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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.
| 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.
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| 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.
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| 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.
| Infrared observations of the Magellanic Clouds. I - The Small Magellanic Cloud
Results of IRAS pointed observations in four infrared wavelength bands(12, 25, 60, and 100 microns) for the SMC are presented. Maps withorthogonal scan directions are shown, and a source list containing 219infrared sources is extracted from the data. Comparison with the IRASPoint Source Catalog (PSC) shows that only three entries in this catalogare spurious. All 13 entries in the IRAS Small Scale Structure Catalog(SSS) in the SMC are confirmed. Seventy-two new infrared sources, notincluded in either the PSC or in the SSS are found. The present SMCinfrared source list is compared to other object lists. Two blueglobular clusters, 28 SAO stars, and seven planetary nebulae areidentified. No SMC stars are found. In general there is a goodcorrelation of infrared emission with the distributiion of H II regionsand dark clouds.
| IRAS Point Source Catalogue cross-identifications
Not Available
| Sequential star formation in the Magellanic Clouds
Joint stellar and nebular studies were carried out for a few adjacentstellar associations in the Magellanic Clouds. The spread in ages of WRstars of different subtypes as deduced from their different environmentsis analyzed. A similar study is briefly sketched for S Dor type stars.It is shown how a starburst propagates in the interstellar medium at ascale of 100-300 pc in about 10 to the 7th yr for three regions of theMagellanic Clouds, using both new observational data and published ones.In view of the results provided by such detailed studies, several trapsin the methods were pointed out based on integrated properties of starclusters and their associated nebulae, or their brightest stars.Finally, the moderately old age found (from their environment) for S Dortype stars and their tendency to enter into multiple systems are pointedout.
| High degree of fragmentation in the nebulae SMC - N83 and N84 and discovery of two O stars
The discovery of unevolved O stars in the youngest parts of the complexN83-84-85, i.e., N83-84, in the SMC is reported. The observationalprocedures are summarized, and the nebular and stellar characteristicsof N83-84 are described. The individual nebulae of ionized gas and theextended ones are discussed, the results of a new BV photometry in N83are presented, and the search for O and WR stars is described. The highdegree of fragmentation of the ionized gas and the method of discoveryof hot stars are discussed.
| 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.
| 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.
| A survey of chemical compositions of H II regions in the Magellanic Clouds
The reported investigation had the objective to extend abundancedeterminations to a larger number of H II regions in the Small Cloud(and a few more in the Large Cloud) in connection with a study regardingthe possible occurrence of large-scale abundance gradients analogous tothose found in Sc galaxies and in the Galaxy, taking into account alsoquestions concerning the existence of a characteristic abundance patternrepresenting the entire young population of either cloud. It is foundthat the laws governing enrichment of the interstellar medium are verysimilar in the Magellanic Clouds to what they are in the outer parts ofSc galaxies, including the very massive system M101. There is no reasonto believe that any special process such as preferential escape of gashas operated to reduce the effective yield in the Magellanic Clouds.
| 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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