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On the Age and Metallicity Estimation of Spiral Galaxies Using Optical and Near-Infrared Photometry
In integrated light, some color-color diagrams that use optical andnear-infrared photometry show surprisingly orthogonal grids as age andmetallicity are varied, and they are coming into common usage forestimating the average age and metallicity of spiral galaxies. In thispaper we reconstruct these composite grids using simple stellarpopulation models from several different groups convolved with someplausible functional forms of star formation histories at fixedmetallicity. We find that the youngest populations present (t<2 Gyr)dominate the light, and because of their presence the age-metallicitydegeneracy can be partially broken with broadband colors, unlike olderpopulations. The scatter among simple stellar population models bydifferent authors is, however, large at ages t<2 Gyr. The dominantuncertainties in stellar population models arise from convective coreovershoot assumptions and the treatment of the thermally pulsingasymptotic giant branch phase and helium abundance may play asignificant role at higher metallicities. Real spiral galaxies areunlikely to have smooth, exponential star formation histories, andburstiness will cause a partial reversion to the single-burst case,which has even larger model-to-model scatter. Finally, it is emphasizedthat the current composite stellar population models need someimplementation of chemical enrichment histories for the proper analysisof the observational data.
| The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
| 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.
| Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) cluster ESO 121-SC03 taken with theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. Ourresulting colour-magnitude diagram (CMD) reaches 3 mag below themain-sequence turn-off, and represents by far the deepest observation ofthis cluster to date. We also present similar photometry from ACSimaging of the accreted Sagittarius dSph cluster Palomar 12, used inthis work as a comparison cluster. From analysis of its CMD, we obtainestimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = -0.97 +/- 0.10 and E(V - I) = 0.04 +/- 0.02, in excellent agreement withprevious studies. The observed horizontal branch (HB) level in ESO121-SC03 suggests this cluster may lie 20 per cent closer to us thandoes the centre of the LMC. ESO 121-SC03 also possesses a significantpopulation of blue stragglers, which we briefly discuss. Our newphotometry allows us to undertake a detailed study of the age of ESO121-SC03 relative to Palomar 12 and the Galactic globular cluster 47Tuc. We employ both vertical and horizontal differential indicators onthe CMD, calibrated against isochrones from the Victoria-Regina stellarmodels. These models allow us to account for the differentα-element abundances in Palomar 12 and 47 Tuc, as well as theunknown run of α-elements in ESO 121-SC03. Taking a straighterror-weighted mean of our set of age measurements yields ESO 121-SC03to be 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the ageof Palomar 12. Palomar 12 is 79 +/- 6 per cent as old as 47 Tuc,consistent with previous work. Our result corresponds to an absolute agefor ESO 121-SC03 in the range 8.3-9.8 Gyr, depending on the age assumedfor 47 Tuc, therefore confirming ESO 121-SC03 as the only known clusterto lie squarely within the LMC age gap. We briefly discuss a suggestionfrom earlier work that ESO 121-SC03 may have been accreted into the LMCsystem.
| Star clusters in late-type galaxies
I present an overview of recent progress in the study of star clustersin nearby late-type galaxies. Colour-magnitude diagrams of star clustersin several nearby spiral galaxies show some differences in the meancolour of the globular clusters and in the magnitude of the brightestclusters. Recent wide field CCD survey of star clusters in M33 foundthat the star cluster system of M33 is dominated by blue clusters, andthat the red clusters are more dispersed in a wider region than bluerclusters. Age distribution of the clusters in the interacting system M51shows that a significant number of clusters in M51 were probably formedduring the interaction of NGC 5194 and NGC 5195. About 50 faint fuzzyclusters are found in NGC 5195, SB0. Recent wide field CCD survey ofstar clusters in NGC 6822 discovered a new kind of extended clusters inthe remote halo of NGC 6822. These new clusters in NGC 6822 shareseveral common features with the faint fuzzy clusters found in theinteracting SB0 galaxies, providing an important clue to understandingthe origin of extended clusters.
| 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.
| 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.
| 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.
| Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster ω Centauri as standard candles
New photometry of RRab and RRc stars in ω Centauri is used tocalibrate their absolute magnitudes MV as a function of (a)metallicity and (b) the Fourier parameters of light curves in the Vband. The zero point of both calibrations relies on the distance modulusto the cluster derived earlier by the Cluster AgeS Experiment (CASE)project based on observations of the detached eclipsing binary OGLEGC17. For RRab variables, we obtained a relation of MV= (0.26+/- 0.08)[ Fe/H ] + (0.91 +/- 0.13). A dereddened distance modulus tothe Large Magellanic Cloud (LMC) based on that formula isμ0= 18.56 +/- 0.14 mag. The second calibration ofMV, which is based on Fourier coefficients of decomposedlight curves, results in the LMC distance of μ0= 18.51 +/-0.07 mag.
