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Stellar Populations of Globular Clusters in the Elliptical Galaxy NGC 1407
We present high-quality Keck spectroscopic data for a sample of 20globular clusters (GCs) in the massive E0 galaxy NGC 1407. A subset of20 line-strength indices of the Lick/IDS system has been measured forboth the GC system and the central integrated starlight of the galaxy.Ages, metallicities, and [α/Fe] ratios have been derived usingseveral different approaches. The majority of GCs in NGC 1407 studiedare old, follow a tight metallicity sequence reaching values slightlyabove solar, and exhibit mean [α/Fe] ratios of ~0.3 dex. Inaddition, three GCs are formally derived to be young (~4 Gyr), but weargue that they are actually old GCs hosting blue horizontal branches.We report, for the first time, evidence for the existence of twochemically distinct subpopulations of metal-rich (MR) GCs. We find thatsome MR GCs exhibit significantly larger [Mg/Fe] and [C/Fe] ratios.Different star formation timescales are proposed to explain thecorrelation between Mg and C abundances. We also find striking CNoverabundances over the entire GC metallicity range. It is interestingto note that the behavior of C and N in metal-poor GCs clearly deviatesfrom that in MR GCs. In particular, for MR GCs, N increases dramaticallywhile C essentially saturates. This may be interpreted as a consequenceof the increasing importance of the CNO cycle with increasingmetallicity.
| Washington Photometry of the Red Giant Branch in NGC 6388
We present Washington photometry of the giant branches of the relativelymetal-rich globular cluster NGC 6388 and compare it with 47 Tuc. Byexamining the spatial distribution of stellar colors across the redgiant branch and using spectroscopic data on several bright giants, wefound that NGC 6388's center is affected by differential reddening of atleast ΔE(B-V)~0.1. Most of the width of the red giant branch iscaused by that differential reddening. The southeastern outer regions ofthe cluster seem to be less affected by variable dust extinction, and wefind that NGC 6388 is slightly more metal-rich than 47 Tuc, with adistinct asymptotic giant branch. We have put an upper limit on possiblevariations in [Fe/H] for NGC 6388, finding that it must be less than~0.2 dex. It is unlikely that such a small range in metallicity could beresponsible for the blue horizontal branch and blue tail stars in acluster with a metallicity similar to the more massive 47 Tuc, whichonly exhibits a red horizontal branch.
| 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.
| NGC 6388: Chemical Composition of Its Eight Cool Giants
Eight cool giants in the unusual globular cluster NGC 6388 have beeninvestigated in order to derive their elemental abundances. Effectivetemperatures from 3500 to 3850 K were derived using the method ofline-depth ratios. Surface gravities were derived in two ways.Spectroscopic gravities, derived by the requirement that the abundanceof iron be the same from Fe I and Fe II lines, were rather low, rangingfrom -0.3 to 0.0. Photometric gravities, derived from the assumedstellar mass of 0.7 Msolar and the luminosity andTeff of the stars, fell between +0.25 and +0.70. Mean [Fe/H]values were -0.8 for spectroscopic gravities and -0.6 for photometricgravities. A test using spectra of the same resolution of the twocoolest red giants in the globular cluster M4 obtained at the ApachePoint Observatory were analyzed for comparison with the definitiveanalysis of Ivans et al. (1999). The very cool metal-poor red giant HD232078 was also analyzed for comparison with the analysis of Gonzalez& Wallerstein (1998). Both comparisons showed that our methods yieldthe same abundance scale as previous works. We have compared thecomposition of stars in NGC 6388 with those of K giants with similar[Fe/H] in 47 Tuc and the Galactic bulge. The observed value of [O/Fe] isnear zero, which is less than in 47 Tuc and bulge stars of similarmetallicity. The α-elements behave similarly to oxygen and showonly small excesses at about the same level as do the α-elementsin the globular clusters associated with the Sgr system. It is unclearwhether these differences are responsible for the unusualcolor-magnitude diagram of NGC 6388.
| Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
| 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
| 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.
