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|Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters|
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters|
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
|A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters|
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
|On the Iron Content of NGC 1978 in the LMC: A Metal-rich, Chemically Homogeneous Cluster|
We present a detailed abundance analysis of giant stars in NGC 1978, amassive, intermediate-age stellar cluster in the Large Magellanic Cloud,characterized by a high ellipticity and suspected to have a metallicityspread. We analyzed 11 giants, all cluster members, by usinghigh-resolution spectra acquired with UVES/FLAMES at the ESO Very LargeTelescope. We find an iron content of [Fe/H] = -0.38 dex with very lowσ[Fe/H]=0.07 dex dispersion, a mean heliocentric radialvelocity vr=293.1+/-0.9 km s-1, and a velocitydispersion σvr=3.1 km s-1, thusexcluding the presence of a significant metallicity, as well asvelocity, spread within the cluster.Based on observations collected at the Very Large Telescope of theEuropean Southern Observatory (ESO), Cerro Paranal, Chile, underprograms 072.D-0342 and 074.D-0369.
|Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters|
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
|The properties of Galactic globular cluster subsystems|
In this paper we compare the properties of three subsystems of Galacticglobular clusters, which are defined according to metallicity andhorizontal branch morphology. We specifically focus on clusterluminosities, structures, surface brightnesses and ellipticities. It isshown that the so-called `young' halo (YH) clusters, which are thoughtto have formed in external satellite galaxies, exhibit characteristicswhich are clearly distinct from those of the `old' halo (OH) andbulge/disc (BD) clusters, the majority of which are believed to beGalactic natives. The properties of the YH objects are, in manyrespects, similar to those of clusters belonging to a number ofpresent-day satellite dwarf galaxies. The OH and BD populations haveapparently been strongly modified by destructive tidal forces and shocksin the inner Galaxy. By comparing the properties of the three clustersubsystems, we estimate that the present population of native Galacticclusters may only represent approximately two-thirds of the originalpopulation. Several clusters with low surface brightnesses are observedto be highly flattened. We briefly speculate on the possibility thatthis ellipticity reflects the intrinsic flattening of dark mattermini-haloes in which these optically dim clusters might be embedded.Finally, we examine the distribution of clusters on the size(logRh) versus luminosity (MV) plane. Threeobjects are seen to fall well above the sharp upper envelope of the maindistribution of clusters on the size-luminosity plane: ω Centauri,M54 and NGC 2419. All three of these objects have previously, andindependently, been suggested to be the stripped cores of former dwarfgalaxies. This suspicion is strengthened by the additional observationthat the massive cluster G1 in M31 plus a number of the most luminousclusters in NGC 5128 also fall in the same region of thelogRh versus MV plane. All of the latter objectshave previously been suggested as the stripped cores of now defunctdwarf galaxies.
|Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo|
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
|ISOCAM Observations of Globular Clusters in the Magellanic Clouds: The Data|
Seventeen globular clusters in the Large and Small Magellanic Cloudswere observed in the mid-infrared wavelength region with the ISOCAMinstrument on board the Infrared Space Observatory (ISO). Observationswere made using the broadband filters LW1, LW2, and LW10, correspondingto the effective wavelengths of 4.5, 6.7, and 12 μm, respectively. Wepresent the photometry of point sources in each cluster, as well astheir precise positions and finding charts.Based on observations with ISO, an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with participation of ISAS andNASA.