| Why hot horizontal branch stars can appear redder than red giants
In this paper we report on a curious feature in the V, (U-B)color-magnitude diagrams of globular clusters. In our database, we findthat a considerable fraction of blue horizontal branch stars, hotterthan the instability strip and cooler than the Grundahl et al. (1999)jump (i.e., 6000 <~ Teff(K)<~ 10 000), have (U-B)colors redder than their red giant progenitors. This red incursion isnot expected on theoretical grounds, as horizontal branch stars (whoseconvective regions are less extended than in red giant structures)should not ``appear'' cooler than a red giant. Analyzing data fromdifferent telescopes we show that: 1) the horizontal branch redincursion is strongly dependent on the shape of the adopted U filter andto a lesser extent, on the B filter; 2) the photometry done with Ufilters that do not encompass the Balmer jump shows the blue horizontalbranch red incursion; 3) the occurrence of this feature is also due tothe peculiar dependence of the U and B magnitudes on star effectivetemperature, gravity, and metallicity; 4) theoretical tracks canreproduce the observed horizontal branch morphology, provided that theappropriate (i.e. exactly responding to the filters effectively used inthe observations) transmission curve efficiencies are used for derivingcolor-Teff transformations; 5) the red incursion extentdepends on metallicity.Based on observations with the ESO/MPI 2.2 m telescope, located at LaSilla Observatory (Chile) and on observations with the NASA/ESA HubbleSpace Telescope.
| 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.
| UBV stellar photometry of bright stars in GC M5 - I. UV colour-magnitude and colour-colour diagrams and some peculiarities in the HB stellar distribution
We present stellar photometry in the UBV passbands for the globularcluster M5≡NGC 5904. The observations, taken from short-exposurephotographic plates and CCD frames, were obtained in the Ritchey-Cretien(RC) focus of the 2-m telescope of the National Astronomy Observatory`Rozhen'. All stars in an annulus with radius 1<=r<=5.5arcmin weremeasured. We show that the ultraviolet (UV) colour-magnitude diagrams(CMDs) describe different evolutionary stages in a better manner thanthe `classical' V, B-V diagram. We use HB stars, with knownspectroscopic Teff to check the validity of the colourzero-point. A review of all known UV-bright star candidates in M5 ismade and some of their parameters are catalogued. Six new stars of thiskind are suspected on the basis of their position on the CMD. Newassessment of the cluster reddening and metallicity is done using theU-B, B-V diagram. We find that [Fe/H]=-1.38, which confirms the Zinn& West value, contrasting with recent spectroscopic estimates. In aneffort to clarify the question of the gap in the blue horizontal branch(BHB) stellar distribution and to investigate some other peculiarities,we use the relatively long-base colour index U-V. A comparison of theobserved V, (U-V)0 distribution of horizontal branch (HB)stars with a canonical zero-age horizontal branch (ZAHB) model revealsthat the hottest stars rise above the model line. This is similar to the`u-jump' found in the Strömgren photometry. 18 BHB stars with(B-V)0∈[-0.02/0.18] are used to estimate theirultraviolet deficiency. It is shown that low-gravity (logg<=2)Kurucz's atmospheric models fit the observed distribution of these starsalong the two-colour diagram well.
| 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.
| Empirical relations for cluster RR Lyrae stars revisited
Our former study on the empirical relations between the Fourierparameters of the light curves of the fundamental mode RR Lyrae starsand their basic stellar parameters has been extended to considerablylarger data sets. The most significant contribution to the absolutemagnitude MV comes from the period P and from the firstFourier amplitude A1, but there are statistically significantcontributions also from additional higher order components, mostimportantly from A3 and in a lesser degree from the Fourierphase varphi51. When different colors are combined inreddening-free quantities, we obtain basically period-luminosity-colorrelations. Due to the log Teff(B-V,log g,[Fe/H]) relationfrom stellar atmosphere models, we would expect some dependence also onvarphi 31. Unfortunately, the data are still not extensiveand accurate enough to decipher clearly the small effect of this Fourierphase. However, with the aid of more accurate multicolor data on fieldvariables, we show that this Fourier phase should be present either inV-I or in B-V or in both. From the standard deviations of the variousregressions, an upper limit can be obtained on the overall inhomogeneityof the reddening in the individual clusters. This yields sigmaE(B-V)<~ 0.012 mag, which also implies an average minimumobservational error of sigmaV >~ 0.018 mag.
