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|The IMF of the field population of 30 Doradus|
The star-formation history and IMF of the field population of the 30Doradus super-association is determined using Wide Field Imagerphotometry. The cluster NGC 2070 and the OB association LH104 are alsostudied and used for comparison. The star-formation history of the 30Doradus super-association appears to be characterized by a largeincrease in star-formation activity 10 Myr to 20 Myr ago. This seems tobe the case across the whole eastern half of the LMC as demonstrated bythe ages of stellar populations as far away as 30 Doradus and Shapley'sConstellation III. Star-formation appears to be occurring at a constantrate in the field and in loose associations, and in bursts in theclusters. The field IMF is found to have almost the exact Salpeter slopein the range 7 ~M_ȯ ≤ M ≤ 40 ~M_ȯ, at odds with previousclaims. We find that, for objects with more complex star-formationhistories, Be stars and selective incompleteness strongly affect thedetermination of the IMF for M > 40~ M_ȯ, naturally explainingthe observed deviation of the high mass IMF slope from the Salpetervalue. The present work supports the idea of a universal IMF.Based on observations obtained with the MPG/ESO 2.2-m telescope at LaSilla Observatory.Tables 1-3 are only available in electronic form athttp://www.edpsciences.org.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/443/851
|Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters|
This work continues our efforts to calibrate model surface brightnessfluctuation luminosities for the study of unresolved stellarpopulations, through a comparison with the data of Magellanic Cloud starclusters. We present here the relation between absoluteKs-band fluctuation magnitude and (V-I) integrated colour,using data from the Two-Micron All-Sky Survey (2MASS) and the DeepNear-Infrared Southern Sky Survey (DENIS), and from the literature. Wecompare the star cluster sample with the sample of early-type galaxiesand spiral bulges studied by Liu et al. We find that intermediate-age toold star clusters lie along a linear correlation with the same slope,within the errors, of that defined by the galaxies in the versus (V-I)diagram. While the calibration by Liu et al. was determined in thecolour range 1.05 < (V-IC)0 < 1.25, oursholds in the interval . This implies, according to Bruzual-Charlot andMouhcine-Lançon models, that the star clusters and the lateststar formation bursts in the galaxies and bulges constitute an agesequence. At the same time, a slight offset between the galaxies and thestar clusters [the latter are ~0.7 mag fainter than the former at agiven value of (V-I)], caused by the difference in metallicity ofroughly a factor of 2, confirms that the versus (V-I) plane maycontribute to break the age-metallicity degeneracy in intermediate-ageand old stellar populations. The confrontation between models and galaxydata also suggests that galaxies with Ks fluctuationmagnitudes that are brighter than predicted, given their (V-I) colour,might be explained in part by longer lifetimes of thermally pulsingasymptotic giant branch stars. A preliminary comparison between the H2MASS data of the Magellanic star clusters and the sample of 47early-type galaxies and spiral bulges observed by Jensen et al. throughthe F160WHubble Space Telescope filter leads to the same basicconclusions: galaxies and star clusters lie along correlations with thesame slope, and there is a slight offset between the star cluster sampleand the galaxies, caused by their different metallicities. Magellanicstar clusters are single populations, while galaxies are compositestellar systems; moreover, the objects analysed live in differentenvironments. Therefore, our findings mean that the relationship betweenfluctuation magnitudes in the near-infrared, and (V-I) might be a fairlyrobust tool for the study of stellar population ages and metallicities,could provide additional constraints on star formation histories, andaid in the calibration of near-infrared surface brightness fluctuationsfor cosmological distance measurements.
|The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample|
In order to determine the physical properties of the hottest and mostluminous stars and understand how these properties change as a functionof metallicity, we have analyzed HST/UV and high-S/N optical spectra ofan additional 20 Magellanic Cloud stars, doubling the sample presentedin the first paper in this series. Our analysis uses non-LTEline-blanketed models that include spherical extension and thehydrodynamics of the stellar wind. In addition, our data set includesFUSE observations of O VI and HST near-UV He I and He II lines to testfor consistency of our derived stellar properties for a few stars. Theresults from the complete sample are as follows: (1) We present aneffective temperature scale for O stars as a function of metallicity. Wefind that the SMC O3-7 dwarfs are 4000 K hotter than Galactic stars ofthe same spectral type. The difference is in the sense expected due tothe decreased significance of line blanketing and wind blanketing at thelower metallicities that characterize the SMC. The temperaturedifference between the SMC and Milky Way O dwarfs decreases withdecreasing temperature, becoming negligible by spectral type B0, inaccord with the decreased effects of stellar winds at lower temperaturesand luminosities. The temperatures of the LMC stars appear to beintermediate between that of the Milky Way and SMC, as expected based ontheir metallicities. Supergiants show a similar effect but are roughly3000-4000 K cooler than dwarfs for early O stars, also with a negligibledifference by B0. The giants appear to have the same effectivetemperature scale as dwarfs, consistent with there being littledifference in the surface gravities. When we compare our scale to otherrecent modeling efforts, we find good agreement with some CMFGENresults, while other CMFGEN studies are discordant, although there arefew individual stars in common. WM-BASIC modeling by others has resultedin significantly cooler effective temperatures than what we find, asdoes the recent TLUSTY/CMFGEN study of stars in the NGC 346 cluster, butour results lead to a far more coeval placement of stars in the H-Rdiagram for