Cluster Tidal Radius Research Articles

A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions

Wide-field and deep DECam multiband photometry, combined with HST data for the core of the Galactic globular cluster NGC 2808, allowed us to study the distribution of various stellar subpopulations and stars in different evolutionary phases out to the cluster tidal radius. We used the C ugi = (u − g) − (g − i) index to identify three chemically distinct subpopulations along the red giant branch and compared their spatial distributions. The most light-element-enriched subpopulation (P3) is more centrally concentrated; however, it shows a more extended distribution in the external regions of the cluster compared to the primordial (P1) and intermediate (P2) composition populations. Furthermore, the P3 subpopulation centroid is off-center relative to those of the P1 and P2 groups. We also analyzed the spatial distribution of horizontal branch stars and found that the relative fraction of red horizontal branch stars increases for radial distances larger than ≈1.′5, while that of the blue and hotter stars decreases. These new observations, combined with literature spectroscopic measurements, suggest that the red horizontal branch stars are the progeny of all the stellar subpopulations in NGC 2808, i.e., primordial and light-element enhanced, while the blue stars are possibly the result of a combination of the “hot-flasher” and the “helium-enhanced” scenarios. A similar distribution of different red giant branch subpopulations and horizontal branch stars was also found for the most massive Galactic globular cluster, ω Cen, based on combined DECam and HST data, which suggests that the two may share a similar origin.

The elusive tidal tails of the Milky Way globular cluster NGC 7099

We present results on the extra-tidal features of the Milky Way globular cluster NGC 7099, using deep gr photometry obtained with the Dark Energy Camera (DECam). We reached nearly 6 mag below the cluster’s main sequence (MS) turnoff, so that we dealt with the most suitable candidates to trace any stellar structure located beyond the cluster tidal radius. From star-by-star reddening corrected color-magnitude diagrams (CMDs), we defined four adjacent strips along the MS, for which we built the respective stellar density maps, once the contamination by field stars was properly removed. The resulting, cleaned, field star stellar density maps show a short tidal tail and some scattered debris. Such extra-tidal features are hardly detected when much shallower Gaia DR2 data sets are used and the same CMD field star cleaning procedure is applied. Indeed, by using 2.5 mag below the MS turnoff of the cluster as the faintest limit (G < 20.5 mag), cluster members turned out to be distributed within the cluster’s tidal radius, and some hints for field star density variations are found across a circle of radius 3.5° centered on the cluster and with similar CMD features as cluster stars. The proper motion distribution of these stars is distinguishable from that of the cluster, with some superposition, which resembles that of stars located beyond 3.5° from the cluster center.

The eye of Gaia on globular clusters structure: tidal tails

ABSTRACT I analyse the projected density distribution of member stars over a wide area surrounding 18 Galactic globular clusters using the photometric and astrometric information provided by the second data release of the Gaia mission. A 5D mixture modelling technique has been employed to optimally isolate the signal of the cluster stellar population from the contamination of the Galactic field, taking advantage of its different distribution in the space formed by colours, magnitudes, parallaxes, and proper motions. In 7 clusters I detect collimated overdensities at a >3σ level above the background density extending well beyond the cluster tidal radius, consistent with the distortion expected as a result of the tidal interaction with the Milky Way potential. In five of these clusters (NGC 288, NGC 2298, NGC 5139, NGC 6341, and NGC 7099) spectacular tidal tails extend up to the border of the analysed field of view at 5 deg from the centre. At large distances from the cluster centre, the orientation of the detected overdensities appears to be systematically aligned with the cluster orbital path, in agreement with the predictions of N-body simulations. The fraction of stars contained in the tidal tails of these clusters is also used to determine the first observational estimate of their present-day destruction rates.

