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Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)
We have compiled the asymptotic giant, horizontal, and upper red giantbranch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809).Using the star counts and the R-parameter we compute the initial heliumabundance Y=0.274+/-0.016. TheR2=NAGB/NHB ratio (0.156+/-0.023) isunusually high for a globular cluster, being almost 2 σ away fromthe predicted values, and theR1=NAGB/NRGB ratio (0.272+/-0.047) isthe highest recorded for a massive globular cluster. We argue that M55'sparticular HB morphology and metallicity have produced long-lived HBstars that are not too blue to avoid producing AGB stars. This resulthints that we are able to map evolutionary effects on the HB. Finally,although we find no evidence of variations in HB morphology withdistance from the center of the cluster, the red HB stars aresignificantly less concentrated than the majority of HB stars, and thebluest HB stars are more centrally concentrated.
| Observational Consequences of the Hypothesized Helium-rich Stellar Population in ω Centauri
In response to the proposed high-helium content stars as an explanationfor the double main sequence observed in ω Centauri, weinvestigated the consequences of such stars elsewhere on thecolor-magnitude diagram. We concentrated on the horizontal branch, wherethe effects of high helium are expected to show themselves more clearly.In the process we developed a procedure for comparing the mass losssuffered by differing stellar populations in a physically motivatedmanner. High-helium stars in the numbers proposed seem absent from thehorizontal branch of ω Cen unless their mass-loss history is verydifferent from that of the majority metal-poor stars. It is possible togenerate a double main sequence with existing ω Cen stars viaaccretion of helium-rich pollution consistent with the latest AGB ejectatheoretical yields and such polluted stars are consistent with theobserved HB morphology of ω Cen. Polluted models are consistentwith observed merging of the main sequences as opposed to our models ofhelium-rich stars. Using the (B-R)/(B+V+R) statistic, we find that thehigh-helium bMS stars require an age difference compared to the rMSstars that is too great, whereas the pollution scenario stars have nosuch conflict for inferred ω Cen mass losses.
| Star Counts in the Globular Cluster ω Centauri. I. Bright Stellar Components
We present a photometric investigation on HB, RGB, and MSTO stars inω Cen=NGC 5139. The center of the cluster was covered with amosaic of F435W, F625W, and F658N band data collected with HST ACS. Theouter reaches were covered with a mosaic of U-, B-, V-, and I-band datacollected with the 2.2 m ESO/MPI telescope. The final catalog includes~1.7 million stars. We identified more than 3200 likely HB stars, thelargest sample ever collected in a globular cluster. We found that theHB morphology changes with the radial distance from the cluster center.The relative number of extreme HB stars decreases from ~30% to ~21% whenmoving from the center toward the outer reaches of the cluster, whilethe fraction of less hot HB stars increases from ~62% to ~72%. Thecomparison between theory and observations indicates that the empiricalstar counts of HB stars are on average larger (30%-40%) than predictedby canonical evolutionary models. Moreover, the rate of HB stars is ~43%larger than the MSTO rate. We also compared theory and observations byassuming a mix of stellar populations made with 70% of canonical He(Y=0.23) stars and 30% of He-enhanced (Y=0.33, 0.42) stars. We foundthat the observed RG/MSTO ratio agrees with the predicted lifetimes ofHe-mixed stellar populations. The discrepancy between theory andobservations decreases by a factor of 2 when compared with ratespredicted by canonical He content models, but still 15%-25% (Y=0.42) and15%-20% (Y=0.33) higher than observed. Furthermore, the ratios betweenHB and MSTO star counts are ~24% (Y=0.42) and 30% (Y=0.33) larger thanpredicted lifetime ratios.During the revision of this manuscript, Vittorio Castellani passed awayon 2006 May 19. His suggestions, ideas, and personality will be greatlymissed.Based on data obtained from the ESO Science Archive Facility and theHubble Space Telescope Archive Facility.
| Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample
We have measured the absolute proper motions of globular clusters NGC2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on theHipparcos system, and they are the first determinations ever made forthese low Galactic latitude clusters. The proper-motion uncertaintiesrange from 0.3 to 0.5 mas yr-1. The inferred orbits indicatethat (1) the single metal-rich cluster in our sample, NGC 5927,dynamically belongs to the thick disk; (2) the remaining metal-poorclusters have rather low-energy orbits of high eccentricity, and amongthese there appear to be two ``pairs'' of dynamically associatedclusters; (3) the most energetic cluster in our sample, NGC 3201, is ona highly retrograde orbit-which had already been surmised from radialvelocity alone-with an apocentric distance of 22 kpc; and (4) none ofthe metal-poor clusters appear to be associated with the recentlydetected SDSS streams or with the Monoceros structure. These are thefirst results of the Southern Proper Motion program where thesecond-epoch observations are taken with the recent CCD camera systeminstalled on the double astrograph at El Leoncito, Argentina.
| The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters
Various Galactic globular clusters display abundance anomalies thataffect the morphology of their color-magnitude diagrams. In this paperwe consider the possibility of helium enhancement in the anomaloushorizontal branch of NGC 2808. We examine the dynamics of aself-enrichment scenario in which an initial generation of stars with atop-heavy initial mass function enriches the interstellar medium withhelium via the low-velocity ejecta of its asymptotic giant branch stars.This enriched medium then produces a second generation of stars whichare themselves helium-enriched. We use a direct N-body approach toperform five simulations and conclude that such two-generation clustersare both possible and would not differ significantly from theirsingle-generation counterparts on the basis of dynamics. We find,however, that the stellar populations of such clusters would differ fromsingle-generation clusters with a standard initial mass function and inparticular would be enhanced in white dwarf stars. We conclude, at leastfrom the standpoint of dynamics, that two-generation globular clustersare feasible.
| On the effect of helium enhancement on bolometric corrections and Teff-colour relations
We evaluate the effects that variations in He content have on bolometriccorrections and T_eff-colour relations. To do this, we computed ATLAS9model atmospheres and spectral energy distributions for effectivetemperatures ranging from 3500 K to 40 000 K for dwarfs and from 3500 Kto 8000 K for giants, considering both “He-non-enhanced” and“He-enhanced” compositions. The variations in He contentthat were considered are of Δ Y = +0.1 and +0.2 for themetallicity [M/H] = +0.5 and Δ Y = +0.1 for [M/H] = -0.5 and -1.5.Synthetic photometry was then carried out for the UBVRIJHK system. Weconclude that the changes in bolometric corrections, caused by theadopted He-enhancements are in general too small (less than 0.01 mag),for both dwarfs and giants, to be affecting present-day tables ofbolometric corrections at a significant level. The only possibleexceptions are found for the U-band at T_eff between 4000 K and 8000 K,where |Δ BCU| amounts to ~0.02 mag, and for T_eff equalto 3500 K, where |Δ BCS_λ| values clearly becomemuch higher (up to 0.06 mag for passbands from U to V). However, even inthe latter case the overall uncertainty caused by variations in the Hecontent may be not so significant, because the ATLAS9 results are stillapproximative at their lowest temperature limit.
| Origin of abundance inhomogeneity in globular clusters
We numerically investigate abundance properties of the Galactic globularclusters (GCs) by adopting a new `external pollution' scenario. In thisframework, GCs are assumed to originate in forming low-mass dwarfsembedded in dark matter subhaloes at very high redshifts (z) and thus bechemically influenced by field asymptotic giant branch (AGB) stars ofthe dwarfs during early GC formation processes. GCs within a dwarfgalaxy therefore can be formed from the mixture of (i) gas ejected fromthe field AGB stars formed earlier in the dwarf and (ii) theinterstellar gas infalling to the central region of the dwarf. In thisexternal pollution scenario, the ratio of the total mass of infallinggas to that of AGB ejecta during GC formation in a dwarf (s) and thetime-scale of gas infall (σI) are the most importantkey parameters that can determine abundance properties of GCs. We mainlyinvestigate the abundance inhomogeneity among light elements (e.g. C, N,O, Na and Al) of stars in GCs by using the latest stellar yield modelsof metal-poor AGB stars with and without third dredge-up. Our principalresults for the models with no third dredge-up, which are moreconsistent with observations, are as follows. (i) Both [N/Fe] and[C/Fe] can be diverse among stars within a GC owing to chemicalpollution from field AGB stars. [N/Fe] distributions in some GCs canclearly show bimodality, whereas [C/Fe] is monomodal in most models.[N/Fe] distributions depend on s such that models with smaller s (i.e.larger mass fraction of AGB ejecta used for GC formation) show the[N/Fe] bimodality more clearly. (ii) N-rich, C-poor stars in GCs alsohave higher He abundances owing to pollution from massive AGB stars withHe-rich ejecta. The number fraction of He-rich stars (Y > 0.30) ishigher for the models with smaller s and shorter σI for3 <= s <= 24 and 105 <= σI <=107 yr. He abundances of stars correlate with [N/Fe] and[Al/Fe] and anticorrelate with [C/Fe], [O/Fe] and [Na/Fe] within GCs inour models. (iii) Although our model can much better explain theobserved C-N and Mg-Al anticorrelations than previous theoreticalmodels, it is in strong disagreement with the observed O-Naanticorrelation. (iv) This model naturally provides an explanation forthe large fraction of CN-strong stars without recourse to an implausibleinitial mass function. Based on these results for the above externalpollution scenario, we discuss the long-standing problem of theCN-bimodality prevalent in the Galactic GCs, the possible heliumabundance inhomogeneity in these systems and their horizontal branchmorphologies.
