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The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. I. New Candidate Be Stars in the LMC, SMC, and Milky Way
We present B, V, R, and Hα photometry of eight clusters in theSmall Magellanic Cloud, five in the Large Magellanic Cloud, and threeGalactic clusters and use two-color diagrams (2-CDs) to identifycandidate Be star populations in these clusters. We find evidence thatthe Be phenomenon is enhanced in low-metallicity environments, based onthe observed fractional early-type candidate Be star content of clustersof age 10-25 Myr. Numerous candidate Be stars of spectral types B0-B5were identified in clusters of age 5-8 Myr, challenging the suggestionof Fabregat & Torrejon that classical Be stars should only be foundin clusters at least 10 Myr old. These results suggest that asignificant number of B-type stars must emerge onto the zero-age mainsequence as rapid rotators. We also detect an enhancement in thefractional content of early-type candidate Be stars in clusters of age10-25 Myr, suggesting that the Be phenomenon does become more prevalentwith evolutionary age. We briefly discuss the mechanisms that mightcontribute to such an evolutionary effect. A discussion of thelimitations of using the 2-CD technique to investigate the roleevolutionary age and/or metallicity play in the development of the Bephenomenon is offered, and we provide evidence that other B-type objectsof very different nature, such as candidate Herbig Ae/Be stars, maycontaminate the claimed detections of Be stars via 2-CDs.
| Triggered star formation in the inner wing of the SMC. Two possible supernova explosions in the N83-84-85 region
In this article we study the N83-84-85 region of the inner wing of theSMC. Direct and low-dispersion objective prism plates taken with the 1.2m UK Schmidt Telescope have been digitized by the SuperCOSMOS machine.Star counts have been performed for our region in selected luminosityslices in the U filter and isodensity contours have been used toidentify the structures with enhanced stellar number density. We findevidence of triggered star formation from massive stars of older to morerecent OB associations. Circular arcs constructed by O and B stars havebeen detected. A study of the population places stars with more recentages in the groups of the arcs than of their centers. These effects canbe explained by supernova explosions. A catalogue of the non-saturateddetected OB stars in this region is given.Table \ref{t3} is only available in electronic form athttp://www.edpsciences.org
| 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.
| Automatic extraction and classification of low-dispersion objective prism stellar spectra
The observing material used for this task is prism spectral plates takenwith Schmidt-class telescopes. Such a plate generally contains thousandsof spectra, and there are prism-plate libraries and digitized databasesin several astronomical centers that can be exploited for this analysis.After a successive detection from the prism plate image, the spectra areautomaticaly extracted in one-dimensional streams containing all thebasic information. These spectra require automated classificationmethods to be analyzed in an objective form. In this article we comparetwo classification methods directly applied to stellar spectra: a linearcorrelation and a minimum distance method.
| Extinction and reddening of HII regions in the Small Magellanic Cloud.
We present absolute Hα and Hβ fluxes, obtained with aFabry-Perot spectrophotometer, of 24 bright HII regions in the SmallMagellanic Cloud. The photographic Hα maps of Kennicutt &Hodge (1986) are re-calibrated using these new Hα fluxes; the newcalibration gives fluxes 25% smaller than those previously published.These photographic and photoelectric Hα data are used inconjunction with radio continuum observations at 843 MHz from theMolonglo Observatory Synthesis Telescope to study the dust associatedwith SMC HII regions. For most regions the derived reddenings andextinctions are compatible with the standard Galactic extinction law anduniform interstellar extinction. A few regions display relatively highreddening and extinction; these are bright compact sources, such asN13AB, N27 and the cores of N81 and N88, all of which probably haveclosely associated dust. Low resolution Hi observations do not detectthese high concentrations of dust.