| 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.
| Stellar Populations in the Large Magellanic Cloud from 2MASS
We present a morphological analysis of the feature-rich 2MASS LMCcolor-magnitude diagram, identifying Galactic and LMC populations andestimating the density of LMC populations alone. We also present theprojected spatial distributions of various stellar populations. Theconditions that prevailed when 2MASS observed the LMC provided 10σ limiting sensitivity for J<~16.3, H<~15.3, andKs<~14.7. Major populations are identified based onmatching morphological features of the color-magnitude diagram withexpected positions of known populations, isochrone fits, and analysis ofthe projected spatial distributions. 2MASS has detected a significantpopulation of asymptotic giant branch (AGB) stars (>~104sources) and obscured AGB stars (>~2×103 sources).The LMC populations along the first-ascent red giant branch (RGB) andAGB are quantified. Comparison of the giant luminosity functions in thebar and the outer regions of the LMC shows that both luminosityfunctions appear consistent with each other. The luminosity function inthe central (bar) field has well-defined drop-off nearKs=12.3 mag corresponding to the location of the RGB tip; thesame feature is seen in the luminosity function of the entire LMC field.Isochrone fits for the corresponding giant branches reveal nosignificant differences in metallicities and ages between central andouter regions of the LMC. This may be evidence for strong dynamicalevolution in the last several gigayears. In particular, the observed LMCgiant branch is well fit by published tracks in the CIT/CTIO system witha distance modulus of μ=18.5+/-0.1, reddeningEB-V=0.15-0.20, metallicityZ=0.004+0.002-0.001, and age 3-13 Gyr. Analysis ofdeep 2MASS engineering data with 6 times the standard exposure producessimilar estimates.
| 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.
| 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.
| Distances, Ages, and Epoch of Formation of Globular Clusters
We review the results on distances and absolute ages of Galacticglobular clusters (GCs) obtained after the release of the Hipparcoscatalog. Several methods aimed at the definition of the Population IIlocal distance scale are discussed, and their results compared,exploiting new results for RR Lyraes in the Large Magellanic Cloud(LMC). We find that the so-called short distance and long distancescales may be reconciled whether or not a consistent reddening scale isadopted for Cepheids and RR Lyrae variables in the LMC. Emphasis isgiven in the paper to the discussion of distances and ages of GCsderived using Hipparcos parallaxes of local subdwarfs. We find that theselection criteria adopted to choose the local subdwarfs, as well as thesize of the corrections applied to existing systematic biases, are themain culprit for the differences found among the various independentstudies that first used Hipparcos parallaxes and the subdwarf fittingtechnique. We also caution that the absolute age of M92 (usuallyconsidered one of the oldest clusters) still remains uncertain due tothe lack of subdwarfs of comparable metallicity with accurateparallaxes. Distances and ages for the nine clusters discussed in aprevious paper by Gratton et al. are rederived using an enlarged sampleof local subdwarfs, which includes about 90% of the metal-poor dwarfswith accurate parallaxes (Δπ/π<=0.12) in the wholeHipparcos catalog. On average, our revised distance moduli are decreasedby 0.04 mag with respect to Gratton et al. The corresponding age of theGCs is t=11.5+/-2.6 Gyr, where the error bars refer to the 95%confidence range. The relation between the zero-age horizontal branch(ZAHB) absolute magnitude and metallicity for the nine program clustersturns out to beMV(ZAHB)=(0.18+/-0.09)([Fe/H]+1.5)+(0.53+/-0.12) Thanks toHipparcos the major contribution to the total error budget associatedwith the subdwarf fitting technique has been moved from parallaxes tophotometric calibrations, reddening, and metallicity scale. This totaluncertainty still amounts to about +/-0.12 mag. We then compare thecorresponding (true) LMC distance modulusμLMC=18.64+/-0.12 mag with other existing determinations.We conclude that at present the best estimate for the distance of theLMC is μLMC=18.54+/-0.03+/-0.06, suggesting that distancesfrom the subdwarf fitting method are ~1 σ too long. Consequently,our best estimate for the age of the GCs is revised to Age=12.9+/-2.9Gyr (95% confidence range). The best relation between ZAHB absolutemagnitude and metallicity isMV(ZAHB)=(0.18+/-0.09)([Fe/H]+1.5)+(0.63+/-0.07). Finally, wecompare the ages of the GCs with the cosmic star formation rate recentlydetermined by studies of the Hubble Deep Field (HDF), exploiting thedeterminations of ΩM=0.3 andΩΛ=0.7 provided by Type Ia supernovae surveys.We find that the epoch of formation of the GCs (at z~3) matches well themaximum of the star formation rate for elliptical galaxies in the HDF asdetermined by Franceschini et al. Based on data from the Hipparcosastrometry satellite.
