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|The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90|
We present the results of our investigation of the intermediate-age starcluster BS 90, located in the vicinity of the H II region N66 in theSMC, observed with HST ACS. The high-resolution data provide a uniqueopportunity for a very detailed photometric study performed on one ofthe rare intermediate-age rich SMC clusters. The complete set ofobservations is centered on the association NGC 346 and contains almost100,000 stars down to V~=28 mag. In this study we focus on the northernpart of the region, which covers almost the whole stellar content of BS90. We construct its stellar surface density profile and derivestructural parameters. Isochrone fits on the CMD of the cluster resultsin an age of about 4.5 Gyr. The luminosity function is constructed andthe present-day mass function of BS 90 has been obtained using themass-luminosity relation, derived from the isochrone models. We found aslope between -1.30 and -0.95, comparable to or somewhat shallower thana typical Salpeter IMF. Examination of the radial dependence of the massfunction shows a steeper slope at larger radial distances, indicatingmass segregation in the cluster. The derived half-mass relaxation timeof 0.95 Gyr suggests that the cluster is mass segregated due to itsdynamical evolution. From the isochrone model fits we derive ametallicity for BS 90 of [Fe/H]=-0.72, which adds an important point tothe age-metallicity relation of the SMC. We discuss our findings on thisrelation in comparison to other SMC clusters.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
|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.
|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 Star Clusters of the Small Magellanic Cloud: Age Distribution|
We present age measurements for 195 star clusters in the SmallMagellanic Cloud based on comparison of integrated colors measured fromthe Magellanic Clouds Photometric Survey with models of simple stellarpopulations. We find that the modeled nonuniform changes of clustercolors with age can lead to spurious age peaks in the cluster agedistribution; that the observed numbers of clusters with age t declinessmoothly as t-2.1 that for an assumed initial cluster massfunction scaling as M-2, the dependence of the clusterdisruption time on mass is proportional to M0.48; thatdespite the apparent abundance of young clusters, the dominant epoch ofcluster formation was the initial one; and that there are significantdifferences in the spatial distribution of clusters of different ages.Because of limited precision in our age measurements, we cannot addressthe question of detailed correspondence between the cluster age functionand the field star formation history. However, this sample provides aninitial guide as to which clusters to target in more detailed studies ofspecific age intervals.
|Infrared Surface Brightness Fluctuations of Magellanic Star Clusters|
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
|Surface brightness profiles and structural parameters for 10 rich stellar clusters in the Small Magellanic Cloud|
As a follow-up to our recent study of a large sample of Large MagellanicCloud (LMC) clusters, we have conducted a similar study of thestructures of 10 Small Magellanic Cloud (SMC) clusters, using archivalHubble Space Telescope snapshot data. We present surface brightnessprofiles for each cluster and derive structural parameters, includingcore radii and luminosity and mass estimates, using exactly the sameprocedure as for the LMC sample. Because of the small sample size, theSMC results are not as detailed as for the larger LMC sample. We do notobserve any post-core-collapse clusters (although we did not expect to),and there is little evidence for any double clusters in our sample. Nonethe less, despite the small sample size, we show for the first time thatthe SMC clusters follow almost exactly the same trend in core radiuswith age observed for the LMC system, including the apparent bifurcationat several hundred Myr. This further strengthens our argument that thisrelationship represents true physical evolution in these clusters, withsome developing significantly expanded cores due to an as yetunidentified physical process. Additional data, both observational andfrom N-body simulations, are still required to clarify many issues.
|A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. I. Small Magellanic Cloud and Bridge|
A survey of extended objects in the Magellanic System was carried out onthe ESO/SERC R and J Sky Survey Atlases. The present work is dedicatedto the Small Magellanic Cloud and to the inter-Magellanic Cloud region("Bridge") totaling 1188 objects, of which 554 are classified as starclusters, 343 are emissionless associations, and 291 are related toemission nebulae. The survey includes cross-identifications amongcatalogs, and we present 284 new objects. We provide accurate positions,classification, homogeneous sizes, and position angles, as well asinformation on cluster pairs and hierarchical relation for superimposedobjects. Two clumps of extended objects in the Bridge and one at theSmall Magellanic Cloud wing tip might be currently forming dwarfspheroidal galaxies.
|Accurate positions for SMC clusters|
Positions of 203 SMC clusters accurate to + or - 5 arcsec are reported.The astrometry method used is briefly described. Plans for futureMagellanic Cloud cluster astrometry are summarized.
|The morphology of star clusters in the SMC|
The projected ellipticities of 34 populous SMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. A pair of J and Rplates taken with the 1.2 m UK Schmidt telescope in Australia were used.Radial ellipticity variations within individual globular clusters seemto be a common phenomenon for the SMC clusters, similar to that observedin the LMC clusters where the innerparts are more elliptical than theouter ones in 95 percent of the cases. The derived ellipticities whichcorrespond to the innermost part of the cluster at radial distances nearto half-mass radii have been found to be statistically more ellipticalthan those of the LMC, known to be more elliptical than those of theGalaxy. The dynamical masses of the clusters seem to correlate withellipticities supporting the hypothesis that, either the gravitationalfield of the parent galaxy being a dominant factor affect slower theshape of the high mass clusters and/or the most massive clusters, beingdynamically younger, retain their original shape.
