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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.
|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 Discovery of a 12th Wolf-Rayet Star in the Small Magellanic Cloud|
We report the discovery of a relatively faint (V=15.5) early-type WNstar in the SMC. The line strength and width of He II λ4686emission are similar to those of the other SMC WN stars, and thepresence of N V λλ4603, 4619 emission (coupled with thelack of N III) suggests that this star is of spectral type WN3-4.5, andthus is similar in type to the other SMC WR stars. Also like the otherSMC WN stars, an early-type absorption spectrum is weakly present. Theabsolute magnitude is comparable to that of other (single) Galacticearly-type WN stars. The star is located in the Hodge 53 OB association,which is also the home of two other SMC WN stars. This star, which wedesignate SMC-WR12, was actually detected at a high significance levelin an earlier interference-filter survey, but the wrong star wasobserved as part of a spectroscopic follow-up, and this case of mistakenidentity resulted in its Wolf-Rayet nature not being recognized untilnow.
|Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar|
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.
|The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds|
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
|H-alpha emission-line star survey in two young Small Magellanic Cloud star clusters.|
|A radio continuum study of the Magellanic Clouds V. Catalogues of radio sources in the Small Magellanic Cloud at 1.42, 2.45, 4.75, 4.85 and 8.55 GHz|
We present catalogues of radio sources in the Small Magellanic Cloudfrom observations with the Parkes radio telescope at 1.42, 2.45, 4.75and 8.55 GHz, and an additional catalogue from the Parkes-MIT-NRAOsurvey at 4.85 GHz. A total of 224 sources were detected at at least oneof these frequencies, 60 of which are reported here for the first timeas radio sources. We compare positions and flux densities of thesesources with previously published results and find no significantpositional displacement or flux discrepancies. Tables 2-7 are onlyavailable electronically at the CDS via ftp 18.104.22.168 or athttp://cdsweb.u-strasbg.fr/Abstract.html
|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.
|On the occurrence of enhanced lithium in Magellanic Cloud red giants|
High-resolution spectra have been obtained for 27 red giants in theMagellanic Clouds with Mbol from about -5 to -9. A previous study (Smithand Lambert, 1989) found that five out of five luminous AGB stars in theSMC showed enhanced Li I resonance doublets (6707-A); here it is shownthat the Li-strong red giants in both Clouds are luminous (Mbol = -6 to-7) stars on the AGB. Lower luminosity AGB stars show no Li I features,while the very luminous red giants that are identified as massivecore-burning supergiants also show no Li I features. The enhanced Liabundances in these luminous AGB stars may be a byproduct of envelopeburning in the more massive AGB stars.
|A southern redshift survey. I - Accurate redshifts for 500 galaxies observed at SAAO|
Radial velocities are presented for about 500 galaxies observed with anintensified Reticon photon-counting detector attached to the 1.9 mtelescope at the Sutherland field station of SAAO. The velocities weredetermined by cross correlation with standard velocity templates, fromthe measured wavelengths of emission lines, or from both techniques,with spectra covering the wavelength range from 3700 to 5400 A. Thefinal velocities appear to have the same zero point as H I radiovelocities, and have standard deviations of the order of 35 km/s, thoughcross-correlation velocities on their own may have significantly largererrors.
|Population-I Pulsating Stars. VI - Ages of Star Clusters and Associations|
On the basis of our age estimations of Population I pulsating stars inour Galaxy (Tsvetkov, 1986a), the mean ages of 6 open star clusterscontaining 21 Delta Scuti-variables and of 8 star clusters andassociations containing 13 classical cepheids, have been evaluated.These mean cluster age estimations weighted according to theprobabilities for different evolutionary phases of the pulsating stars,are obtained in the evolutionary track systems of Iben (1967) andPaczyÃ±ski (1970); the cluster ages are larger in theformer system. Our results are compared with those obtained from variousmethods by other authors. Clusters with classical cepheids and withDelta Scuti-stars have ages, respectively, in the ranges 107_108 yearsand 106_109 years. It is shown that the use of simpleperiod-age(-colour) relations for Population I pulsating stars givessufficiently accurate cluster age estimations. By use of our period-agerelations for classical cepheids (Tsvetkov, 1986a), the mean ages of 56other star clusters and associations in our Galaxy, the MagellanicClouds, and M 31 galaxy have been estimated in both systems of tracks.The results are generally in agreement with those obtained from variousmethods by other authors. The use of Population I pulsating stars instar clusters and associations is one of the simplest and most easilyapplied methods for determining cluster ages; but there are somelimitations in its application
|The long-period variables of Fornax. I - Search, discovery and periods|
Twenty-one visual UKST plates, measured by the APM, were used toidentify 30 long-period variables in Fornax. Periods are determined for26 of them. No large-amplitude Mira variable has been detected; a groupof low-amplitude variables is observed. These Population II variableshave essentially the same absolute visual magnitude as the LPVs ingalactic globular clusters and the LPVs near NGC 371 in the SMC. Theirperiod and amplitude distributions are, however, different. The visualphotometry is not extensive enough to reveal more of the physicalproperties of these variables, but from their color it can be statedthat nearly a quarter of the variables appear to be carbon stars.