| A Triple Main Sequence in the Globular Cluster NGC 2808
Accurate photometry with HST's ACS shows that the main sequence (MS) ofthe globular cluster NGC 2808 splits into three separate branches. Thethree MS branches may be associated with the complexities of thecluster's horizontal branch and of its abundance distribution. Weattribute the MS branches to successive rounds of star formation, withdifferent helium abundances; we discuss possible sources of heliumenrichment. Some other massive globulars also appear to have complexpopulations; we compare them with NGC 2808.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555.
| Kinematic Decoupling of Globular Clusters with the Extended Horizontal Branch
About 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion.
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| Overluminous Blue Horizontal-Branch Stars Formed by Low-Mass Companions
We construct a speculative scenario for rotation-induced extra heliummixing to the envelope of horizontal-branch (HB) stars. This scenariodiffers from previous ones in that the mixing occurs after the star hasleft the red giant branch (RGB). We follow the evolution of alow-metallicity star from the RGB to the HB, and examine the densityprofile and radius in the core-envelope boundary region. In thetransition from the RGB to the HB the envelope shrinks by 2 orders ofmagnitude in size and the core swells, such that any nonnegligiblerotation on the RGB will result in a strong rotational shear at thecore-envelope boundary. For a nonnegligible rotation to exist on theRGB, the star has to be spun up by a companion spiraling inside itsenvelope (a common-envelope evolution). We speculate that shearinstabilities on the HB might mix helium-rich core material to theenvelope. The shallow density profile on the HB is less likely toprevent mixing. As previously shown, extra helium mixing can account forthe overluminous blue HB stars found in some globular clusters. Althoughbeing speculative, this study supports the idea that the presence oflow-mass companions, from planets to low-mass main-sequence stars,influence the evolution of stars, and can explain some properties of thecolor-magnitude (Hertzsprung-Russel) diagram of globular clusters.Namely, low-mass companions can be an ingredient in the so-called secondparameter of globular clusters.
| Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters
Aims.We propose the Wind of Fast Rotating Massive Stars scenario toexplain the origin of the abundance anomalies observed in globularclusters. Methods: We compute and present models of fast rotating starswith initial masses between 20 and 120 M_ȯ for an initialmetallicity Z = 0.0005 ([Fe/H]≃-1.5). We discuss thenucleosynthesis in the H-burning core of these objects and present thechemical composition of their ejecta. We consider the impact ofuncertainties in the relevant nuclear reaction rates. Results: Fastrotating stars reach critical velocity at the beginning of theirevolution and remain near the critical limit during the rest of the mainsequence and part of the He-burning phase. As a consequence they loselarge amounts of material through a mechanical wind which probably leadsto the formation of a slow outflowing disk. The material in this slowwind is enriched in H-burning products and presents abundance patternssimilar to the chemical anomalies observed in globular cluster stars. Inparticular, the C, N, O, Na and Li variations are well reproduced by ourmodel. However the rate of the 24Mg(p,γ) has to beincreased by a factor 1000 around 50 × 106 K in orderto reproduce the amplitude of the observed Mg-Al anticorrelation. Wediscuss how the long-lived low-mass stars currently observed in globularclusters could have formed out of the slow wind material ejected bymassive stars.