|Infrared observations of the Magellanic Clouds. II - The Large Magellanic Cloud|
Results of IRAS pointed observations in four infrared wavelength bands(12, 25, 60, and 100 microns) of the LMC are presented. Maps are shownwith orthogonal scan directions, and a source list containing 1823infrared sources is extracted from the data. Comparison with the IRASPoint Source Catalog (PSC) shows that wenty-eight entries in thiscatalog are spurious. All 49 entries in the IRAS Small Scale StructureCatalog (SSS) in the LMC are confirmed. Eight hundred and two newinfrared sources, not included in either the PSC or in the SSS arefound. The LMC infrared source list and the infrared maps are comparedto other object lists. Three LMC globular clusters and 57 SAO stars havebeen identified, and infrared emission has been detected from thedirection of 13 planetary nebulae. Infrared emission has been detectedfrom four Radcliffe LMC-stars and five out of ten small size supernovaremnants. In general, there is a good correlation of infrared emissionwith the distribution of H II regions and dark clouds.
|Some studies of Magellanic Cloud clusters|
Four photoelectric sequences in the UBV system for areas in the Largeand Small Magellanic Clouds are presented. The stars range in brightnessfrom V = 8.33 to V = 19.42 and in color from (B-V) = 0.08 to (B-V) = +1.80. Photographic color-magnitude diagrams for seven clusters in theClouds are derived from CTIO 4-m telescope plates.
|Spectral classification of bright stars in SMC clusters. I.|
The identification charts and spectral-classification catalogs forfifteen SMC globular clusters and their adjoining fields are presented.Film copies of plates taken with the 1.2-m UK Schmidt telescope wereexamined in Athens by means of a binocular microscope. The studiedclusters are disk, halo, and intermediate, located in various places ofthe parent galaxy. All classified stars are brighter than B = 18.5 magand are located within the cluster's tidal radii.
|Distribution of spectral types of stars in the SMC clusters|
Spectral classification of stars brighter than B of about 18.5 mag in 15SMC clusters was carried out using U.K. Schmidt Telescope objectiveprism plates. The studied clusters represent all ages of clusters (disk,intermediate and halo) and are located at various areas of the parentgalaxy. All classified stars are situated within the tidal radius ofeach cluster. For comparison, stars in adjoining fields of each clusterwere also studied. It was found that M stars are the main contributorsof the halo clusters, whereas early type stars are mainly found in diskclusters. M stars are also detected in the disk clusters as it isexpected from their HR diagrams. A significant number of carbon starswere found in two (out of three) intermediate age clusters. Carbon starswere also detected in the regions of the halo clusters L8 and L59 and ofthe disk cluster L16. The ratios of C to M stars derived herestrengthened the assumption that there is a deficiency of M type starsin the SMC.
|Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud|
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
|Ellipticities of 'disk' and 'halo' globular clusters in the SMC|
Isodensity contours are used to determine the projected ellipticities of24 'disk' and 'halo' globular clusters of various ages and compositionsin the Small Magellanic Cloud (SMC). The ellipticities derived from Band V plates that were scanned with an isodensitometer indicate that theglobular clusters of the SMC are more elliptical than their counterpartsin the Large Magellanic Cloud, as well as those of the Galaxy; since thecomparison of the observed distribution of the ellipticities for bothcluster types, disks and haloes, do not show a statistically significantdifference, the observations are held not to support age dependence.Results also show that the ellipticities of the inner part of theclusters are greater than those of the outer parts.
|Integrated magnitudes and colors of clusters in the magellanic clouds and Fornax system|
Data from PV, six-color, and four-color photometric observations ofclusters (38 in the Small Magellanic Cloud, 16 in the Large MagellanicCloud, four in the Fornax system, and NGC 1841) are reported. Theobservations were made in 1951, 1960-1961, 1959-1966, and 1974-1975using various telescopes and photometer setups at Mount StromloObservatory in Australia. Tables of integrated magnitudes and colors(both as originally observed and as reduced to the BV system) arepresented, and comparable published data are shown. The combined V dataare fitted to the theoretical luminosity profiles of King (1966) toestimate the total magnitudes and surface brightness distributions of 33of the clusters. Several sample profile fits are shown. A color-colorplot (V-B vs. G-R) is discussed in terms of identification of clustertypes by color: it is found that globular clusters can be separated fromother types, if all have the same amount of reddening.
|Age calibrations of Magellanic Cloud clusters|
Using primarily main sequence photometry, this paper provides acompilation of age estimates for 81 star clusters in the MagellanicClouds. These ages are used to calibrate the photometric age classes ofSearle, Wilkinson, and Bagnuolo and of van den Bergh. Previouslypublished calibrations require systematic revisions, especially thosebased on carbon star membership.
|UBV photometry for star clusters in the Small Magellanic Cloud.|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978A&AS...34..431A&db_key=AST
|Magnitudes of Clusters in the Small Magellanic Cloud|
|The cluster system of the Small Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958MNRAS.118..172L&db_key=AST
|Star Clusters in the Small Magellanic Cloud: I. Identification of 69 Clusters|