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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.
| 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 hot and cool component of the symbiotic nova SMC 3. A supersoft X-ray variable and a small-amplitude red variable
The ˜6 year supersoft X-ray lightcurve of the symbiotic nova SMC 3(=RX J0048.4-7332) in the Small Magellanic Cloud is derived fromarchival ROSAT PSPC and HRI data. It shows one deep X-ray eclipse duringwhich the count rate decreased by a factor of ≈gt80. In MACHO B-banddata sinusoidal variation is found with a quasi-periodicity of ˜4years. The minimum of the B-band flux occurs during the X-ray eclipse.In OGLE II I-band observations performed after the ROSAT observations wedetect 110±2 day oscillations which we interpret as pulsations ofthe M0 giant star in the symbiotic system. The observed duration of thesupersoft X-ray eclipse of ˜0.4-1.8 years is explained by theoccultation of the white dwarf by the giant companion with an orbitalperiod of ˜(4.0-4.8) years and a strong wind blown from itssurface with a mass loss rate of ˜(2.6-8.2)× 10-7 Mȯ yr-1, assuming that˜(1-3.5)% of the ionized phase is neutral (e.g. due to dust) andassuming a terminal velocity of ˜30 km s-1. The˜4 year quasi-periodicity found in the optical is explained as thebinary orbital period of the system. It is less likely that it reflectsthe activity (or mass-loss) time scale of the red giant star. A˜(700-800) day quasi-periodicity found in the OGLE II and MACHOdata is explained as the first harmonic of a binary orbital cycle. SMC 3therefore may be classified as a small-amplitude red variable star(SARV). The hot star most likely is in a state of steady nuclear burningwith an accretion rate somewhat below the upper critical value of˜10-7 Mȯ yr-1.Based on archival data from the ROSAT, OGLE-II and MACHO project.
| 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.
| Magellanic Cloud X-Ray Sources. III. Completion of a ROSAT Survey
This paper concludes a series of three papers presenting ROSAT HighResolution Imager (HRI) observations of unidentified Einstein andserendipitous ROSAT X-ray sources in the direction of the MagellanicClouds. Accurate positions and fluxes have been measured for thesesources. Optical photometry and spectroscopy were obtained to search foridentifications in order to determine the physical nature of thesesources. The present paper includes new data for 24 objects;identifications are given or confirmed for 30 sources. For six sources,optical finding charts showing the X-ray positions are provided. Theresults from this program are summarized, showing that the populationsof luminous X-ray sources in the Magellanic Clouds are quite differentfrom those in the Galaxy.
| 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.
| Cepheids in MC Clusters: New Observations
Not Available
| Magellanic Cloud X-ray sources observed with ROSAT
This paper is the second in a series which presents ROSATHigh-Resolution Imager (HRI) observations of previously known X-raysources in the Magellanic Clouds. This paper includes new data on 31X-ray sources discovered by Einstein Observatory as well as 16serendipitous ROSAT sources. Optical photometry and spectroscopy wereobtained to search for identifications and to determine the physicalnature of these sources. Optical finding charts showing the ROSAT-HRIpositions are given for 35 sources, and where appropriate candidates oridentified counterparts are marked. Twenty-nine of these sources areoptically identified with objects including X-ray binaries and supernovaremnants in both the Small and Large Magellanic Clouds, foreground(Galactic) stars, and background active galactic nuclei. For previouslyknown sources, there is evidence that more than half of them exhibitX-ray variability. (SECTION: Stars)
| Mid-infrared properties of globular clusters using the IRAS data base
We present an analysis of the mid-IR properties of 18 globular clusters(GCs) [15 in the Galaxy and three in the Large Magellanic Cloud (LMC)]using the IRAS photometric data at 12 and 25 mum. Eight of the nineGalactic GCs with central escape velocities greater than 50 km s^-1 haveIRAS sources within a radius of 60 arcsec from the centre, in agreementwith the expectation that interstellar gas and dust should indeed bepresent in the central regions of the most massive clusters owing tomass-loss processes occurring in the late stages of the stellarevolution. No other significant correlation is found between IRAS sourceincidence and any intrinsic GC parameters. Warm dust (T~300K) isdetectable mostly around unresolved giant stars, but in three massiveGCs it is also present as diffuse emission. However, most of the dustmight be cold (T<50K) and it was thus notdetected by IRAS because of its limited sensitivity at 60 and 100 mum.The inferred mass-loss rates and statistical considerations arecompatible with a non-steady mass-loss process with several episodes ofejection lasting a few times 10^5 yr.
| 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.
| A New Catalogue of Hα Emission Line Stars and Small Nebulae in the Small Magellanic Cloud
An objective-prism survey of the Small Magellanic Cloud has beenperformed through an Hα + [N II] interference filter, using the0.90 m Curtis Schmidt telescope of Cerro Tololo. 1898 emission-lineobjects have been detected in the main body of this galaxy, almostquadrupling the number of those found, in the same region, by theprevious objective-prism surveys. Among these objects are newlydiscovered planetary nebulae, compact HII regions and late-type stars.Continuum intensity, as well as the shape and relative strength of theHα emission-line have been estimated; coordinates, cross-identifications for the listed objects and 2.25 arcmin square findingcharts for all the objects are provided.
| 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 asymptotic giant branch of Magellanic Cloud clusters
The present search for carbon and M-type asymptotic giant branch (AGB)stars in the 39 clusters of the Magellanic Clouds has yieldedidentifications and near-IR photometry for about 400 such stars. TheSearle et al. (1980) cluster-age-related classification scheme is abasic element of the present analysis of these data. In a C-M diagram,the cluster M stars shift steadily redward as one proceeds from clustersof SWB type I to VI, due to the increasing age of the clusters along thesequence. Luminous carbon stars are present only in SWB IV-VI clusters,and are easily distinguished from M stars by their color and luminosity.
| 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.
| Photometric studies of composite stellar systems. V - Infrared photometry of star clusters in the Magellanic clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1983ApJ...266..105P
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. II
In order to obtain a complete sample of upper asymptotic giant branch(AGB) stars in the red globular clusters of the Magellanic Clouds, aphotographic near-infrared survey of the clusters was conducted. Theresults are compared with previous photometry and the problem of errorarising from variability of carbon stars is addressed. Stars withoutspectra are tentatively classified based on their JHK colors. Apparentand absolute bolometric magnitudes and effective temperatures werecalculated from the IR colors, allowing for the location of the redstars and of the cluster giant branches in the physical H-R diagram tobe determined. Stellar evolution on the AGB is discussed, leading toimproved estimates of the extent of the upper AGB. A carbon star censusis presented and the ages of the clusters is derived with suitablycomplete photometry. On this basis, the chemical enrichment history ofthe Clouds is discussed.
| 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
Not Available
| 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
Not Available
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