| Na-O anticorrelation and horizontal branches. VI. The chemical composition of the peculiar bulge globular cluster NGC 6388
We present the LTE abundance analysis of high resolution spectra for redgiant stars in the peculiar bulge globular cluster NGC 6388. Spectra ofseven members were taken using the UVES spectrograph at the ESO VLT2 andthe multiobject FLAMES facility. We exclude any intrinsic metallicityspread in this cluster: on average, [Fe/H]=-0.44±0.01±0.03dex on the scale of the present series of papers, where the first errorbar refers to individual star-to-star errors and the second issystematic, relative to the cluster. Elements involved in H-burning athigh temperatures show large spreads, exceeding the estimated errors inthe analysis. In particular, the pairs Na and O, Al and Mg areanticorrelated and Na and Al are correlated among the giants in NGC6388, the typical pattern observed in all galactic globular clustersstudied so far. Stars in NGC 6388 shows an excess of α-processelements, similar to the one found in the twin bulge cluster NGC 6441.Mn is found underabundant in NGC 6388, in agreement with the averageabundance ratio shown by clusters of any metallicity. Abundances ofneutron-capture elements are homogeneously distributed within NGC 6388;the [Eu/Fe] ratio stands above the value found in field stars of similarmetallicity.Based on observations collected at ESO telescopes under programme073.D-0211. Full Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/967
| Na-O anticorrelation and horizontal branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra
Aims.We present an analysis of FLAMES-Giraffe spectra for several brightgiants in NGC 6441, to investigate the presence andextent of the Na-O anticorrelation in this anomalous globular cluster. Methods: The field of NGC 6441 is very crowded, withsevere contamination by foreground (mainly bulge) field stars.Appropriate membership criteria were devised to identify a group of 25likely cluster members among the about 130 stars observed. Combined withthe UVES data obtained with the same observations (Gratton et al. 2006),high dispersion abundance analyses are now available for a total of 30stars in NGC 6441, 29 of them having data for both Oand Na. The spectra were analyzed by a standard line analysis procedure;care was taken to minimize the impact of the differential interstellarreddening throughout the cluster, and to extract reliable informationfrom crowded, and moderately high S/N (30-70), moderately highresolution (R˜ 23 000) spectra. Results: NGC6441 has the typical abundance pattern seen in several otherglobular clusters. It is very metal-rich ([Fe/H] = -0.34 ± 0.02± 0.04 dex). There is no clear sign of star-to-star scatter inthe Fe-peak elements. The α-elements Mg, Si, Ca, and Ti areoverabundant by rather large factors, suggesting that the cluster formedfrom material enriched by massive core collapse SNe. The O-Naanticorrelation is well defined, with about 1/4 of the stars beingNa-rich and O-poor. One of the stars is a Ba-rich and moderately C-richstar. Such stars are rare in globular clusters. Conclusions: .Thedistribution of [Na/O] ratios among RGB stars in NGC6441 appears similar to the distribution of colors of starsalong the horizontal branch. The fraction of Na-poor, O-rich stars foundin NGC 6441 agrees well with that of stars on the redhorizontal branch of this cluster (in both cases about 80%), with asloping distribution toward lower values of [O/Na] (among RGB stars) andbluer colors (among HB stars).Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO Program 073.D-0211). Full Table [see fulltextsee full textsee full textsee full text] is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/953
| Na-O anticorrelation and horizontal branches. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218
We used the multifiber spectrograph FLAMES on the ESO Very LargeTelescope UT2 to derive atmospheric parameters, metallicities andabundances of O and Na for 79 red giant stars in the Galactic globularcluster NGC 6218 (M 12). We analyzed stars in the magnitude range fromabout 1 mag below the bump to the tip of the Red Giant Branch. Theaverage metallicity we derive is [Fe/H]=-1.31± 0.004±0.028 dex (random and systematic errors, respectively), with a verysmall star-to-star scatter (rms=0.033 dex), from moderatelyhigh-resolution Giraffe spectra. This is the first extensivespectroscopic abundance analysis in this cluster. Our results indicatethat NGC 6218 is very homogeneous as far as heavy elements areconcerned. On the other hand, light elements involved in the well knownproton-capture reactions of H-burning at high temperature, such as O andNa, show large variations, anticorrelated with each other, at allluminosities along the red giant branch. The conclusion is that the Na-Oanticorrelation must be established in early times at the clusterformation. We interpret the variation of Na found near the RGB-bump asthe effect of two distinct populations having different bumpluminosities, as predicted for different He content. To our knowledge,NGC 6218 is the first GC where such a signature has beenspectroscopically detected, when combined with consistent andhomogeneous data obtained for NGC 6752 to gain in statisticalsignificance.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables 2, 3, and 5 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