| 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.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| The initial mass function for massive stars in the Magellanic Clouds. 2: Interstellar reddening toward 14 OB associations
We have used UBV CCD photometry to determine the interstellar reddeningtoward 14 OB associations in the Magellanic Clouds. The tworeddening-free indices available in the UBV system were used to obtainthe reddening estimates. The mean color excesses of the associationsrange from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC)and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). Wehave modeled the observed scatter in the color excesses of individualstars within the associations and find that statisically significantdifferential reddening exists in at least some of the associations.
| The initial mass function for massive stars in the Magellanic Clouds. 1: UBV photometry and color-magnitude diagrams for 14 OB associations
UBV charge coupled device (CCD) photometry has been obtained for 14 OBassociations in the Magellanic Clouds using the University of Toronto's0.6 m telescope and the Carnegie Institution of Washington's 1.0 mreflector, both on Las Campanas, Chile. The data are presented and usedto construct color-magnitude diagrams for the purposes of investigatingthe massive-star content of the associations.
| 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.
| Two new catalogues of Small Magellanic Cloud members coming soon.
Not Available
| 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.
| Spectral types of stars in the SMC associations NGC456, NGC460a, B and NGC465
Spectral types of stars for an extended area (1200 sq arcmin) of theSMC, containing the associations NGC456, NGC460 a, b and NGC465 havebeen derived here. High-quality UJ and YJ film copies of objective prismplates taken with the 1.2-m UK Schmidt telescope were used. The mediumdispersion spectral (830 A/mm) were digitized with a PDSmicrodensitometer and spectra types of stars were obtained down to about16.5 mag. A computer interactive method of reduction has been developed,and the spectral types obtained are accurate to 0.3 spectral subtypes.Spectral classification for the low (2440 A/mm) dispersion spectra downto 18.5 mag was carried out visually using a binocular microscope. Thefrequency distributions of stars in the associations region, aftersubtracting the field stars, have revealed that only blue main sequencestars are their members.
| The Magellanic Clouds - Their evolution, structure and composition
Recent data related to the history of the Magellanic Clouds as galaxiesare described, and attempts to determine accurate distances to theMagellanic Clouds are discussed, with special attention given to thegeometry of the Magellanic Clouds and different methods of distancedeterminations. Consideration is given to the various star generationspresent in the Clouds (i.e., the oldest generation, of greater than 10Gyr; the intermediate-age generations, between 7 and 0.2 Gyr, and theyoungest generation, the formation of which started only about 50 Myrago) and to their occurrences in the LMC and SMC populations, as well asto the interstellar medium in the Magellanic Clouds. The structure ofthe Magellanic System, which comprises the Magellanic Clouds, theIntercloud Region, and the Magellanic Stream is described, withparticualr consideration given to the complex structure of the LMC andSMC and the kinematics of their populations.
| 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.
| The Small Magellanic Cloud. I - A study of the structure revealed by the supergiants
The structure of the Small Magellanic Cloud (SMC) is studied on thebasis of the distribution of 465 supergiants belonging to that galaxy.The distribution of the supergiants according to spectral type is shownto be 64 per cent O9-B2, 11 per cent B3-B6, 20 per cent B7-A9, and 5 percent F-G. It is found that these stars are concentrated in fiveaccumulation zones located in the southwest part of the bar, at thenorthwest border of the bar, in the northeast part of the bar, and (twozones) in the wing. The location of emission-line stars, red and blueclusters, and H II regions relative to the supergiant accumulation zonesis investigated, and a correlation between O-B2 supergiants and neutralhydrogen is determined which indicates a star formation rate similar tothat for other galaxies. Analysis of interstellar reddening shows thatthe mean absorption in the SMC is low and that the gas/dust ratio is 15times higher than in the Galaxy.
| Equivalent widths of Hγ in stellar spectra of the Magellanic Clouds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966MNRAS.132..433H&db_key=AST
| A new catalogue of emission-line stars and planetary nebulae in the Small Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1961AJ.....66..169L
| The distribution of stars in an outlying part of the Small Magellanic Cloud.
Not Available
| The distribution of stars in an outlying part of Small Magellanic Cloud.
Not Available
| 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
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