The tale of the Milky Way globular cluster NGC 6362 – I. The orbit and its possible extended star debris features as revealed by Gaia DR2

ABSTRACTWe report the identification of possible extended star debris candidates beyond the cluster tidal radius of NGC 6362 based on the second Gaia data release (Gaia DR2). We found 259 objects possibly associated with the cluster lying in the vicinity of the giant branch and 1–2 magnitudes fainter/brighter than the main-sequence turn-off in the cluster colour–magnitude diagram and which cover an area on the sky of ∼4.1 deg2 centred on the cluster. We traced back the orbit of NGC 6362 in a realistic Milky Way potential, using the gravpot16 package, for 3 Gyr. The orbit shows that the cluster shares similar orbital properties as the inner disc, having peri-/apogalactic distances, and maximum vertical excursion from the Galactic plane inside the corotation radius (CR), moving inwards from CR radius to visit the inner regions of the Milky Way. The dynamical history of the cluster reveals that it has crossed the Galactic disc several times in its lifetime and has recently undergone a gravitational shock, ∼15.9 Myr ago, suggesting that less than 0.1 per cent of its mass has been lost during the current disc-shocking event. Based on the cluster’s orbit and position in the Galaxy, we conclude that the possible extended star debris candidates are a combined effect of the shocks from the Galactic disc and evaporation from the cluster. Lastly, the evolution of the vertical component of the angular momentum shows that the cluster is strongly affected dynamically by the Galactic bar potential.

Discovery of Tidal RR Lyrae Stars in the Bulge Globular Cluster M62∗

Abstract The RR Lyrae (RRL) rich globular cluster M62 (NGC 6266) is one of the most massive globular clusters in the Milky Way, located in the dense region of the Galactic bulge, where dynamical processes that affect the survival of globular clusters are maximized. Using Gaia second data release data we have found clear evidence for an excess of RRLs beyond the cluster tidal radius of M62, associated partly with stars stripped into the Galaxy field. This is confirmed with new Visible and Infrared Survey Telescope for Astronomy Variables in the Via Lactea Extended survey observations, which discard any differential reddening effect as the possible cause of the observed RRL density excess. We also determined the orbit of M62 using accurate new measurements of its distance, radial velocity, and proper motions, finding that its orbit is prograde with respect to the direction of the Galactic rotation. Orbits are integrated in the non-axisymmetric galactic model GravPot16, which includes the perturbations due to the central Galactic bar. M62 shows a particular orbital behavior, having a dynamical signature of the bar-bulge region. The small extra-tidal RRLs extensions that are observed are roughly aligned toward the galactic center, and the direction is almost perpendicular to the galactic plane, not with its motion along its orbit. This may be a clear sign of bulge-crossing shocks during the last passage close of the cluster toward its perigalacticon. M62 would be the first clear observed case of bulge shocking in the inner Galaxy acting on a globular cluster.

Radial Velocities of RR Lyrae Stars in and around NGC 6441

Abstract Detailed elemental abundance patterns of metal-poor ([Fe/H] ∼ −1 dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of GCs. Here, an attempt is made to identify such presumptive stripped stars originating from the massive, inner Galaxy GC NGC 6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of 40 RRLs centered on the GC NGC 6441. All 13 of the RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 ± 5 km s−1 and a star-to-star scatter of 11 km s−1. This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extra-tidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC 6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster’s orbit. Therefore, either the tidal radius of NGC 6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC 6441 that are building up the old spheroidal bulge.

On the extended stellar structure around NGC 288

We report on observational evidence of an extra-tidal clumpy structure around NGC 288 from an homogeneous coverage of a large area with the Pan-STARRS PS1 database. The extra-tidal star population has been disentangled from that of the Milky Way field by using a cleaning technique that successfully reproduced the stellar density, luminosity function and colour distributions of MW field stars. We have produced the cluster stellar density radial profile and a stellar density map from independent approaches, from which we found results in excellent agreement : the feature extends up to 3.5 times the cluster tidal radius. Previous works based on shallower photometric data sets have speculated on the existence of several long tidal tails, similar to that found in Pal 5. The present outcome shows that NGC 288 could hardly have such tails, but favours the notion that interactions with the MW tidal field has been a relatively inefficient process for stripping stars off the cluster. These results point to the need of a renewed overall study of the external regions of Galactic globular clusters (GGCs) in order to reliably characterise them. Hence, it will be possible to investigate whether there is any connection between detected tidal tails, extra-tidal stellar populations, extent diffuse halo-like structures with the GGCs' dynamical histories in the Galaxy.