| Na-O anticorrelation and horizontal branches. II. The Na-O anticorrelation in the globular cluster NGC 6752
We are studying the Na-O anticorrelation in several globular clusters ofdifferent Horizontal Branch (HB) morphology in order to derive apossible relation between (primordial) chemical inhomogeneities andmorphological parameters of the cluster population. We used themultifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 andderived atmospheric parameters and elemental abundances of Fe, O and Nafor about 150 red giant stars in the Galactic globular cluster NGC 6752.The average metallicity we derive is [Fe/H] = -1.56, in agreement withother results from red giants, but lower than obtained for dwarfs orearly subgiants. In NGC 6752 there is not much space for an intrinsicspread in metallicity: on average, the rms scatter in [Fe/H] is 0.037± 0.003 dex, while the scatter expected on the basis of the majorerror sources is 0.039 ± 0.003 dex. The distribution of starsalong the Na-O anticorrelation is different to what was found in thefirst paper of this series for the globular cluster NGC 2808: in NGC6752 it is skewed toward more Na-poor stars, and it resembles more theone in M 13. Detailed modeling is required to clarify whether thisdifference may explain the very different distributions of stars alongthe HB.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables [see full textsee full textsee full text], [seefull textsee full textsee full text] and [see full textsee full textseefull text] are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/927
| NGC 6441: another indication of very high helium content in globular cluster stars
Context: The metal-rich bulge globular cluster NGC6441 shows both a well-developed blue horizontal branch (Richet al.), and a strong slope upward from the red clump to the blue of theRR Lyrae region. The former corresponds to the well-known secondparameter problem. Both features are not explained by conventionalevolutionary models. Aims: Helium self-enrichment is proposed as apossible solution to both questions, a mechanism already invoked for theinterpretation of the peculiarities in NGC 2808 andM 13. Methods: We make use of horizontal branchsimulations that cover a wide range of main sequence helium abundance toinvestigate whether the main features of the NGC 6441 horizontal branchpopulation, including the RR Lyrae variables period, can be reproduced. Results: To describe the horizontal branch of NGC 6441, the followingmust be met. The helium content Y in the red clump must reach at least0.35; values up to Y˜0.37 are necessary to populate the RR Lyrregion, also reproducing the observed mean long periods; depending onthe dispersion in mass loss assumed in the simulations, values up toY˜0.38-0.40 are necessary to populate the blue HB. The totalself-enriched population amounts to ~60% of the whole stellar content. Conclusions: . Self-enrichment and multiple star-formation episodes inthe early evolution of globular clusters appear more and more able toaccount for many of the chemical and population peculiarities observedin these systems. The very large helium abundances (Y⪆0.35) requiredfor ~14% of the horizontal branch population pose some problems for theenrichment mechanisms.
| Integrated colours of Milky Way globular clusters and horizontal branch morphology
Broadband colours are often used as metallicity proxies in the study ofextragalactic globular clusters. A common concern is the effect ofvariations in horizontal branch (HB) morphology - the second-parametereffect - on such colours. We have used U BV I, Washington, and DDOphotometry for a compilation of over 80 Milky Way globular clusters toaddress this question. Our method is to fit linear relations betweencolour and [Fe/H], and study the correlations between the residualsabout these fits and two quantitative measures of HB morphology. Whilethere is a significant HB effect seen in U-B, for the commonly usedcolours B-V, V-I, and C-T_1, the deviations from the baselinecolour-[Fe/H] relations are less strongly related to HB morphology.There may be weak signatures in B-V and C-T_1, but these are at thelimit of observational uncertainties. The results may favour the use ofB-I in studies of extragalactic globular clusters, especially when itshigh [Fe/H]-sensitivity is considered.