|Globular clusters and the formation of the outer Galactic halo|
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
|Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old Large Magellanic Cloud clusters NGC 1928, 1939 and Reticulum|
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) globular clusters NGC 1928, 1939 andReticulum taken with the Advanced Camera for Surveys on the Hubble SpaceTelescope. Exposures through the F555W and F814W filters result inhigh-accuracy colour-magnitude diagrams (CMDs) for these three clusters.This is the first time that CMDs for NGC 1928 and 1939 have beenpublished. All three clusters possess CMDs with features indicating themto be >10 Gyr old, including main-sequence turn-offs at V~ 23 andwell-populated horizontal branches (HBs). We use the CMDs to obtainmetallicity and reddening estimates for each cluster. NGC 1939 is ametal-poor cluster, with [Fe/H]=-2.10 +/- 0.19, while NGC 1928 issignificantly more metal rich, with [Fe/H]=-1.27 +/- 0.14. The abundanceof Reticulum is intermediate between the two, with [Fe/H]=-1.66 +/-0.12- a measurement which matches well with previous estimates. Allthree clusters are moderately reddened, with values ranging from E(V-I)= 0.07 +/- 0.02 for Reticulum and E(V-I) = 0.08 +/- 0.02 for NGC 1928,to E(V-I) = 0.16 +/- 0.03 for NGC 1939. After correcting the CMDs forextinction we estimate the HB morphology of each cluster. NGC 1928 and1939 possess HBs consisting almost exclusively of stars to the blue ofthe instability strip, with NGC 1928 in addition showing evidence for anextended blue HB. In contrast, Reticulum has an intermediate HBmorphology, with stars across the instability strip. Using a variety ofdating techniques we show that these three clusters are coeval with eachother and the oldest Galactic and LMC globular clusters, to within ~2Gyr. The census of known old LMC globular clusters therefore now numbers15 plus the unique, younger cluster ESO121-SC03. The NGC 1939 fieldcontains another cluster in the line of sight, NGC 1938. A CMD for thisobject shows it to be less than ~400 Myr old, and it is thereforeunlikely to be physically associated with NGC 1939.
|Physical Characteristics of the RR Lyrae Stars in the Very Metal Poor Globular Cluster NGC 5053|
The physical characteristics of the 10 RR Lyrae stars in the verymetal-poor globular cluster NGC 5053 are derived from photometry of~1000 B and V CCD frames acquired from 1994 to 2002 with the DominionAstrophysical Observatory 1.8 m Plaskett Telescope. Revised pulsationperiods and light curves, mean magnitudes, colors, amplitudes, andFourier parameters are presented. Periods accurate to<~10-5 days are now known for all 10 RR Lyrae stars. Usingtimes of maximum light dating back to Baade's original 1923-1927observations, period change rates, dP/dt, accurate to <~0.07 daysMyr-1, have been derived for the 10 stars. Seven stars haveincreasing periods, and three have decreasing periods, with theestimated period change rates for V1, V2, V9, and V10 being very closeto zero. The mean dP/dt is equal to 0.04+/-0.04 days Myr-1and is consistent with Lee's evolutionary model predictions for acluster with horizontal-branch type ~0.5. Mean B - V colors range from0.20 to 0.40 and are more consistent with near-zero reddening thanalternative higher estimates. A reddening EB-V=0.018+/-0.003is derived from the 1998 SFD maps. Mean effective temperatures vary from6040 K (V10) to 7290 K (V6), with 2.6<=logg<=3.1. Visual andbolometric absolute magnitudes, bolometric corrections, and luminositiesare derived using Fourier methods and using intensity- andmagnitude-averaged mean magnitudes. Mean locations of the stars in theH-R diagram tend to progress from hotter, lower L stars to cooler,higher L stars and are consistent with theoretical blue and red edges ofthe instability strip. Masses estimated assuming zero reddening andDorman's oxygen-enhanced models range from 0.68 Msolar(V6) to 0.78 Msolar (V10) for the 10 stars. The meanmetal abundance for NGC 5053 derived using the Jurcsik-Kovácsmethod lies significantly higher than the range -2.3 to -2.6 dexdetermined using other, more well established methods. This findingsupports recent suggestions that metallicities derived fromFourier-based [Fe/H] calibrations need to be revised downward by atleast 0.3 dex for RR Lyrae stars with very low metal abundances.