Apparent Trend of the Iron Abundance in NGC 3201: The Same Outcome with Different Data

Abstract We further study the unusual trend we found at statistically significant levels in some globular clusters, including NGC 3201: a decreasing iron abundance in red giants toward the cluster centers. We first show that recently published new estimates of iron abundance in the cluster reproduce this trend, in spite of the authors’ statement about no metallicity spread due to a low scatter achieved in the [Fe ii/H] ratio. The mean of [Fe ii/H] within from the cluster center is lower, by Δ[Fe ii/H] = 0.05 ± 0.02 dex, than in the outer region, in agreement with our original estimate for a much larger sample size within . We found that an older data set traces the trend to a much larger radial distance, comparable with the cluster tidal radius, at Δ[Fe/H] ∼ 0.2 dex, due to higher metallicity of distant stars. We conclude the trend is reproduced by independent data sets and find that it is accompanied by both a notable same-sign trend of oxygen abundance that can vary by up to Δ[O/Fe] ∼ 0.3 dex within and an opposite-sign trend of sodium abundance.

Close encounters involving RAVE stars beyond the 47 Tucanae tidal radius

The most accurate 6D phase-space information from the Radial Velocity Experiment (RAVE) was used to integrate the orbits of 105 stars around the galactic globular cluster 47 Tucanae, to look for close encounters between them in the past, with a minimum distance approach less than the cluster tidal radius. The stars are currently over the distance range 3.0 kpc $<$ d $<$ 5.5 kpc. Using the uncertainties in the current position and velocity vector for both, star and cluster, 105 pairs of star-cluster orbits were generated in a Monte Carlo numerical scheme, integrated over 2 Gyr and considering an axisymmetric and non-axisymmetric Milky-Way-like Galactic potential, respectively. In this scheme, we identified 20 potential cluster members that had close encounters with the globular cluster 47 Tucanae, all of which have a relative velocity distribution (V$_{rel}$) less than 200 km s$^{-1}$ at the minimum distance approach. Among these potential members, 9 had close encounters with the cluster with velocities less than the escape velocity of 47 Tucanae, therefore a scenario of tidal stripping seems likely. These stars have been classified with a 93\% confidence level, leading to the identification of extratidal cluster stars. For the other 11 stars, V$_{rel}$ exceeds the escape velocity of the cluster, therefore they were likely ejected or are unassociated interlopers.

Wavenumber spectra of oscillations of open star cluster models

A correlation and spectral analysis of the phase-space density oscillations and oscillations of absolute values of stellar velocities is performed for six open cluster models (for cluster model 1 different smoothing parameters e for force functions in equations of stellar motions is considered). The mutual correlation functions are computed for the fluctuations of phase-space density f and absolute values of stellar velocities v. The spatial Fourier transform of the correlation functions with zero time shift is used to compute the wavenumber spectra of the oscillations of f and v. The wavenumber spectrum of phase-space density oscillations has a more simple structure than the wavenumber spectrum of oscillations of absolute velocities v. The most powerful oscillations of f and v are located in the domain of small wavenumbers k (and large wavelengths λ > 0.91–1.25 pc) with homologous cluster oscillations contributing significantly to the mean power of oscillations of f and v. Recurrent oscillations of v at equally spaced k are found in five cluster models. The wavenumber spectra of oscillations of f and v for cluster model 1 are found to depend on e, and the corresponding dependences are analyzed. These wavenumber spectra are found to be appreciably less dependent on e than the frequency spectra of oscillations of f. An increase of e has a weaker effect on the wavenumber spectra of oscillations of f than on those of the oscillations of absolute velocity v. The slope of the wavenumber spectrum increases, on the average, with increasing e because of the change of the radial structure of the cluster model. Astrophysical applications of the results (the dissimilarity of our spectra from the Kolmogorov spectrum, discrete structure of the wavelength spectrum of oscillations of v in five cluster models, widths of the spectra of the most powerful oscillations of cluster models, estimates of cluster tidal radii, etc.) are discussed.