| Surface Pollution of Main-Sequence Stars through Encounters with AGB Ejecta in ω Centauri
The origin of a double main sequence (MS) in ω Centauri isexplored. We have shown from theoretical calculations of stellarevolution that the colors of MS stars are shifted to those of theobserved blue MS if the surface layers are polluted by He-rich materialswith a mass of ~0.1 Msolar. Stars are thought to be pollutedthrough numerous encounters with the ejecta descended from massiveasymptotic giant branch (AGB) stars. Two populations of stars withdifferent kinematics observed especially in ω Cen indicate thatkinematically cooler stars are more polluted through encounters with AGBejecta than kinematically hotter ones because the accretion rate isinversely proportional to the cube of the relative velocity. We proposethat both of these factors split the MS in ω Cen. This theoreticalscheme explains why only ω Cen exhibits a double MS and matchesthe amount of He necessary to produce the blue MS to that supplied frommassive AGB stars. Furthermore, we predict that even if globularclusters (GCs) possess only one generation of stars, the velocitydispersion of stars broadens the MS in the color-magnitude diagram aslong as the GCs are massive enough to retain the AGB ejecta after theburst of star formation. This view explains the broad MS recently foundin the GC NGC 2808, which exhibits no scatter in [Fe/H] and thus islikely to consist of a single generation of stars, unlike the case ofω Cen.
| A Robust Test of Evolution near the Tip of the Red Giant Branch and Missing Giants in NGC 2808
We describe a new method for robustly testing theoretical predictions ofred giant evolution near the tip of the giant branch. When theoreticalcumulative luminosity functions are shifted to align the tip in the Iband and normalized at a luminosity level slightly brighter than the redgiant bump, virtually all dependence on age and composition (heavyelements and helium abundance) is eliminated. While significantcomparisons with observations require large samples of giant stars, suchsamples are available for some of the most massive Milky Way globularclusters. We present comparisons with the clusters NGC 2808 and M5 andfind that NGC 2808 has a deficiency of bright giants (with a probabilityof less than about 3% that a more extreme distribution of giant starswould have happened by chance). We discuss the possibilities thatunderestimated neutrino losses or strong mass loss could be responsiblefor the deficit of giants. While we cannot rule out the neutrinohypothesis, it cannot explain the apparent agreement between the M5observations and models. On the other hand, strong mass loss provides apotential link between the giant star observations and NGC 2808'sunusually blue horizontal branch. If the mass loss hypothesis is true,there is likely a significant population of He white dwarfs that couldbe uncovered with slightly deeper UV observations of the cluster.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| 2D Fokker-Planck models of rotating clusters
Globular clusters rotate significantly, and with the increasing amountof detailed morphological and kinematical data obtained in recent yearson galactic globular clusters many interesting features show up. We showhow our theoretical evolutionary models of rotating clusters can be usedto obtain fits, which at least properly model the overall rotation andits implied kinematics in full 2D detail (dispersions, rotationvelocities). Our simplified equal mass axisymmetric rotating modelprovides detailed two-dimensional kinematical and morphological data forstar clusters. The degree of rotation is not dominant in energy, butalso non-negligible for the phase-space distribution function, shape andkinematics of clusters. Therefore, the models are well applicable forgalactic globular clusters. Since previously published papers on thatmatter by us made it difficult to do detailed comparisons withobservations, we provide a much more comprehensive and easy-to-use setof data here, which uses as entries dynamical age and flattening ofobserved cluster and then offers a limited range of applicable models infull detail. The method, data structure and some exemplary comparisonwith observations are presented. Future work will improve modelling anddata base to take a central black hole, a mass spectrum and stellarevolution into account.
| The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
We investigate the suggestion that there are stellar populations in someglobular clusters with enhanced helium (Y~0.28-0.40) compared to theprimordial value. We assume that a previous generation of massiveasymptotic giant branch (AGB) stars have polluted the cluster. Twoindependent sets of AGB yields are used to follow the evolution ofhelium and CNO using a Salpeter initial mass function (IMF) and twotop-heavy IMFs. In no case are we able to produce the postulated largeY~0.35 without violating the observational constraint that the CNOcontent is nearly constant.