|The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions|
Recently obtained kinematic data have shown that the Large MagellanicCloud (LMC) possesses an old stellar halo. In order to furthercharacterize the properties of this halo, parametric King models arefitted to the surface density of RR Lyrae stars. Using data from boththe MACHO and OGLE II microlensing surveys, the model fits yield thecenter of their distribution at α=5h21.1'+/-0.8',δ=-69deg45'+/-6' (J2000.0) and acore radius of 1.42+/-0.12 kpc. As a check, the halo model is comparedwith RR Lyrae star counts in fields near the LMC's periphery previouslysurveyed with photographic plates. These data, however, require acautious interpretation. Several topics regarding the LMC stellar haloare discussed. First, the properties of the halo imply a globalmass-to-light ratio of M/LV=5.3+/-2.1 and a total mass of(1.6+/-0.6)×1010 Msolar for the LMC, in goodagreement with estimates based on the rotation curve. Second, althoughthe LMC's disk and halo are kinematically distinct, the shape of thesurface density profile of the halo is remarkably similar to that of theyoung disk. For example, the best-fit exponential scale length for theRR Lyrae stars is 1.47+/-0.08 kpc, which compares to 1.46 kpc for theLMC's blue light. In the Galaxy, the halo and disk do not resemble eachother like this. Finally, a local maximum in the LMC's microlensingoptical depth due to halo-on-disk stellar self-lensing is predicted. Forthe parameters of the stellar halo obtained, this maximum is locatednear MACHO events LMC-4 and LMC-23 and is large enough to possiblyaccount for these two events but not for all of the observedmicrolensing.
|RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy|
We have surveyed four of the globular clusters in the Fornax dwarfgalaxy (clusters 1, 2, 3 and 5) for RR Lyrae stars, using archival F555Wand F814W Hubble Space Telescope observations. We identify 197 new RRLyrae stars in these four clusters, and 13 additional candidatehorizontal-branch variable stars. Although somewhat restricted by ourshort observational baseline, we derive periods and light curves for allof the stars in the sample, and calculate photometric parameters such asmean magnitudes and colours. This is the first time that RR Lyrae starsin the Fornax globular clusters have been quantitatively identified andmeasured. We find that the Fornax clusters have exceptionally largespecific frequencies of RR Lyrae stars, in comparison with the galacticglobular clusters. It is likely that Fornax 1 has the largest specificfrequency measured in any globular cluster. In addition, the Fornaxclusters are unusual in that their RR Lyrae populations possess meancharacteristics intermediate between the two Oosterhoff groups definedby the galactic globular clusters. In this respect the RR Lyraepopulations in the Fornax clusters most closely resemble the fieldpopulations in several dwarf galaxies. Fornax 5 has an unusually largefraction of RRc stars, and also possesses several strong RRe (secondovertone pulsator) candidates.With a large sample of horizontal-branch variable stars available to us,we revise previous measurements of the horizontal-branch morphology ineach cluster. The Fornax clusters most closely resemble the `young'galactic halo population defined by Zinn in that their horizontal-branchmorphologies are systematically redder than many galactic clusters ofsimilar metallicity. We also confirm the existence of thesecond-parameter effect among the Fornax clusters, most markedly betweenclusters 1 and 3. The edges of the instability strip are well defined inseveral of the Fornax clusters, and we are able to make measurements ofthe intrinsic V-I colours of these edges. Finally, we determineforeground reddening and distance estimates for each cluster. We find amean distance modulus to the Fornax dwarf of (m-M)0= 20.66+/- 0.03 (random) +/-0.15 (systematic). Our measurements are consistentwith a line-of-sight depth of ~8-10 kpc for this galaxy, which is inaccordance with its dimensions as measured in the plane of the sky. Thisapproximately spherical shape for Fornax is incompatible with tidalmodel explanations for the observed high internal stellar velocitydispersions in many dwarf spheroidal galaxies. Dark matter dominance issuggested.