Dynamics of the coronas of open star clusters

A method for distinguishing coronas in models of open star clusters is proposed. The method uses trajectories of stars that do not leave the coronas over time intervals t comparable to the mean lifetime τ of the clusters. Corona models are constructed for six numerical cluster models, and the direction and character of the dynamical evolution of the coronas are determined. Retrograde stellar motions are dominant in the coronas. In spite of some signs of dynamical instability of the coronas (small densities compared to the critical density and accelerated expansion of the coronas), the formation of close-toequilibrium density and phase-density distributions at distances from one to three cluster tidal radii from the cluster center can be seen. Approximations are constructed for the corona and cluster phase density using distributions that depend on three parameters (the parameters of the stellar motion in the Lindblad rotating coordinate system). This temporary equilibrium of the corona is due to balance in the number of starsmoving from the central areas of the cluster to the corona, and from the corona to the corona periphery or beyond. Evidence that corona stars can be gravitationally bound at distances out to four tidal radii from the cluster center is found: the presence of nearly periodic retrograde mean motions of a large number of corona stars in the Galactic plane; 91–99% of corona stars satisfy the gravitational binding criterion of Ross, Mennim and Heggie over time intervals that are close to the mean cluster lifetime. The escape rate from the corona is estimated for t ≥ τ, and found to be from 0.03 to 0.23 of the number of corona stars per violent relaxation time.

Globular cluster interstellar media: ionized and ejected by white dwarfs

UV radiation from white dwarfs can efficiently clear Galactic globular clusters (GCs) of their intra-cluster medium (ICM). This solves the problem of the missing ICM in clusters, which is otherwise expected to build up to easily observable quantities. To show this, we recreate the ionizing flux in 47 Tuc, following randomly generated stars through their AGB, post-AGB and white dwarf evolution. Each white dwarf can ionize all the material injected into the cluster by stellar winds for ~3 Myr of its evolution: ~40 such white dwarfs exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15 should be caused by a temporary overdensity. A pressure-supported ICM will expand over the cluster's tidal radius, where it will be truncated, allowing Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass should equal the mass injected by its stars over the sound-travel time between the cluster core and tidal radius. We predict ~11.3 solar masses of ICM within 47 Tuc, cleared over ~4 Myr, compared to a dynamical timescale of 4.3 Myr. We present a new mass hierarchy, discussing the transition between globular clusters dwarf galaxies.

STREGA: STRucture and Evolution of the GAlaxy – I. Survey overview and first results★

STREGA (STRucture and Evolution of the GAlaxy) is a Guaranteed Time survey being performed at the VST (the ESO VLT Survey Telescope) to map about 150 square degrees in the Galactic halo, in order to constrain the mechanisms of galactic formation and evolution. The survey is built as a five-year project, organized in two parts: a core program to explore the surrounding regions of selected stellar systems and a second complementary part to map the southern portion of the Fornax orbit and extend the observations of the core program. The adopted stellar tracers are mainly variable stars (RR~Lyraes and Long Period Variables) and Main Sequence Turn-off stars for which observations in the g,r,i bands are obtained. We present an overview of the survey and some preliminary results for three observing runs that have been completed. For the region centered on $\omega$~Cen (37 deg^2), covering about three tidal radii, we also discuss the detected stellar density radial profile and angular distribution, leading to the identification of extratidal cluster stars. We also conclude that the cluster tidal radius is about 1.2 deg, in agreement with values in the literature based on the Wilson model.