| Hubble Space Telescope Observations of the Oldest Star Clusters in the Large Magellanic Cloud
We present V, V-I color-magnitude diagrams (CMDs) for three old starclusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, andHodge 11. Our data extend ~3 mag below the main-sequence turnoff,allowing us to determine accurate relative ages and the blue stragglerfrequencies. Based on a differential comparison of the CMDs, any agedifference between the three LMC clusters is less than 1.5 Gyr.Comparing their CMDs to those of M92 and M3, the LMC clusters, unlesstheir published metallicities are significantly in error, are the sameage as the old Galactic globulars. The similar ages to Galacticglobulars are shown to be consistent with hierarchial clustering modelsof galaxy formation. The blue straggler frequencies are also similar tothose of Galactic globular clusters. We derive a true distance modulusto the LMC of (m-M)0=18.46+/-0.09 [assuming(m-M)0=14.61 for M92] using these three LMC clusters.
| The Giant, Horizontal, and Asymptotic Branches of Galactic Globular Clusters. I. The Catalog, Photometric Observables, and Features
A catalog including a set of the most recent color-magnitude diagrams(CMDs) is presented for a sample of 61 Galactic globular clusters(GGCs). We used this database to perform a homogeneous systematicanalysis of the evolved sequences (namely, the red giant branch [RGB],horizontal branch [HB], and asymptotic giant branch [AGB]). Based onthis analysis, we present (1) a new procedure to measure the level ofthe zero-age horizontal branch (V_ZAHB) and a homogeneous set ofdistance moduli obtained by adopting the HB as standard candle; (2) anindependent estimate for RGB metallicity indicators and new calibrationsof these parameters in terms of both spectroscopic ([Fe/H]_CG97) andglobal metallicity ([M/H], including also the α-elementenhancement), such that the set of equations presented can be used tosimultaneously derive a photometric estimate of the metal abundance andthe reddening from the morphology and the location of the RGB in the(V,B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and theAGB bump (in nine GGCs). The dependence of these features on metallicityis discussed. We find that by using the latest theoretical models andthe new metallicity scales, the earlier discrepancy between theory andobservations (~0.4 mag) completely disappears.
| The absolute magnitudes of RR Lyrae stars from BT HIPPARCOS parallaxes parallaxes
Using the method of ``reduced parallaxes'' for the Halo RR Lyrae starsin the hipparcos catalogue we derive a zero point of 0.77 +/- 0.26 magfor an assumed slope of 0.18 in the M_V-[Fe/H] relation. This is 0.28magnitude brighter than the value Fernley et al. (1998a) derived byemploying the method of statistical parallax for the identical sampleand using the same slope. We point out that a similar difference existsbetween the ``reduced parallaxes'' method and the statistical parallaxmethod for the Cepheids in the hipparcos catalogue. We also determinethe zero point for the M_K-log P0 relation, and obtain avalue of -1.16 +/- 0.27 mag (for a slope of -2.33). The distance modulito the hipparcos RR Lyrae stars derived from the two relations agreewell. The derived distance scale is in good agreement with the resultsfrom the Main Sequence fitting distances of Galactic globular clustersand with the results of theoretical Horizontal Branch models, andimplies a distance modulus to the LMC of 18.61 +/- 0.28 mag. Based ondata from the ESA Hipparcos astrometry satellite.}
| Spectroscopic analysis of the candidate globular clusters NGC 1928 and 1939 in the Large Magellanic Cloud
The integrated spectral properties in the range 3600-6700 A of thecandidate old clusters NGC 1928 and 1939 in the LMC bar are comparedwith those of old- and intermediate-age reference LMC clusters, theproperties of which are better established. It has been possible toinfer the age of the sample clusters by means of absorption features andthe continuum distribution, in particular in the plane W_M x W_B (whereW_B is the average of Hdelta, Hγ and H beta equivalent widths, andW_M that of Ca II K, G band and Mg i). The results indicate that NGC1928 and 1939 are compatible with old clusters. The metallicity isderived with respect to galactic globular cluster templates: [Fe/H]~-1.2 and -2.0 for NGC 1928 and 1939, respectively. We also discuss thecensus of Population II clusters in the LMC, their spatial distributionand the possibility of a LMC core and a transient morphologicalclassification for interacting late-type disc galaxies.
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