| A Pulsational Distance to ω Centauri Based on Near-Infrared Period-Luminosity Relations of RR Lyrae Stars
We present new near-infrared (J and K) magnitudes for 114 RR Lyrae starsin the globular cluster ω Centauri (NGC 5139), which we combinewith data from the literature to construct a sample of 180 RR Lyraestars with J and K mean magnitudes on a common photometric system. Thisis currently the largest such sample in any stellar system. We alsopresent updated predictions for J- and K-band period-luminosityrelations for both fundamental and first-overtone RR Lyrae stars, basedon synthetic horizontal branch models with metal abundance ranging fromZ=0.0001 to 0.004. By adopting for the ω Cen variables withmeasured metal abundances an α-element enhancement of a factor of3 (~0.5 dex) with respect to iron, we find a true distance modulusμ0=13.70+/-0.06+/-0.06 (random and systematic errors,respectively), corresponding to a distance d=5.5+/-0.03+/-0.03 kpc. Ourestimate is in excellent agreement with the distance inferred for theeclipsing binary OGLEGC 17, but differ significantly from the recentdistance estimates based on cluster dynamics and on high-amplitudeδ Scuti stars.Based in part on observations collected with the New TechnologyTelescope, operated at ESO, La Silla, under programs 066D-0557 and068D-0545.
| Fluorine Abundance Variations as a Signature of Enhanced Extra Mixing in Red Giants of the Globular Cluster M4
We show that enhanced extra mixing in low-mass red giants can result ina fluorine abundance that is correlated with abundance variations ofother elements participating in H burning, such as C, N, O, and Na. Thisfinding is used to explain the fluorine abundance variations recentlyfound in bright red giants of the globular cluster M4.
| On the self-enrichment scenario of galactic globular clusters: constraints on the IMF
Aims.Galactic globular cluster (GC) stars exhibit abundance patternsthat are not shared by their field counterparts, e.g. thewell-documented O-Na and Mg-Al anticorrelations. Recent spectroscopicobservations of GC turnoff stars have provided compelling evidence thatthese abundance anomalies were already present in the gas from which theobserved stars formed. A widely held hypothesis is that the gas was"polluted" by stars that were more massive (and evolving faster) thanthe presently observed low-mass stars. In the framework of this"self-enrichment" scenario for GCs, we present a new method of derivingthe initial mass function (IMF) of the polluters, by using the O/Naabundance distribution. Methods: .We focus on NGC 2808, a GC forwhich the largest sample of O and Na abundance determinations ispresently available. We use the abundance distribution of [O/Na] toderive the amount of polluted material with respect to the originalcomposition. We explore two scenarios in detail for the self-enrichmentof the cluster, which differ by the assumptions made on the compositionof the polluter ejecta. In each case we consider two classes of possible"culprits": massive asymptotic giant branch (AGB) stars (4-9Mȯ) and winds of massive stars (WMS) in the mass range10-100 Mȯ. Results: .We obtain upper limits forthe slope of the IMF (assumed to be given by a power-law) of the starsinitially more massive than the present turnoff mass. We also derivelower limits for the amount of stellar residues in NGC 2808.Conclusions: .We find that the polluter IMF had to be much flatter thanthe presently observed IMFs in stellar clusters, which agrees with theresults of two other GC IMF determination methods, which we alsodiscuss. Likewise, we find that the present mass of the GC should betotally dominated by stellar remnants if the polluters were AGB stars,which is not the case if the culprits are WMS. We critically analyse theadvantages and shortcomings of each potential polluter class and findthe WMS scenario more attractive.
| Does the oxygen-sodium anticorrelation in globular clusters require a lowering of the 23Na(p,α)20Ne reaction rate?
Context: .The chemical content of the ejecta of Asymptotic Giant Branchstars (AGBs) is important to understand whether intermediate mass starsplayed a role in the "self-enrichment scenario", to explain the chemicalanomalies observed among Globular Clusters stars. One difficulty of thismodel is that it is not fully consistent with the observed oxygen-sodiumanticorrelation. Aims: .We look for the combinations of theinput-physics parameters by which the yields of massive AGBs aresodium-rich and oxygen-poor. Methods: .Many evolutions from theearly evolutionary phases until the mass of the envelope drops below~0.5~Mȯ are calculated for a 5~MȯZ=0.001 model for various assumptions concerning the extra mixing fromthe bottom of the envelope and the relevant cross sections involvingsodium. Results: .A modest amount of extra mixing from the bottomof the surface convective zone leads to ejecta that are moderatelysodium rich and oxygen depleted. A different but appealing modelizationallows us to reproduce the observations of stars with a lower oxygencontent: extra mixing is not included, but the cross sections of thereaction 23Na(p, α)20Ne must be a factor of~4 lower than the recommended values. If the initial neon content of thestar is not solar-scaled but enhanced as the other α elements by afactor ~2, the afore mentioned cross section must be lessened by only afactor ~2.