DOES THE OOSTERHOFF DICHOTOMY EXIST IN THE ANDROMEDA GALAXY? I. THE CASE OF G11

We present the first evidence that Oosterhoff type II globular clusters exist in the Andromeda galaxy (M31). On the basis of time-series photometry of the moderately metal-poor ([Fe/H]\sim -1.6 dex) M31 globular cluster G11, obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we detected and derived periods for 14 RR Lyrae stars, of which five are found to lie inside the cluster tidal radius. They include three fundamental-mode (RRab) and two first-overtone (RRc) pulsators, with average periods <Pab> = 0.70 d, and <Pc> = 0.40 d, respectively. These mean periods and the position of the cluster variable stars in the period-amplitude and period-metallicity diagrams, all suggest that G11 is likely to be an Oosterhoff type II globular cluster. This appears to be in agreement with the general behavior of Milky Way globular clusters with similar metallicity and horizontal branch morphology.

Nuclear clusters in dwarf irregular galaxies and their connection to massive galactic clusters

AbstractWe compare nuclear globular clusters (nGCs) in dwarf galaxies and Galactic GCs with extended (hot) horizontal branches (EHB–GCs) to test the suggested external origin of the latter and the conditions under which GC self-enrichment can operate. We show that the present-day escape velocity (vesc) of stellar ejecta to reach the cluster tidal radius compares with those of EHB–GCs. For EHB–GCs, we find a correlation between the present-day vesc and their metallicity as well as (V − I) colour. The similar vesc and (V − I) distribution of nGCs and EHB–GCs implies that nGCs could also have complex stellar populations. The vesc–[Fe/H] relation could reflect the known relation of increasing stellar-wind velocity with metallicity, which in turn explains why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass.

Does Collinder 236 host a Cepheid calibrator?

Photoelectric UBV photometry and star counts are presented for the previously unstudied open cluster Collinder 236, supplemented by observations for stars near the Cepheid WZ Car. Collinder 236 is typical of groups associated with Cepheids, with an evolutionary age of (3.4+-1.1)x10^7 years, but it is 1944+-71 pc distant, only half the predicted distance to WZ Car. The cluster is reddened by E(B-V)~0.26, and has nuclear and coronal radii of rn~2 arcmin (1.1 pc) and Rc~8 arcmin (4.5 pc), respectively. The Cepheid is not a member of Collinder 236 on the basis of location beyond the cluster tidal radius and implied distance, but its space reddening can be established as E(B-V)=0.268+-0.006 s.e. from 5 adjacent stars. Period changes in WZ Car studied with the aid of archival data are revised. The period of WZ Car is increasing, its rate of +8.27+-0.19 s yr^(-1) being consistent with a third crossing of the instability strip.

HIERARCHICAL STAR FORMATION IN THE MILKY WAY DISK

Hierarchical star formation leads to a progressive decrease in the clustering of star clusters both in terms of spatial scale and age. Consistently, statistical analysis of the positions and ages of clusters in the Milky Way disk strongly suggests that a correlation between the duration of star formation in a region and its size does exist. The average age difference between pairs of open clusters increases with their separation as the ∼0.16 power. In contrast, for the Large Magellanic Cloud, Efremov & Elmegreen found that the age difference scales with the ∼0.35 power of the region size. This discrepancy may be tentatively interpreted as an argument in support of intrinsically shorter (faster) star formation timescales in smaller galaxies. However, if both the effects of cluster dissolution and incompleteness are taken into consideration, the average age difference between cluster pairs in the Galaxy increases with their separation as the ∼0.4 power. This result implies that the characteristic timescale for coherent, clustered-mode star formation is nearly 1 Myr. Therefore, the overall consequence of ignoring the effect of cluster dissolution is to overestimate the star formation timescale. On the other hand, in the Galactic disk and for young clusters separated by less than three times the characteristic cluster tidal radius (10 pc), the average age difference is 16 Myr, which suggests common origin. A close pair classification scheme is introduced and a list of 11 binary cluster candidates with physical separation less than 30 pc is compiled. Two of these pairs are likely primordial: ASCC 18/ASCC 21 and NGC 3293/NGC 3324. A triple cluster candidate in a highly hierarchical configuration is also identified: NGC 1981/NGC 1976/Collinder 70 in Orion. We find that binary cluster candidates seem to show a tendency to have components of different size—evidence for dynamical interaction.

