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|Young star clusters immersed in intermediate-age fields in the Small Magellanic Cloud|
We present CCD photometry in the Washington C and T1 filtersfor six star clusters (B34, NGC256, NGC265, NGC294, IC1611 and NGC376)in the Small Magellanic Cloud (SMC) and their surrounding fields. Theresultant colour-magnitude diagrams (CMDs) extend from T1 ~14 to as faint as T1 ~ 22 revealing the main-sequenceturnoffs of the clusters. Adopting a metallicity of Z = 0.004, wecompare our cluster photometry with theoretical isochrones in theWashington system in order to derive ages. To facilitate agedetermination of the surrounding fields, we use the magnitude differencebetween the helium-burning red clump stars and the main-sequenceturnoff. Finally, we estimate mean metallicities for the field stars bycomparing the location of the field red giant branch with standard giantbranches for Galactic globular clusters of known abundance, correctedfor age effects. Combining these results with our previous work, we finda clear trend of younger clusters being located closer to the centre ofthe SMC. In addition, there is a tendency for the mean metallicity andits dispersion to be greater inside 4° of the SMC's centre ascompared to outside this radius. As far as the properties of the fieldstars are concerned, we find little correlation between the ages of theclusters and those of the field stars against which they are projected.Clearly, more work needs to be done to clarify these trends.
|Age distribution of young clusters and field stars in the Small Magellanic Cloud|
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
|Integrated spectral analysis of 18 concentrated star clusters in the Small Magellanic Cloud|
We present in this study flux-calibrated integrated spectra in the range(3600-6800) Å for 18 concentrated SMC clusters. Cluster reddeningvalues were estimated by interpolation between the extinction maps ofBurstein & Heiles (1982, AJ, 87, 1165) and Schlegel et al. (1998,ApJ, 500, 525). The cluster parameters were derived from the templatematching procedure by comparing the line strengths and continuumdistribution of the cluster spectra with those of template clusterspectra with known parameters and from the equivalent width (EW) method.In this case, new calibrations were used together with diagnosticdiagrams involving the sum of EWs of selected spectral lines. A verygood agreement between ages derived from both methods was found. Thefinal cluster ages obtained from the weighted average of values takenfrom the literature and the present measured ones range from 15 Mr (e.g.L 51) to 7 Gyr (K 3). Metal abundances have been derived for only 5clusters from the present sample, while metallicity values directlyaveraged from published values for other 4 clusters have been adopted.Combining the present cluster sample with 19 additional SMC clusterswhose ages and metal abundances were put onto a homogeneous scale, weanalyse the age and metallicity distributions in order to explore theSMC star formation history and its spatial extent. By considering thedistances of the clusters from the SMC centre instead of theirprojections onto the right ascension and declination axes, the presentage-position relation suggests that the SMC inner disk could have beenrelated to a cluster formation episode which reached the peak ~2.5 Gyrago. Evidence for an age gradient in the inner SMC disk is alsopresented.
|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.
|An estimate of the supernova kick velocities for high-mass X-ray binaries in the Small Magellanic Cloud|
This work investigates the possible supernova kick velocities imposed onhigh-mass X-ray binary (HMXB) systems in the Small Magellanic Cloud.Comparisons are made between the location of such systems and thelocations of young stellar clusters on the premise that these mayrepresent the birthplace of many of these systems. Measurements of theseparation of clusters and HMXBs, and an estimate of the typicallifetimes of these systems, lead to a minimum average space velocity of30 kms-1. This value is compared to theoretical estimates.
|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.
|Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud|
An isophotal atlas of 75 star cluster pairs and multiplets in the SmallMagellanic Cloud is presented, comprising 176 objects. They areconcentrated in the SMC main body. The isophotal contours were made fromDigitized Sky Survey* images and showed relevant structuralfeatures possibly related to interactions in about 25% of the sample.Previous N-body simulations indicate that such shapes could be due totidal tails, bridges or common envelopes. The diameter ratio between themembers of a pair is preferentially in the range 1 - 2, with a peak at1. The projected separation is in the range ~ 3 - 22 pc with apronounced peak at ~ 13 pc. For 91 objects it was possible to deriveages from Colour-Magnitude Diagrams using the OGLE-II photometricsurvey. The cluster multiplets in general occur in OB stellarassociations and/or HII region complexes. This indicates a common originand suggests that multiplets coalesce into pairs or single clusters in ashort time scale. Pairs in the SMC appear to be mostly coeval andconsequently captures are a rare phenomenon. We find evidence that starcluster pairs and multiplets may have had an important role in thedynamical history of clusters presently seen as large single objects.The images in this study are based on photographic data obtained usingthe UK Schmidt Telescope, which was operated by the Royal ObservatoryEdinburgh, with funding from the UK Science and Engineering ResearchCouncil, until 1988 June, and thereafter by the Anglo-AustralianObservatory. Original plate material is copyright by the RoyalObservatory Edinburgh and the Anglo-Australian Observatory. The plateswere processed into the present compressed digital form with theirpermission. The Digitized Sky Survey was produced at the Space TelescopeScience Institute under US Government grant NAG W-2166.
|Updating the Census of Star Clusters in the Small Magellanic Cloud|
Surveys using CCD detectors are retrieving bright and faint catalogedclusters and revealing new ones in the Magellanic Clouds. This paperdiscusses the contribution of the OGLE Survey to the overall census ofstar clusters in the Small Magellanic Cloud (SMC). A detailedcross-identification indicates that the new objects in the SMC OGLEcatalog are 46. The increase in the number of cataloged clusters is ~7%,the total sample being ~700. This updated census includes embeddedclusters in H II regions and a density range attaining loose systems.