| Variable Stars in Metal-Rich Globular Clusters. III. NGC 6539
We present the first CCD-based search for variable stars in themetal-rich globular cluster NGC 6539 ([Fe/H]~-0.7). Our VI photometry ofstars within 7' of the cluster center shows a color-magnitude diagramwith a normal red clump horizontal-branch morphology without theextension of blue stars seen in two globular clusters of comparablemetallicity, NGC 6388 and NGC 6441. We have discovered 13 variablestars, 12 of which are long-period variables (LPVs), and have usedinfrared photometry from the Two Micron All Sky Survey to furtherclarify their position in the color-magnitude diagram. We find thespecific frequency of these stars, SLPV=0.32, to becomparable to those found in other clusters recently surveyed. We havealso detected one RR Lyrae variable star in our data, although it isunlikely to be a cluster member. The color of this star, when comparedwith low-reddening RR Lyrae stars near the Sun, indicates aninterstellar reddening of E(V-I)=1.22 mag toward NGC 6539, in excellentagreement with estimates from the literature.
| 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).
| Deep HST Photometry of NGC 6388: Age and Horizontal-Branch Luminosity
Using the Hubble Space Telescope, we have obtained the firstcolor-magnitude diagram (CMD) to reach the main-sequence turnoff of theGalactic globular cluster NGC 6388. From a comparison between thecluster CMD and that of 47 Tucanae, we find that the bulk of the starsin these two clusters have nearly the same age and chemical composition.On the other hand, our results indicate that the blue horizontal branchand RR Lyrae components in NGC 6388 are intrinsically overluminous,which must be due to one or more, still undetermined, noncanonicalsecond parameter(s) affecting a relatively minor fraction of the starsin NGC 6388.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 (AURA), Inc.,under NASA contract NAS 5-26555, and on observations retrieved from theESO ST-ECF Archive.
| Complex stellar populations in massive clusters: trapping stars of a dwarf disc galaxy in a newborn stellar supercluster
Some of the most-massive globular clusters of our Milky Way, such as,for example, ω Centauri, show a mixture of stellar populationsspanning a few Gyr in age and 1.5 dex in metallicities. In contrast,standard formation scenarios predict that globular and open clustersform in one single starburst event of duration <~10 Myr and thereforeshould exhibit only one age and one metallicity in its stars. Here, weinvestigate the possibility that a massive stellar supercluster may trapolder galactic field stars during its formation process that are laterdetectable in the cluster as an apparent population of stars with a verydifferent age and metallicity.With a set of numerical N-body simulations, we are able to show that, ifthe mass of the stellar supercluster is high enough and the stellarvelocity dispersion in the cluster is comparable to the dispersion ofthe surrounding disc stars in the host galaxy, then up to about 40 percent of its initial mass can be additionally gained from trapped discstars. We also suggest that a supercluster may capture in excess of itsown mass under certain conditions.
| Stellar Populations and Mass Loss in M15: A Spitzer Space Telescope Detection of Dust in the Intracluster Medium
We present Spitzer Space Telescope IRAC and MIPS observations of theGalactic globular cluster M15 (NGC 7078), one of the most metal-poorclusters, with a [Fe/H]=-2.4. Our Spitzer images reveal a population ofdusty red giants near the cluster center, a previously detectedplanetary nebula designated K648, and a possible detection of theintracluster medium arising from mass-loss episodes from the evolvedstellar population. Our analysis suggests that(9+/-2)×10-4 Msolar of dust is present inthe core of M15, and that this material has accumulated over a period of~106 yr, a timescale 10 times shorter than the last Galacticplane crossing event. We also present Spitzer IRS follow-up observationsof K648, including the detection of the [Ne II] 12.81 μm line, anddiscuss abundances derived from infrared fine-structure lines.
| Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441
We have applied Alard's image-subtraction method (ISIS ver. 2.1) to theobservations of the globular clusters NGC 6388 and NGC 6441 previouslyanalyzed using standard photometric techniques (DAOPHOT and ALLFRAME).In this reanalysis of observations obtained at the Cerro TololoInter-American Observatory, besides recovering the variables previouslydetected on the basis of our ground-based images, we have also been ableto recover most of the RR Lyrae variables previously detected only inthe analysis of Hubble Space Telescope WFPC2 observations of the innerregion of NGC 6441. In addition, we report five possible new variablesnot found in the analysis of the Hubble Space Telescope observations ofNGC 6441. This dramatically illustrates the capabilities ofimage-subtraction techniques applied to ground-based data to recovervariables in extremely crowded fields. We have also detected 12 newvariables and 6 possible variables in NGC 6388 not found in our previousground-based studies. The revised mean period for RRab stars in NGC 6388is 0.676 days, while the mean period of RRab stars in NGC 6441 isunchanged at 0.759 days. These values are among the largest known forany Galactic globular cluster. Additional probable type II Cepheids wereidentified in NGC 6388, confirming its status as a metal-rich globularcluster rich in Cepheids.
| Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC 6388. Indications of helium enrichment and a lesson in crowded field spectroscopy
Context: .The metal-rich bulge globular cluster NGC 6388 shows adistinct blue horizontal-branch tail in its colour-magnitude diagram(Rich et al. 1997) and is thus a strong case of the well-known 2ndParameter Problem. In addition, its horizontal branch (HB) shows anupward tilt toward bluer colours, which cannot be explained by canonicalevolutionary models. Aims: .Several noncanonical scenarios havebeen proposed to explain these puzzling observations. In order to testthe predictions of these scenarios, we have obtained medium resolutionspectra to determine the atmospheric parameters of a sample of the blueHB stars in NGC 6388. Methods: .Using the medium resolutionspectra, we determine effective temperatures, surface gravities andhelium abundances by fitting the observed Balmer and helium lines withappropriate theoretical stellar spectra. As we know the distance to thecluster, we can verify our results by determining masses for the stars.During the data reduction we took special care in subtracting thebackground, which is dominated by the overlapping spectra of coolstars. Results: .The cool blue tail stars in our sample witheffective temperatures Teff ≈ 10 000 K have lower thancanonical surface gravities, suggesting that these stars are, onaverage, ≈0.4 m brighter than canonical HB stars in agreement withthe observed upward slope of the HB in NGC 6388. Moreover, the mean massof these stars agrees well with theoretical predictions. In contrast,the hot blue tail stars in our sample with Teff ≥ 12 000 Kshow significantly lower surface gravities than predicted by anyscenario, which can reproduce the photometric observations. Their massesare also too low by about a factor of 2 compared to theoreticalpredictions. Conclusions: .The physical parameters of the blue HBstars near 10 000 K support the helium pollution scenario. The lowgravities and masses of the hot blue tail stars, however, are probablycaused by problems with the data reduction, most likely due to remainingbackground light in the spectra, which would affect the fainter hot bluetail stars much more strongly than the brighter cool blue tail stars.Our study of the hot blue tail stars in NGC 6388 illustrates theobstacles which are encountered when attempting to determine theatmospheric parameters of hot HB stars in very crowded fields usingground-based observations. We discuss these obstacles and offer possiblesolutions for future projects.
| The period-luminosity relation for type II Cepheids in globular clusters
We report the result of our near-infrared observations (JHKs)for type II Cepheids (including possible RV Tau stars) in galacticglobular clusters. We detected variations of 46 variables in 26 clusters(10 new discoveries in seven clusters) and present their light curves.Their periods range from 1.2 d to over 80 d. They show a well-definedperiod-luminosity relation at each wavelength. Two type II Cepheids inNGC 6441 also obey the relation if we assume the horizontal branch starsin NGC 6441 are as bright as those in metal-poor globular clusters inspite of the high metallicity of the cluster. This result supports thehigh luminosity which has been suggested for the RR Lyr variables inthis cluster. The period-luminosity relation can be reproduced using thepulsation equation assuming that all the stars have the same mass.Cluster RR Lyr variables were found to lie on an extrapolation of theperiod-luminosity relation. These results provide important constraintson the parameters of the variable stars.Using Two Micron All-Sky Survey (2MASS) data, we show that the type IICepheids in the Large Magellanic Cloud (LMC) fit our period-luminosityrelation within the expected scatter at the shorter periods. However, atlong periods (P > 40 d, i.e. in the RV Tau star range) the LMC fieldvariables are brighter by about one magnitude than those of similarperiods in galactic globular clusters. The long-period cluster starsalso differ from both these LMC stars and galactic field RV Tau stars ina colour-colour diagram. The reasons for these differences arediscussed.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Na-O anticorrelation and HB. III. The abundances of NGC 6441 from FLAMES-UVES spectra
Aims.The aim of the present work is to determine accurate metallicitiesfor a group of red giant branch stars in the field of the bulge GlobularCluster NGC 6441. This is the third paper in a seriesresulting from a large project aimed at determining the extent of theNa-O anticorrelation among Globular Cluster stars and exploring itsrelationship with HB morphology. Methods: .We present an LTEabundance analysis of these objects, based on data gathered with theFLAMES fiber facility and the UVES spectrograph at VLT2. Results:.Five of the thirteen stars observed are members of the cluster. Theaverage Fe abundance for these five stars is [Fe/H] = -0.39±0.04± 0.05 dex, where the first error bar includes theuncertainties related to star-to-star random errors, and the second onethe systematic effects related to the various assumptions made in theanalysis. The overall abundance pattern is quite typical of GlobularClusters, with an excess of the α-elements and of Eu. There isevidence that the stars of NGC 6441 are enriched inNa and Al, while they have been depleted of O and Mg by H-burning athigh temperatures, analogous with extensive observations of otherGlobular Clusters: in particular, one star is clearly Na and Al-rich andO and Mg-poor. We also obtained quite high V abundances, but it ispossible that this is an artifact of the analysis, since similar high Vabundances are also derived for the field stars. These last are all moremetal-rich than NGC 6441 and probably belong to thebulge population.
