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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.
| 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.
| 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.
| 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.
| 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.
| The INTEGRAL Galactic bulge monitoring program: the first 1.5 years
Aims.The Galactic bulge region is a rich host of variable high-energypoint sources. Since 2005, February 17 we are monitoring the sourceactivity in the Galactic bulge region regularly and frequently, i.e.,about every three days, with the instruments onboard INTEGRAL. Thanks tothe large field of view, the imaging capabilities and the sensitivity athard X-rays, we are able to present for the first time a detailedhomogeneous (hard) X-ray view of a sample of 76 sources in the Galacticbulge region. Methods: We describe the successful monitoring programand show the first results from the start of the monitoring up to 2006,April 21, i.e., for a period of about one and a half year, during threevisibility seasons. We focus on the short (hour), medium (month) andlong-term (year) variability in the hard X-ray bands, i.e., 20-60 keVand 60-150 keV. When available, we discuss the simultaneous observationsin the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost allthe sources in the Galactic bulge region we detect in the 20-60 keV and60-150 keV bands are variable. During the last two and a half weeks ofthe third visibility season most of the known persistent (hard) X-raysources in the Galactic Center region were not detected. Of our sampleof sources, per visibility season we detect 32/33 sources in the 20-60keV band and 8/9 sources in the 60-150 keV band above a signal to noiseof 7. On average, we find per visibility season one active bright(⪆100 mCrab, 20-60 keV) black-hole candidate X-ray transient andthree active weaker (⪉25 mCrab, 20-60 keV) neutron star X-raytransients. Most of the time a clear anti-correlation can be seenbetween the soft and hard X-ray emission in some of the X-ray bursters.Hard X-ray flares or outbursts in X-ray bursters, which have a durationof the order of weeks are accompanied by soft X-ray drops. On the otherhand, hard X-ray drops can be accompanied by soft X-rayflares/outbursts. During the course of our program we found a number ofnew sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGRJ17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here onsome of the high-energy properties of these sources. Conclusions: Thehigh-energy light curves of all the sources in the field of view, andthe high-energy images of the region, are made available through theWWW, as soon as possible after the observations have been performed, athttp://isdc.unige.ch/Science/BULGE/.Appendices are only available in electronic form at http://www.aanda.org
| 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
| 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
| 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.
| Computation of the Fourier parameters of RR Lyrae stars by template fitting
Aims.Due to the importance of accurate Fourier parameters, we devise amethod that is more appropriate for deriving these parameters forlow-quality data than the traditional Fourier fitting. Methods: Basedon the accurate light curves of 248 fundamental mode RR Lyrae stars, wetest the power of a full-fetched implementation of the template methodin the computation of the Fourier decomposition. The applicability ofthe method is also demonstrated on data sets of filter passbandsdifferent from those of the template set. Results: We examine in moredetail the question of the estimation of Fourier-based iron abundance[Fe/H] and average brightness. We get, for example, for light curvessampled randomly in 30 data points with σ=0.03 mag observationalnoise, that optimized direct Fourier fits yield σ([Fe/H])=0.33,whereas the template fits result in σ([Fe/H])=0.18. Tests made onthe RR Lyrae database of the Large Magellanic Cloud of the OpticalGravitational Lensing Experiment support the applicability of the methodon real photometric time series. These tests also show that the dominantpart of error in estimating the average brightness comes from othersources, most probably from crowding effects, even for under-sampledlight curves.
| 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).
| The Not So Extraordinary Globular Cluster 037-B327 in M31
A velocity dispersion has been measured for the luminous globularcluster M31 037-B327, claimed to be the most massive star cluster in theLocal Group and to be a young ``super star cluster'' that has survivedto an old age. M31 037-B327 has a mass comparable to that of M31 G1 butnot significantly larger. Although near the upper end for the massdistribution of globular clusters, it is not an unprecedentedextraordinary object.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration.