|The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds|
We present Cepheids located in the close neighborhood of star clustersfrom the Magellanic Clouds. 204 and 132 such stars were found in the LMCand SMC, respectively. The lists of objects were constructed based oncatalogs of Cepheids and star clusters, recently published by theOGLE-II collaboration. Location of selected Cepheids on the skyindicates that many of them are very likely cluster members. Photometricdata of Cepheids and clusters are available from the OGLE Internetarchive.
|The Optical Gravitational Lensing Experiment. Age of Star Clusters from the SMC|
We present determination of age of clusters from 2.4 square degreeregion of the SMC bar. The photometric data were taken from the BVI mapsof the SMC and catalog of clusters in this galaxy obtained during theOGLE-II microlensing survey. For 93 well populated SMC clusters theirage is derived with the standard procedure of isochrone fitting. Thedistribution of age of cluster from the SMC is presented. It indicateseither non-uniform process of cluster formation or very effectivedisruption of clusters.
|The Optical Gravitational Lensing Experiment. Variable Stars in Star Clusters of the Magellanic Clouds. I.Eclipsing Systems in the Clusters of the SMC|
The list of 127 eclipsing stars in optical coincidence with starclusters from the SMC is presented. It was prepared using the catalogsof eclipsing systems and star clusters from the SMC based onobservations collected during the OGLE-II microlensing project. Locationof 12 eclipsing stars in the color-magnitude diagram of clusters allowsto exclude their membership. Photometric data of 73 systems supporttheir membership. The remaining 42 objects were found in loose, faintclusters and therefore no conclusive statement about their membershipcan be made. All presented data are available from the OGLE archive.
|Dynamical studies of cluster pairs in the Magellanic Clouds|
We performed N-body simulations of star cluster encounters withHernquist's TREECODE in a CRAY YMP-2E computer under different initialconditions (relative positions and velocities, cluster sizes, masses andconcentration degrees). The total number of particles per simulationranged from 1024 to 20480. These models are compared with a series ofisodensity maps of cluster pairs in the Magellanic Clouds. Evidence isfound that during the interactions, transient morphological effects suchas an expanded halo, isophotal deformation and isophotal twisting canoccur as a result of tidal effects and dynamical friction. Thesimulations also show that different outcomes are possible depending onthe initial parameters: (i) long-standing changes of concentrationdegree can occur after the collision; (ii) one member can disaggregate;or (iii) the pair can coalesce into a single cluster with a distinctstructure compared with the original ones. These simulations canreproduce a wide range of morphological structures in observed clusterpairs.
|Optical Gravitational Lensing Experiment. The Catalog of Clusters in the Small Magellanic Cloud|
We present the catalog of clusters found in the area of approx 2.4square degree in the central regions of the Small Magellanic Cloud. Thecatalog contains data for 238 clusters, 72 of them are new objects. Foreach cluster equatorial coordinates, radii, approximate number ofmembers, cross-identification, finding chart and color magnitudediagrams: V-(B-V) and V-(V-I) are provided. Photometric data for allclusters presented in the catalog are available from the OGLE Internetarchive.
|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.
|The evolution of carbon stars in the Magellanic Clouds|
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
|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.
|The evolution of the Magellanic Clouds. I - The ages of globular clusters|
Theoretical and observed maximum luminosities of AGB stars in theMagellanic Cloud clusters are compared in order to obtain cluster ageestimations. The ages of 10 clusters in the SMC and 25 in the LMC areconsidered for the cases of several rates of mass loss by AGB stars. Itis demonstrated that discrepancies between ages derived from AGB peakluminosities and from the Main-Sequence turn off and maximum luminositycan be accounted for by the intensive mass loss during the AGBevolutionary phase.
|A preliminary survey of collapsed cores in the Magellanic Clouds' globular clusters|
A preliminary report on a surface photometry survey for collapsed coresin the Magellanic Clouds' globular clusters is presented. Coremorphology classifications are given for the 33 globular clustersexamined so far. One cluster, NGC 2019, shows definite signs of acollapsed core, and two others, NGC 1774 and NGC 1951, appear as strongcandidates. This detection of postcollapse cores outside the Milky Wayopens some interesting prospects for future dynamical studies. However,the fraction of collapsed-core clusters appears to be smaller in theMagellanic Clouds than in the Galaxy. This may be due in part to theiryounger ages, or to the limitations imposed by the seeing effects. It isalso possible that the relative scarcity reflects the physicaldifference in the tidal field environments between the Galaxy and theClouds, in agreement with a trend found earlier, viz., that the tidalshocks from disk passages accelerate dynamical evolution and enhance thepropensity for core collapse.
|Observed radii and structural parameters of star clusters in the SMC. III|
The tidal radii, concentration parameters, and masses of 24 starclusters in the SMC have been derived by means of star counts. In thisinvestigation the seven most distant clusters (with projected distancesfrom the rotation center similar to the halo radius) are included aswell, and their masses are found to be about one hundred times lowerthan those of the halo clusters of the Galaxy.
|The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. IV|
A complete survey is available for asymptotic giant-branch stars in therich star clusters of the Magellanic Clouds. Although data on themain-sequence turnoffs of these clusters are still incomplete, somesystematic properties of these stars emerge, when grouped by clusterage. Clusters younger than approximately 8 billion years have carbonstars at the tip of the giant branch, produced by the third dredge-upmechanism. Clusters younger than approximately 0.8 billion years havegiant branches populated by M stars. It is suggested that in stars ofthis mass range thermal pulses have not commenced before mass losscompletely erodes the stellar envelope. Cluster stars of 5 solar mass(turnoff approximately 80 million years) suffer about 80-percent massloss in the course of their evolution, compared with approximately 30percent for the oldest stars.
|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
|NGC and IC Objects in S. M. C.|
|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
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