrelation in Figure 6 could therefore be consistent with the CN-strong and CN-weak giants having roughly the same O þ N abundance, as might be expected if the atmospheres of the former group of stars are the product of O N FIG. 9. Measurements of [O/Fe] abundance (bottom) and [Na/Fe] (top) for 47 Tuc giants from Carretta et al. (29) vs. cyanogen excess index δcð41 42Þ Briley s (1997) model calculations did not incorporate an oxygen abundance difference between the CN-strong and CN-weak stars, and as such may overestimate the [N/Fe] difference between these groups. By virtue of their lower oxygen abundances, there will be less molecular CO in the photospheres of the CN-strong giants than in Briley s models. As such, the amount of free carbon available for CN formation would be underestimated, hence leading to a nitrogen overestimate for a given CN band strength.

9 47 TUC RED GIANT BRANCH INHOMOGENEITIES 833 processing of the CN-weak composition mix. In practice, the O N process would likely be accompanied within a stellar nucleosynthesis site by the C N cycle of hydrogen burning, such that CN-strong giants containing such processed material would have depleted carbon abundances relative to CN-weak giants. Norris & Cottrell (1979), Norris & Freeman (1982), Norris et al. (1984), and Briley et al. (1994) found evidence of such an abundance pattern not only on the RGB but also on the red horizontal branch and the main-sequence turnoff of 47 Tuc. Thus the combined CNO bicycle could lead to CN-strong giants having nitrogen abundances more than 1. dex higher than the CN-weak counterparts. How did the CN-strong giants manage to acquire a composition that appears to be a CNO-processed version of the composition of the CN-weak giants? There have usually been two ways of approaching this question: (1) either the two groups of stars formed with similar abundances, and the CN-strong giants have been altered by interior processes capable of transporting mass from the region of the hydrogen-burning shell to the stellar surface, or (2) the abundance differences between CN-strong and CN-weak giants reflect differences in the gas from which these stars formed. The former of these alternatives was the first to be explored theoretically (e.g., Sweigart & Mengel 1979). However, 47 Tuc was the first cluster for which it was discovered that CN inhomogeneities extend to main sequence and main sequence turnoff stars (Hesser 1978; Bell et al. 1983; Briley et al. 1991, 1994; Cannon et al. 1998; Harbeck et al. 23), as do the O and Na inhomogeneities (Briley et al. 1996; Carretta et al. 24; D Orazi et al. 21; Dobrovolskas et al. 214). Furthermore, D Orazi et al. (21) found that the cumulative distribution of [Na/O] among the dwarfs of 47 Tuc is indistinguishable from that of the red giants. Therefore, the origin of the CN O Na inhomogeneities in 47 Tuc evidently preceded the red giant phase of evolution. As such, a primordial scenario in which the CN-strong/Na-rich stars actually formed from N- and Na-enriched material has received much attention in the literature, not just in the case of 47 Tuc (Carretta et al. 213; Ventura et al. 214), but other heterogeneous globular clusters as well (e.g., Cottrell & Da Costa 1981; Smith & Norris 1982; Brown & Wallerstein 1993; Denissenkov et al. 1997, 1998; Fenner et al. 24; Ventura & D Antona 25, 28, 29; Smith 26; Bekki et al. 27; Prantzos et al. 27; Decressin et al. 27, 29; D Ercole et al. 28; Carretta et al. 29, 21; Bekki 211; Krause et al. 213; Bastian et al. 215). 6. DISCUSSION Correlations between CN and Na, as well as CN O anticorrelations, typical of those in 47 Tuc have been also found in the more metal-poor globular clusters of the Milky Way halo (e.g., Cottrell & Da Costa 1981; Norris & Pilachowski 1985; Ivans et al. 21; Sneden et al. 24; Smith & Briley 25, 26; Smith 27; Smith et al. 213, 214). The orbit of 47 Tuc within the Galaxy is such that the cluster is typically located within 3 kpc of the midplane of the Galactic disk (Lane et al. 212a,b). As such, the cluster spends a considerable time orbiting within the vicinity of the Galactic disk, unlike halo clusters such as M13 and M5. Nonetheless, despite differences in formation environment that this might have entailed, 47 Tuc was able to establish internal CN, O, and Na abundance inhomogeneities that are analogous to those of halo globular clusters. The CN-strong/Na-rich population of giants is more centrally concentrated within 47 Tuc than the CN-weak/Na-poor population (Norris & Freeman 1979; Briley 1997; Milone et al. 212; Cordero et al. 214). Kucinskas et al. (214) found that there are also differences between the velocity dispersions of these populations. An interesting idea was proposed by Lane et al. (21) to explain the internal kinematics of 47 Tuc, namely, that the present-day cluster is the result of a merger of two initially separate subclusters that had experienced different chemical evolution histories. Such a scenario might envision a gas-rich progenitor system much more massive than the current 47 Tuc cluster. The CN-weak subcluster may have formed first, during the early stages of the chemical evolution of the progenitor system. A CN-strong subcluster may have formed at a later time, perhaps in a different spatial location within the parent system, as it evolved through a later phase of chemical evolution that managed to enrich the parent cloud in elements such as nitrogen and sodium. If the two subclusters did form at different locations, and/or at different times, within a more massive gas-rich parent system, then the stars that enriched the CN-strong subcluster need not have been associated with the CN-weak subcluster. At some later time, the two subclusters merged as the parent system lost gas and the ability to sustain further enriched star formation. Merging might have been made more likely if the subclusters formed in a rotating disk-like progenitor system at comparable distances from the disk center, such that small differences in rotation speed about the progenitor center could bring the two systems into eventual proximity. One constraint on such a scenario is that the chemical evolution experienced by the parent system of 47 Tuc must have largely been confined to elements lighter than silicon. Not only that, given that bimodal CN distributions and O Na anticorrelations are commonplace within Milky Way globular clusters, a merger origin for 47 Tuc would suggest a merger origin for other globular clusters with multiple populations. Such hypothetical parent objects that gave rise to clusters like 47 Tuc would need to have been massive enough to sustain multiple subcluster formation, and some level of chemical evolution in lighter elements. Searle & Zinn (1978) originally suggested that globular clusters formed in very massive progenitor gas clouds, of the type that may have been comparable in mass to modest dwarf galaxies. Bekki & Freeman (23) and Bekki (212) have discussed scenarios in which very exotic globular clusters such as ω Centauri, M22, and M54 (which are inhomogeneous in

10 834 SMITH iron-peak elements and other elements heavier than silicon), formed within gas-rich dwarf satellite galaxies that merged with the Milky Way at an early time. The gas complexes that produced dwarf spheroidal satellites of the Milky Way such as the Draco and Sculptor systems, also sustained a certain degree of evolution in the iron-peak elements (e.g., Kirby et al. 211a, 211b). In the Bekki & Freeman (23) scenario, clusters such as ω Cen and M54 are the products of chemical evolution within the core or nucleus of a gas-rich dwarf galaxy. Sustained gas flow into the core attended by supernova-induced enrichment, may have been what built up these chemically evolved clusters over an extended period of time. 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