| Multicolour CCD photometry of the variable stars in globular cluster M3
We present time-series data on the variable stars of the galacticglobular cluster Messier 3 (M3). We give BVIC light curvesfor 226 RR Lyrae, 2 SX Phe and 1 W Vir type variables, along withestimated fundamental photometric parameters such as intensity andmagnitude-averaged brightness and pulsation periods. In some cases, theperiods we have found significantly differ from the previously publishedones. This is the first published light curve and period determinationfor variable V266. The I-band light curve has not been observedpreviously for numerous (76) variables. Three new RR Lyrae variableshave been discovered. Groups of RR Lyrae variables that belong todifferent evolutionary stages and have been separated previously on thebasis of V data were found here for all colours and colour indices bycluster analysis. The I-band period-luminosity relation is alsodiscussed. From the 66 modulated (Blazhko type) RR Lyrae stars weinvestigated, six are newly identified and two of them are firstovertone pulsators. In the case of 13 RR Lyrae, the period of Blazhkocycle has been estimated for the first time. V252 is identified as a newRRd variable. Amplitude ratios of RRd stars have been investigated tosearch possible mode content changes. In contrast to previouspublications no changes have been found. Problems with the sampling ofthe time-series of typical cluster variability surveys are demonstrated.
| The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy
Using HST ACS, we have obtained deep optical images reaching stars wellbelow the oldest main-sequence turnoff in the spheroid, tidal stream,and outer disk of Andromeda. We have reconstructed the star formationhistory in these fields by comparing their color-magnitude diagrams to agrid of isochrones calibrated to Galactic globular clusters observed inthe same bands. Each field exhibits an extended star formation history,with many stars younger than 10 Gyr but few younger than 4 Gyr.Considered together, the star counts, kinematics, and populationcharacteristics of the spheroid argue against some explanations for itsintermediate-age, metal-rich population, such as a significantcontribution from stars residing in the disk or a chance intersectionwith the stream's orbit. Instead, it is likely that this population isintrinsic to the inner spheroid, whose highly disturbed structure isclearly distinct from the pressure-supported metal-poor halo thatdominates farther from the galaxy's center. The stream and spheroidpopulations are similar, but not identical, with the stream's mean agebeing ~1 Gyr younger; this similarity suggests that the inner spheroidis largely polluted by material stripped from either the stream'sprogenitor or similar objects. The disk population is considerablyyounger and more metal-rich than the stream and spheroid populations,but not as young as the thin-disk population of the solar neighborhood;instead, the outer disk of Andromeda is dominated by stars of age 4-8Gyr, resembling the Milky Way's thick disk. The disk data areinconsistent with a population dominated by ages older than 10 Gyr andin fact do not require any stars older than 10 Gyr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS 5-26555. These observations areassociated with proposals 9453 and 10265. Some of the data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California, and NASA. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
| 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.
| Planetary nebula candidates in extragalactic young star clusters
Context: .During an analysis of optical spectra of 80 young starclusters in several nearby spiral galaxies, [O III] and [N II] emissionlines were noted in some cases. Three of these emission line sources areassociated with clusters older than 30 Myr, and are identified as likelyplanetary nebula (PN) candidates. Aims: .These objects mayrepresent a rare opportunity to study PNe whose progenitor stars areknown to be of intermediate masses, although detailed analysis ischallenging because of the underlying strong continuum from the clusterstars. This paper presents and discusses basic properties of the PNcandidates and their host clusters. Methods: .Based on theobserved emission line fluxes, the excitation parameters andluminosities of the nebulae are derived. This allows a crude placementof the central stars in two of the objects on the H-R diagram. Clusterages and masses are estimated from broad-band colours and by fittingmodel SSP spectra to the observed spectra. Results: .The two PNcandidates where central star luminosities and temperatures can beestimated are found to be consistent with post-AGB model tracks for acentral star mass of about 0.60 Mȯ. One of the hostclusters has an age of 32-65 Myr, corresponding to a main sequenceturn-off mass of M_TO = 6.6-9.0 M_ȯ. For the other cluster the ageis 282-407 Myr, corresponding to M_TO = 3.2-3.6 M_ȯ. By estimatingthe number of stars evolving off the main sequence per year, a total of6 PNe are expected in the full sample of 80 clusters for a PN lifetimeof 104 years. The factor of two disagreement with the actualobserved number may be due, among other things, to uncertainties in PNlifetimes. It is interesting to note that all three PN candidates areassociated with clusters which are more diffuse than average. Conclusions: .While PNe have previously been found in some old globularclusters, the candidates identified here are among the first identifiedin young star clusters.
| 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.
| An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG), operated on the island La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| 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.
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