| Near-Field Cosmology with Horizontal Branch and RR Lyrae Stars
The importance of horizontal branch and RR Lyrae stars is discussed inthe context of cosmological arguments for the formation of the Galactichalo and its satellite dwarf galaxies. It is shown, in particular, thatthe Galactic halo globular cluster system cannot have formed from theaccretion of ``protogalactic fragments'' resembling the very earlycounterparts of the present-day dwarf satellite galaxies of the MilkyWay, or else its RR Lyrae properties would be very different from whatis currently observed.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| The lack of close binaries among hot horizontal branch stars in NGC 6752
Aims.We present the results of a spectroscopic search for close binariesamong horizontal branch (HB) stars in NGC 6752.Methods: .We used the ESO VLT-FORS2 instrument to obtain mediumresolution (R=4100) spectra of 51 hot HB stars with 8 000 K ≤ T_eff≤ 32 000 K during four consecutive nights. Eighteen of our targetsare extreme horizontal branch (EHB) stars with {T_eff}≥ 22 000 K.Radial velocity variations were measured with cross-correlationtechniques, previously evaluated the statistical and systematic errorsassociated with them. Results: .No close binary system has beendetected among our 51 targets. The data corrected for instrumentaleffects indicate that the radial velocity variations are always below≈15 km s-1 (3σ level). From a statistical analysisof our results, we conclude that (at 95% confidence level) the fractionof binaries with a 0.5 {Mȯ} companion among EHB starsin NGC 6752 is smaller than 20%. Conclusions:.This empirical evidence sharply contrasts with what has been found forhot subdwarfs in the field, and opens new questions about the formationof EHB stars in globular clusters (and possibly in the field as well).
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1. Analysis of two giants in HP-1
Context: .Metal-poor globular clusters in the bulge are importanttracers of early chemical evolution. HP-1 is among the six metal-poorclusters within 5° of the Galactic center, and could be the oneclosest to the centerAims. The main purpose of this study is thedetermination of metallicity and elemental ratios. Methods.Highresolution spectra of two giants of the bulge globular cluster HP-1 wereobtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph.This is the second metal-poor globular cluster in the bulge for which adetailed abundance analysis is presented. MultibandV,I,J,H,Ks photometry was used to derive effectivetemperatures.Results. The present analysis provides a metallicity [Fe/H]= -1.00±0.2. The α-elements oxygen and silicon show [α/Fe] ≈ +0.3, whereas magnesium, calcium and titanium showsolar ratios. A proper motion analysis indicates that the two stars arecluster members. Conclusions.The metallicity is unexpected for a blueHorizontal Branch (BHB) cluster. HP-1 is the first known cluster withsuch a high metallicity combined with a BHB and a steep Red Giant Branch(RGB). Together with NGC 6388 and NGC 6441 of [Fe/H]-0.6 it wouldbe third with such characteristics, but it differs from them, sincethese two other clusters have also a populous Red HB, and a normal slopeof the RGB for their metallicity, which is not the case of HP-1.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
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