Globular cluster systems in nearby dwarf galaxies - II. Nuclear star clusters and their relation to massive Galactic globular clusters

(Abridged) Using luminosities and structural parameters of globular clusters (GCs) in the nuclear regions (nGCs) of low-mass dwarf galaxies from HST/ACS imaging we derive the present-day escape velocities (v_esc) of stellar ejecta to reach the cluster tidal radius and compare them with those of Galactic GCs with extended (hot) horizontal branches (EHBs-GCs). For EHB-GCs, we find a correlation between the present-day v_esc and their metallicity as well as (V-I)_0 colour. The similar v_esc, (V-I)_0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The v_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus the cluster v_esc can be used as parameter to describe the degree of self-enrichment. The nGCs populate the same Mv vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like wCen and NGC 2419. We argue that during accretion the rh of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. Our results support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.

A SPECTROSCOPIC SURVEY FOR ω CENTAURI MEMBERS AT AND BEYOND THE CLUSTER TIDAL RADIUS

We have used the two-degree field (2dF) multi-fiber spectrograph of the Anglo-Australian Telescope (AAT) to search for candidate members of the unusual globular cluster ω Centauri at and beyond the cluster tidal radius. Velocities with an accuracy of ~10 km s–1 were obtained for 4105 stars selected to lie in the vicinity of the lower giant branch in the cluster color-magnitude diagram (CMD) and which cover an area on the sky of ~2.4 × 3.9 deg2 centered on the cluster. Within the velocity interval 190-270 km s–1, the cluster member candidates have a steeply declining surface density distribution consistent with the adopted cluster tidal radius of 57'. For the stars in the sample beyond the tidal radius, an analysis of line strengths from the spectra, as well as radial velocities, identifies only six stars as possible candidates for extra-tidal association with the cluster. If all six of these stars are indeed related to the cluster, then a maximum of 0.7 ± 0.2% of the total cluster mass is contained in the region between 1 and 2 tidal radii. Given this limit, we conclude that there is no compelling evidence for any significant extra-tidal population in ω Cen. The effects of tidal shocks on the outer parts of the cluster are consistent with this limit. Theories for the origin of ω Cen frequently suggest that the cluster is the former nucleus of a tidally stripped dwarf galaxy. Our results require that the stripping process must have been largely complete at early epochs, consistent with the current dynamical models of the process. The stripped former dwarf galaxy stars are therefore now widely distributed around the Galaxy.

Comparing CN and CH line strengths in a homogeneous spectroscopic sample of 8 Galactic globular clusters

Our work focuses on the understanding of the origin of CNO-anomalies, which have been detected in several Galactic globular clusters (GCs). This study is based on a homogeneous data set of hundreds of medium resolution spectra of stars in eight GCs. Two of the clusters are believed to be former members of the Sagittarius dSph galaxy. Our sample comprises stars in different evolutionary states, namely the main sequence turn-off (MSTO) region, the subgiant branch (SGB), and the base of the red giant branch (RGB). We compare the relative CN and CH line strengths of stars in the same evolutionary states. The majority of the examined clusters show significant variations in their CN and CH abundances at the base of the RGB. The two former Sgr dSph clusters do not exhibit any CN-strong stars. Our results suggest that the environment in which the clusters influences existence of CN-strong stars. We confirm the known anticorrelation between CN and CH for most of the observed GCs. Although the signal of CN absorption is weaker for the hotter stars on the MSTO and SGB we observed the same anticorrelation in these less evolved stars for the CN-bimodal clusters. The inclusion of structural parameters taken from literature reveals that the existence of the CN-bifurcation seems to be independent of most other cluster characteristics. In particular, we do not confirm the correlation between cluster ellipticity and number of CN-strong stars. However, there may be a trend of an increased percentage of CN-strong stars with increasing cluster tidal radius and total luminosity. We argue that our findings are consistent with pollution by intermediate AGB stars and/or fast rotating massive stars and two generations of star formation in luminous clusters with larger tidal radii at larger Galactocentric distances.