|Surface brightness profiles and structural parameters for globular clusters in the Fornax and Sagittarius dwarf spheroidal galaxies|
We present radial surface brightness profiles for all five globularclusters in the Fornax dwarf spheroidal galaxy, and for the four presentmembers of the Sagittarius dwarf spheroidal galaxy. These profiles arederived from archival Hubble Space Telescope observations, and have beencalculated using the same techniques with which we measured profiles inour previous studies of Large and Small Magellanic Cloud (LMC and SMC)clusters, apart from some small modifications. From the surfacebrightness profiles, we have determined structural parameters for eachcluster, including core radii and luminosity and mass estimates. We alsoprovide a brief summary of literature measurements of other parametersfor these clusters, including their ages, metallicities and distances.Our core radius measurements are mostly in good agreement with thosefrom previous lower resolution studies, although for several clustersour new values are significantly different. The profile for Fornaxcluster 5 does not appear to be well fitted by a King-type model and wesuggest that it is a post-core-collapse candidate. We examine thedistribution of cluster core radii in each of the two dwarf galaxysystems and compare these with the distribution of core radii for oldLMC clusters. The three distributions match within the limits ofmeasurement errors and the small-sample sizes. We discuss theimplications of this in the context of the radius-age trend we havepreviously highlighted for the Magellanic Cloud clusters.
|Distance to the Large Magellanic Cloud: The RR Lyrae Stars|
New photometry and spectroscopy for more than a hundred RR Lyrae starsin two fields located close to the bar of the Large Magellanic Cloud(LMC) are used to derive new accurate estimates of the averagemagnitude, the local reddening, the luminosity-metallicity relation, andthe distance to the LMC. The average apparent luminosity of the RRLyraes with complete V and B light curves is=19.412+/-0.019 (σ=0.153),=19.807+/-0.022 (σ=0.172) in our field A (62 stars)and =19.320+/-0.023 (σ=0.159),=19.680+/-0.024 (σ=0.163) in our field B (46 stars).The average V apparent luminosity of the clump stars in the same areasis 0.108 and 0.029 mag brighter than the RR Lyrae level(=19.304+/-0.002 and 19.291+/-0.003, in fieldA: 6728 stars, and B: 3851 stars, respectively). Metallicities fromlow-resolution spectra obtained with the Very Large Telescope have beenderived for 101 RR Lyrae stars, finding an average value of[Fe/H]=-1.48+/-0.03 (σ=0.29, on the Harris metallicity scale). Anestimate of the reddening within the two fields was obtained (1) fromthe Sturch method applied to the fundamental-mode pulsators (RRab's)with known metal abundance and (2) from the colors of the edges of theinstability strip defined by the full sample of RR Lyrae variable stars.We obtained E(B-V)=0.116+/-0.017 and 0.086+/-0.017 mag in fields A andB, respectively, with a clear-cut indication of a 0.03 mag differentialreddening between the two fields. We find that reddening in field A is0.028 mag smaller than derived by OGLE-II in the same area. On average,the new reddenings are also 0.035 mag larger than derived from Cepheidswith projected distances within 2° from the centers of our fields.The new metallicities were combined with the apparent averageV0 luminosities to determine the slope of theluminosity-metallicity relation for the RR Lyrae stars. We derivedΔMV(RR)/Δ [Fe/H]=0.214+/-0.047, with no clearevidence for the change in slope at [Fe/H]=-1.5, as recently suggestedby evolutionary/pulsation and horizontal-branch models.The dereddened apparent average luminosity of the RR Lyraes defined bythe present photometry is 0=19.064+/-0.064 at[Fe/H]=-1.5. When coupled with the absolute magnitude derived from theBaade-Wesselink and the statistical parallaxes methods(MV(RR)=0.68+/-0.15 and 0.76+/-0.13 mag at [Fe/H]=-1.5), bothmethods known to favor the short distance scale, this value leads todistance moduli for the LMC of μLMC=18.38+/-0.16 andμLMC=18.30+/-0.14, respectively. If we use instead theabsolute magnitude from the new main-sequence fitting of Galacticglobular clusters from Gratton et al. [MV(RR)=0.61+/-0.07 magat [Fe/H]=-1.5], we derive μLMC=18.45+/-0.09.The average I apparent luminosity of the clump stars derived by thepresent photometry is =18.319+/-0.002 and18.307+/-0.003, in field A (σ=0.190, 6728 stars) and B(σ=0.184, 3851 stars), respectively. These values, once correctedfor our new reddening estimates, lead to0=18.12+/-0.06 mag and move the clump distancemodulus to the LMC to 18.42+/-0.07 and 18.45+/-0.07 when Udalski orPopowski metallicity-I luminosity relations for the clump stars areadopted.All these values are only 1 σ shorter than provided by thePopulation I distance indicators and make it possible to reconcile theshort- and long-distance scale on a common value for the distancemodulus of the LMC of μLMC=18.515+/-0.085 mag.Based on observations collected at the European Southern Observatory,proposals 62.N-0802, 66.A-0485, and 68.D-0466.
|Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud|
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
|The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI|
We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion toM31, for variable stars by using F450W and F555W observations obtainedwith the Hubble Space Telescope. A total of 118 variables were found,including 111 RR Lyrae stars, six anomalous Cepheids, and one variablethat we were unable to classify. We find that the Andromeda VI anomalousCepheids have properties consistent with those of anomalous Cepheids inother dwarf spheroidal galaxies. We revise the existingperiod-luminosity relations for these variables. Further, using theseand other available data, we show that there is no clear differencebetween fundamental and first-overtone anomalous Cepheids in aperiod-amplitude diagram at shorter periods, unlike the RR Lyrae stars.For the Andromeda VI RR Lyrae stars, we find that they lie close to theOosterhoff type I Galactic globular clusters in the period-amplitudediagram, although the mean period of the RRab stars,=0.588 days, is slightly longer than that of thetypical Oosterhoff type I cluster. The mean V magnitude of the RR Lyraestars in Andromeda VI is 25.29+/-0.03, resulting in a distance 815+/-25kpc on the Lee, Demarque, & Zinn distance scale. This is consistentwith the distance derived from the I magnitude of the tip of the redgiant branch. Similarly, the properties of the RR Lyrae stars indicate amean abundance for Andromeda VI that is consistent with that derivedfrom the mean red giant branch color. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555.
|Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)|
We carried out Washington system photometry of the intermediate-ageLarge Magellanic Cloud (LMC) star clusters NGC2155 and SL896 (LW480). Wederive ages and metallicities from the T1 versusC-T1 colour-magnitude diagrams (CMDs). For the first time anage has been obtained for SL896, 2.3+/-0.5Gyr. For NGC2155 we derive3.6+/-0.7Gyr. The two clusters basically define the lower age limit ofthe LMC age gap. In particular, NGC2155 is confirmed as the oldestintermediate-age LMC cluster so far studied. The derived metallicitiesare [Fe/H]=-0.9+/-0.2 and -0.6+/-0.2 for NGC2155 and SL896,respectively. We also studied the CMDs of the surrounding fields, whichhave a dominant turn-off comparable to that of the clusters themselves,and similar metallicity, showing that one is dealing with anintermediate-age disc where clusters and field stars have the sameorigin. We inserted the present clusters in the LMC and Small MagellanicCloud (SMC) age-metallicity relations, using a set of homogeneousdeterminations with the same method as in our previous studies, nowtotalling 15 LMC clusters and four SMC clusters, together with someadditional values from the literature. The LMC and SMC age-metallicityrelations appear to be remarkably complementary, since the SMC wasactively star-forming during the LMC quiescent age gap epoch.
