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|Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K 29 and NGC 290 and their field population|
Aims.We determine the age, metallicity and initial mass function ofthree clusters, namely NGC 265, K29, NGC 290, located in the main body ofthe Small Magellanic Cloud. In addition, we derive the history of starformation in the companion fields. Methods: We make use of ACS/WFC HSTarchive data. For the clusters, the age and metallicity are derivedfitting the integrated luminosity function with single synthetic stellarpopulation by means of the χ2 minimization. For thecompanion fields, the history of star formation is derived using theχ2 minimization together with the downhill-simplexmethod. Results: For the clusters we find the following ages andmetallicities: NGC 265 has log(Age)=8.5±0.3 yrand metallicity 0.004±0.003 (or [ Fe/H]=-0.62); K29 has log(Age)=8.2±0.2 yr and metallicityZ=0.003±0.002 (or [ Fe/H]=-0.75); NGC 290 haslog(Age)=7.8±0.5 yr and metallicity 0.003±0.002 (or [Fe/H]=-0.75). The superior quality of the data allows the study of theinitial mass function down to M ˜ 0.7 Mȯ. Theinitial mass function turns out to be in agreement with the standardKroupa model. The comparison of the NGC 265luminosity function with the theoretical ones from stellar models bothtaking overshoot from the convective core into account and neglectingit, seems to suggest that a certain amount of convective overshoot isrequired. However, this conclusion is not a strong one because thiscluster has a certain amount of mass segregation which makes itdifficult to choose a suitable area for this comparison. The starformation rate of the field population presents periods of enhancementsat 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relativelyquiescent at ages older than 6 Gyr. This result suggests that at olderages, the tidal interaction between the Magellanic Clouds and the MilkyWay was not able to trigger significant star formation events.
|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.
|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.
|The Optical Gravitational Lensing Experiment. Catalog of RRLyr Stars from the Small Magellanic Cloud|
We present the catalog of RRLyrae stars from 2.4 square degrees ofcentral parts of the Small Magellanic Cloud (SMC). The photometric datawere collected during four years of the OGLE-II microlensing survey.Photometry of each star was obtained using the Difference Image Analysis(DIA) method. The catalog contains 571 objects, including 458RRab, 56RRcvariables, and 57 double mode RRLyr stars (RRd). Additionally we attacha list of variables with periods between 0.18-0.26 days, which areprobably delta Sct stars. Period, BVI photometry, astrometry, amplitude,and parameters of the Fourier decomposition of the I-band light curveare provided for each object. We also present the Petersen diagram fordouble mode pulsators.We found that the SMC RRLyr stars are fairly uniformly distributed overthe studied area, with no clear correlation to any location. The mostpreferred periods for RRab and RRc stars are 0.589 and 0.357 days,respectively. We noticed significant excess of stars with periods ofabout 0.29 days, which might be second-overtone RRLyr stars (RRe). Themean extinction free magnitudes derived for RRab stars are 18.97, 19.45and 19.73 mag for the I, V and B-band, respectively.All presented data, including individual BVI observations, are availablefrom the OGLE Internet archive.
|Dating star clusters in the Small Magellanic Cloud by means of integrated spectra|
In this study flux-calibrated integrated spectra in the range(3600-6800) Å are presented for 16 concentrated star clusters inthe Small Magellanic Cloud (SMC), approximately half of which constituteunstudied objects. We have estimated ages and foreground interstellarreddening values from the comparison of the line strengths and continuumdistribution of the cluster spectra with those of template clusterspectra with known parameters. Most of the sample clusters are youngblue clusters (6-50 Myr), while L 28, NGC 643 and L 114 are found to beintermediate-age clusters (1-6 Gyr). One well known SMC cluster (NGC416) was observed for comparison purposes. The sample includes clustersin the surroundings and main body of the SMC, and the derived foregroundreddening values are in the range 0.00 <= E(B-V) <= 0.15. Thepresent data also make up a cluster spectral library at SMC metallicity.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba and San Juan, Argentina.
|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.
|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.
|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.
|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.
|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.
|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
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