| Hot flashers and He dwarfs in galactic globulars
Context: .We revisit the evolutionary scenario for hot flasher low-massstructures, where mass loss delays the He flash till the initial phasesof their white dwarf cooling sequence. Aims: .Our aim has been totest the theoretical results vis-a-vis different assumptions about theefficiency of mass loss. Methods: .To this purpose, we presentevolutionary models covering a fine grid of masses, as obtained assuminga single episode of mass loss in a red giant model of 0.86Mȯ with Z= 0.0015. Results: .We find reasonableagreement with previous evolutionary investigations, showing that forthe given metallicity late hot flashers are predicted to cover the massrange M= 0.4975 to M= 0.4845 (±0.0005) Mȯ, allmodels igniting the He-flash with a mass of the H-rich envelope as givenby Me = 0.00050 ±0.00002 Mȯ. Theignition mechanism is discussed in some detail, showing the occurrenceof a bifurcation in the evolutionary history of stellar structures atthe lower-mass limit for He ignition. Below such a critical mass, thestructures miss the He ignition, cooling down as a hotflasher-manqué He white dwarf. We predict that these structureswill cool down, reaching the luminosity log L/Lȯ=-1 in atime five times longer at the very least than the corresponding coolingtime of a normal CO white dwarf. Conclusions: .On very generalgrounds, one expects that old stellar clusters with a sizeablepopulation of hot flashers should produce at least a similar amount ofslow-cooling He white dwarfs. According to this result, in a clusterwhere 20% of red giants escape the He burning phase, one expects roughlytwice as white dwarfs than in a normal cluster where all red giantsundergo their He flash.
| Wolf-Rayet and OB Star Self-Enrichment of Globular Clusters?
Nitrogen abundance inhomogeneities are present among main-sequence starsin globular clusters of the Milky Way. Since N-rich cluster dwarfs areunlikely to have nucleosynthesized nitrogen within their own interiors,they presumably obtained their excess N from elsewhere. If the abundanceinhomogeneities are due to the self-enrichment of a globular cluster,then the earliest source of nitrogen could have been massive cluster OBstars and Wolf-Rayet stars of type N. Several obstacles to WN stars'being a viable enrichment mode for globular clusters are discussed: (1)a W-R phase may have been inhibited for metal-poor OB stars, due to thelow mass-loss rates expected of their radiatively driven winds, and (2)unless globular clusters had top-heavy stellar mass functions, their WNstars would have been too few in number to explain the amount ofCNO-processed material in their nitrogen-rich, low-mass stars. Incontrast, it is pointed out that OB stars in their main-sequence andsupergiant phases of evolution might also eject CNO-processed materialeven if they do not evolve through a WN phase, while W-R stars have beendiscovered in some metal-poor dwarf galaxies and starburst galaxies. Inaddition, low metallicities could favor the production of WN stars overWC stars, and this could explain why the abundance inhomogeneities inMilky Way globular clusters generally involve excesses of CNO-processedmaterial. One scenario is discussed in which globular clusters form atthe interface between massive, colliding gas clouds. If such clouds havemasses comparable to dwarf galaxies, with relative velocitiescharacteristic of the Galactic halo, then the collision process coulddrive gas into an accreting protocluster at a rate that is sufficient tocomplete cluster formation within several million years. This timescalecould allow nitrogen-rich wind ejecta from WN or OB stars to beincorporated into later cluster star formation. The ram pressureassociated with a colliding-cloud environment might not only confine thestellar wind ejecta, but also funnel it into sites of active starformation within the protocluster. In this picture, globular clustersform as open systems with gas continuing to flow into them from agreater cloud-merger environment while star formation is occurring. Insuch a circumstance, not only may cluster WN and OB stars have promotedenrichment, but wind ejecta from massive stars that are external to theprotocluster may also have been acquired.
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