|A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters|
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
|The Star Cluster Systems of the Magellanic Clouds|
The characteristics of the cluster systems of the Magellanic Clouds, asinferred from integrated properties, are compared with those fromindividual cluster studies and from the field population. The agreementis generally satisfactory though in the case of the LMC, the lack ofclusters older than ˜3 Gyr is not reflected in the fieldpopulation. The possible origin(s) for this cluster ``age-gap'' arediscussed. The SMC cluster age-metallicity relation is also presentedand discussed.
|Ages and metallicities of five intermediate-age star clusters projected towards the Small Magellanic Cloud|
Colour-magnitude diagrams are presented for the first time for L32, L38,K28 (L43), K44 (L68) and L116, which are clusters projected on to theouter parts of the Small Magellanic Cloud (SMC). The photometry wascarried out in the Washington system C and T1 filters,allowing the determination of ages by means of the magnitude differencebetween the red giant clump and the main-sequence turn-off, andmetallicities from the red giant branch locus. The clusters have ages inthe range 2-6Gyr, and metallicities in the range-1.65<[Fe/H]<-1.10, increasing the sample of intermediate-ageclusters in the SMC. L116, the outermost cluster projected on to theSMC, is a foreground cluster, and somewhat closer to us than the LargeMagellanic Cloud. Our results, combined with those for other clusters inthe literature, show epochs of sudden chemical enrichment in theage-metallicity plane, which favour a bursting star formation history asopposed to a continuous one for the SMC.
|Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams|
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
|Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818|
We present colour-magnitude diagrams for two rich(~104Msolar) Large Magellanic Cloud star clusterswith ages ~107yr, constructed from optical and near-infrareddata obtained with the Hubble Space Telescope. These data are part of anHST project to study LMC clusters with a range of ages. In this paper weinvestigate the massive star content of the young clusters, anddetermine the cluster ages and metallicities, paying particularattention to Be-star and blue-straggler populations and evidence of agespreads. We compare our data with detailed stellar-populationsimulations to investigate the turn-off structure of ~25Myr stellarsystems, highlighting the complexity of the blue-straggler phenomenon.
|The Ages of Globular Clusters|
We examine the luminosity levels of the main-sequence turnoffs,MTOv, and horizontal branches, Mv(HB),in 16 globular clusters. An entirely new approach of inferring theluminosity levels by utilizing high-amplitude δ Scuti variables(HADS) is introduced. When the MTOv values arecompared with theoretical values inferred from models, we find all 16clusters (metal-strong to metal-poor) are coeval with an average age of~11.3 Gyr. A considerable scatter of Mv(HB) values ofclusters at similar [Fe/H] values is found. A trend for clusters withblue horizontal branches to have brighter Mv(HB) thanclusters with blue-red horizontal branches is suggested by the data. TheMv(HB) values appear to depend on another or other parametersin addition to the [Fe/H] values. In spite of this problem, we derive anequation relating Mv(HB) values of globular clusters to their[Fe/H] values. We also derive an equation relating theMTOv values of clusters to their [Fe/H] values.Both of these equations can be utilized to find cluster distances. Thedistance modulus of the LMC is found to be 18.66 from the VTOvalues of three LMC globular clusters; RR Lyrae stars in seven globularclusters yield 18.61, and RR Lyrae stars in the LMC bar yield 18.64.