|The brightest stars in nearby galaxies. VII - The Pegasus and Leo A Im V dwarfs|
The present photometry for the most brightly resolved stars in thePegasus and Leo A dwarf galaxies yields color-magnitude diagrams forboth systems that are similar to those of other dwarf galaxies. Each hasa well populated sequence of blue stars, a Hertzsprung gap, and many redsupergiants that reach approximately the same V magnitude as thebrightest blue stars. Variables have been found in both galaxies, buthave not been studied due to insufficient data. The data suggest theuncertain modulus values of m-M = 27 + or - 1 for Pegasus and m-M = 26 +or - 1 for Leo A, thereby giving absolute blue magnitudes of -14.4 and-12.9, respectively.
|Age determination of extragalactic H II regions|
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
|Bright red variables of large amplitude in the Magellanic Clouds|
Periods and epochs of maximum light are given for 17 red variables inthe Magellanic Clouds. Spectroscopic observations of six SMC variablesof large amplitude confirm membership and show that a wide range ofcarbon and heavy element enrichment occurs: in particular two stars areof type CS. The high luminosity of these stars and the absence of starswith a similar moderate degree of carbon enrichment from theintermediate-age clusters is explicable in terms of stellar evolutiontheory. A new discussion of the Harvard data for bright red variables inboth Magellanic Clouds confirms the presence of two groups, of high andintermediate mass, and indicates that stars in the latter group have Mless than or equal to approximately 6 solar masses. The infrared colorsshow the same dependence on amplitude and on chemical composition as dothose of red giant variables in the Galaxy. The large amplitudevariables of high luminosity follow a linear relation in the logP-M(bol)diagram, with a slope steeper than that found for ordinary Miravariables in the LMC.
|M and S stars in the Magellanic Clouds|
The present consideration of digital spectra for 46 red stars in theSmall Magellanic Cloud (SMC), as well as in globular clusters of bothMagellanic Clouds, has yielded identifications of eight K stars, 18 Mstars, 19 early S stars, and a foreground dwarf. K, M, or S types arefound in the SMC among stars with B-V values of about 2, and most of thenoncarbon stars brighter than M(bol) of -4.3 in the clusters are foundto be S stars which evidently represent an intermediate stage in themodification of atmospheric composition. Tentative systematic trendswith cluster age indicate that the M-S and S-C transitions occur athigher luminosity and lower surface temperature in the younger, moremassive stars.
|Long-period variables in the Magellanic Clouds - Supergiants, AGB stars, supernova precursors, planetary nebula precursors, and enrichment of the interstellar medium|
IR JHK magnitudes and low dispersion red spectra for 90 long periodvariables (LPVs) in the Magellanic Clouds show the LPVs to fall intodistinct groups of core He or C burning supergiants and asymptotic giantbranch (AGB) stars. The existence of stars at the AGB limit providesdirect evidence that the more massive AGB stars produce supernovae.Carbon star LPVs are confined to luminosities fainter than about -6, inagreement with other Magellanic cloud carbon star studies. The absenceof carbon stars among the upper AGB stars is interpreted as due to CNOcycling of dredged-up C to N during quiescent AGB evolution. The AGBstars are therefore sources of primary N, as well as of C and s-processelements. It is deduced that stars with initial masses greater thanabout 5 solar masses produce supernovae, while less massive starsproduce planetary nebulae with nebula masses from 0.1 to 2.1 solarmasses.
|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.
|Spectra of red supergiant variables in the SMC|
Spectra of eight bright red variables in the SMC show that five are Mstars, one shows no strong molecular bands, and two are carbon stars.The latter are variables of large amplitude, and it appears that thereare both M stars and carbon stars among those SMC stars which appear tobe supergiant analogues of Mira variables. Current theory predicts theweak C2 bands of these luminous carbon stars, but also requires strongC-13 features which are not observed.
|Spectral classification of carbon stars in Magellanic Cloud clusters|
Image tube spectra of 33 stars in 17 Magellanic Cloud clusters and sixstars in the field of the SMC are discussed. Nineteen of the 21 carbonstars have been classified on Yamashita's (1972) system. None of thesestars, which have M(v) not less than -3, has strong C-13 features. Theweakness of CN relative to C2 suggests an affinity to CH stars in theGalaxy, in accordance with the idea that intermediate age clusters inthe Magellanic Clouds are metal-deficient. Stars with strong H-alphaemission are usually variables of large amplitude, and a period of 115day is found for a new red variable in NGC121 which has bright H-alpha.
|Preliminary colour-magnitude diagrams of 20 star clusters and their adjoining fields in the small Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1980A&AS...40..151K
|Red variable stars in the Magellanic Clouds. III - Carbon stars in the field of NGC 419 /SMC/|
Photographic photometry in the BVI(K) system was obtained on red starsin a field centered on NGC 419 in the SMC. The location of the stars inthe color diagram and the very red B-V color of the red giant tip of thegiant branch suggest that these stars are carbon stars. The meanabsolute visual magnitude of the stars reported is -2.1. The stars inthis field and also the field around NGC 371 having V greater than about17 and very red B-V appear comparable with N-type variables of smallamplitude belonging to the old disk population in the Galaxy. Starscomparable with the young disk group of Eggen (1972) are found atbrighter magnitudes.
|Red variable stars in the Magellanic Clouds. II - The field of NGC 371 in the SMC|
Abstract image available at:http://adsabs.harvard.edu/abs/1978MNRAS.183..305L
|Photometry of cepheid variables in the Small Magellanic Cloud.|
Abstract image available at:http://adsabs.harvard.edu/abs/1976A&AS...24..299B
|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
|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
|Star Clusters in the Small Magellanic Cloud: I. Identification of 69 Clusters|
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