|The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud|
We report new metallicity determinations for 39 red giants in a 220arcmin2 region, 1.8d southwest of the bar of the LargeMagellanic Cloud. These abundance measurements are based on spectroscopyof the Ca II infrared triplet. We have carefully considered the effectsof abundance ratios, the physics of Ca II line formation, the variationof red clump magnitude, and the contamination by foreground stars in ourabundance analyses. The metallicity distribution function (MDF) shows astrong peak at [Fe/H]=-0.57+/-0.04 a tail to abundances at least as lowas [Fe/H]~-1.6 brings the average abundance down to [Fe/H]=-0.64+/-0.02.Half the red giants in our field fall within the range-0.83<=[Fe/H]<=-0.41. The MDF appears to be truncated at[Fe/H]~-0.25 the exact value of the maximum abundance is subject to ~0.1dex uncertainty in the calibration of the Ca II IR triplet for young,metal-rich stars. We find a striking contrast in the shape of the MDFbelow [Fe/H]<=-1 between our inner disk field and the distant outerfield studied by Olszewski: red giants deficient by more than a factorof 10 in heavy elements relative to the Sun are extremely scarce in theinner disk of the LMC. Our field star sample does not reproduce the fullMDF of the LMC star clusters but seems similar to that of theintermediate-age (1-3 Gyr) clusters. We have also obtained abundanceestimates using Strömgren photometry for ~103 red giantsin the same field. Photometry is the only practical way to measureabundances for the large numbers of stars necessary to liftage-metallicity degeneracy from our high-precision color-magnitudediagrams. The Strömgren measurements, which are sensitive to acombination of cyanogen and iron lines, correlate well with the Ca IImeasurements, but a metallicity-dependent offset is found. The offsetmay be due either to variations in the elemental abundance ratios due togalactic chemical evolution or to a metal-dependent mixing mechanism inRGB stars. An empirical relation between photometric and spectroscopicabundance estimates is derived. This will allow photometric abundancemeasurements to be placed on a consistent metallicity scale withspectroscopic metallicities, for very large numbers of stars. Based onobservations obtained at Cerro Tololo Inter-American Observatory, adivision of the National Optical Astronomy Observatories, which areoperated by the Association of Universities for Research in Astronomy,Inc. under cooperative agreement with the National Science Foundation.
|Age and metallicity for six LMC clusters and their surrounding field population|
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H] = -0.65 dex to -0.41 dex), NGC 1711 (-0.57 ∓ 0.17dex), NGC 1806 (-0.71 ∓ 0.23 dex), NGC 2031 (-0.52 ∓ 0.21dex) and NGC 2136/37 (-0.55 ∓ 0.23 dex) and NGC 2257 (-1.63∓ 0.21 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 650 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR). Ourmethod is to estimate ages for individual stars on the basis ofStrömgren isochrones with individually measured metallicities. Withthis method we are able to sample the AMR of the field population up to8 Gyr. Our metallicity data are incompatible with models predicting manymetal-poor stars (G-dwarf problem). The metallicity of the fieldpopulation increased by a factor of six, starting around 2 Gyr ago. Theproposed AMR is consistent with the AMR of the LMC cluster system(including ESO 121 SC03 and three clusters with an age of 4 Gyr). Theproposed AMR is incompatible with the recently proposed AMR by Pagel& Tautvaisiene.
|The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems|
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
|Updated Information on the Local Group|
The present note updates the information published in my recentmonograph on The Galaxies of the Local Group. Highlights include (1) theaddition of the newly discovered Cetus dwarf spheroidal as a certainmember of the Local Group; (2) an improved distance for the Sagittariusdwarf irregular galaxy (SagDIG), which now places this object very closeto the edge of the Local Group zero-velocity surface; (3) moreinformation on the evolutionary histories of some individual Local Groupmembers; and (4) improved distance determinations to, and luminositiesfor, a number of Local Group members. These data increase the number ofcertain (or probable) Local Group members to 36. The spatialdistribution of these galaxies supports Hubble's claim that the LocalGroup ``is isolated in the general field.'' Currently available evidencesuggests that star formation continued much longer in many dwarfspheroidals than it did in the main body of the Galactic halo. It issuggested that ``young'' globular clusters, such as Ruprecht 106, mighthave formed in now defunct dwarf spheroidals. Assuming SagDIG, which isthe most remote Local Group galaxy, to lie on, or just inside, thezero-velocity surface of the Local Group yields a dynamical age>~17.9+/-2.7 Gyr. However, this value is meaningful only if the outerregions of the local Group are in virial equilibrium.
|A catalogue of helium abundance indicators from globular cluster photometry|
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
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