this cluster. (2) We find that the wind momentum of thesestars scales with luminosity and metallicity in the ways predicted byradiatively driven wind theory, supporting the use of photosphericanalyses of hot luminous stars as a distance indicator for galaxies withresolved massive star populations. (3) A comparison of the spectroscopicmasses with those derived from stellar evolutionary theory showsrelatively good agreement for stars with effective temperatures below45,000 K; however, stars with higher temperatures all show a significantmass discrepancy, with the spectroscopic masses a factor of 2 or moresmaller than the evolutionary masses. This problem may in part be due tounrecognized binaries in our sample, but the result suggests a possiblesystematic problem with the surface gravities or stellar radii derivedfrom our models. (4) Our sample contains a large number of stars of theearliest O types, including those of the newly proposed O2 subtype. Weprovide the first quantitative descriptions of their defining spectralcharacteristics and investigate whether the new types are a legitimateextension of the effective temperature sequence. We find that the NIII/N IV emission line ratio used to define the new classes does not, byitself, serve as an effective temperature indicator within a givenluminosity class: there are O3.5 V stars that are as hot or hotter thanO2 V stars. However, the He I/He II ratio does not fair much better forstars this hot, as we find that He I λ4471/He II λ4542,usually taken primarily as a temperature indicator, becomes sensitive toboth the mass-loss rate and surface gravities for the hottest stars.This emphasizes the need to rely on all of the spectroscopic diagnosticlines, and not simply N III/N IV or even He I/He II, for these extremeobjects. (5) The two stars with the most discordant radial velocities inour sample happen to be O3 ``field stars,'' i.e., found far from thenearest OB associations. This provides the first compellingobservational evidence as to the origin of the field O stars in theMagellanic Clouds, i.e., that these are classic runaway OB stars,ejected from their birthplaces.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withprograms 6417, 7739, and 9412.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, operated for NASA by John Hopkins Universityunder NASA contract NAS5-32985. These observations are associated withprogram C002.
|Near-Infrared Imaging Observations of the N159/N160 Complex in the Large Magellanic Cloud: Large Clusters of Herbig Ae/Be Stars and Sequential Cluster Formation|
We have carried out deep near-infrared imaging observations of theN159/N160 star-forming region in the Large Magellanic Cloud. We observedan area of ~380 arcmin2 (~80,000 pc2 at thedistance of the LMC) in the J, H, and Ks bands. Theobservations are deep enough to detect Herbig Ae/Be stars down to ~3Msolar in the LMC. We discovered a total of 338 and 464candidate Herbig Ae/Be and OB stars, respectively, based on thenear-infrared colors and magnitudes. The Herbig Ae/Be candidatescomprise 10 clusters, the OB star candidates 13. We discovered anembedded Herbig Ae/Be cluster in the N159 East giant molecular cloud(GMC) and a Herbig Ae/Be cluster at the northeast tip of the N159 SouthGMC. Together with two neighboring H II regions, the Herbig Ae/Becluster at the tip of the N159S GMC provides a hint of the beginning ofsequential cluster formation in N159S. The spatial distributions of theHerbig Ae/Be and OB clusters, in conjunction with previously knownoptical clusters and embedded massive stars, indicate (1) sequentialcluster formation within each of the N159 and N160 star-forming regionsand (2) large-scale sequential cluster formation over the entireobserved region from N160 to N159S. Possible triggers for thelarge-scale cluster formation are the supergiant shell SGS 19 and anexpanding superbubble. Some of the Herbig Ae/Be clusters in theN159/N160 complex are significantly larger in spatial extent thanpre-main-sequence clusters of similar age in the Milky Way. Highlyturbulent gas motion in the LMC is probably responsible for forming thelarge young clusters.
|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 Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds|
We have obtained Hubble Space Telescope (HST) and ground-basedobservations of a sample of 20 O-type stars in the LMC and SMC,including six of the hottest massive stars known (subtypes O2-O3) in theR136 cluster. In general, these data include (1) the HST UV spectra inorder to measure the terminal velocities of the stellar winds, (2) highsignal-to-noise, blue-optical data where the primary temperature- andgravity-sensitive photospheric lines are found, and (3) nebular-freeHα profiles, which provide the mass-loss rates. We find that theolder (Faint Object Spectrograph) HST data of the R136 stars (which wereobtained without the benefits of sky measurements) suffered fromsignificant nebular emission, which would increase the derived mass-lossrates by factors of ~3, all other factors being equal. We also findseveral stars in the SMC for which the N III λλ4634, 4642and He II λ4686 emission ``f'' characteristics do not appear tofollow the same pattern as in Galactic stars. Since He II emission isdue to the stellar wind (which will be weaker in SMC for stars of thesame luminosity), while N III emission is a complex non-LTE (NLTE)effect affected mostly by temperature, it would not be surprising tofind that these features do not correlate with each other or withluminosity in SMC stars in the same was as they do in Galactic stars,but theory does not provide a clean answer, and analysis of more stars(both SMC and Galactic) is needed to resolve this issue. Theline-blanketed NLTE atmosphere code FASTWIND was then used to determinethe physical parameters of this sample of stars. We find good agreementbetween the synthetic line profiles for the hydrogen, He I, and He IIlines in the majority of the stars we analyzed; the three exceptionsshow evidence of being incipiently resolved spectroscopic binaries orotherwise spectral composites. One such system is apparently an O3 V+O3V eclipsing binary, and a follow-up radial velocity study is planned toobtain Keplerian masses. Although we did not use them to constrain thefits, good agreement is also found for the He I λ3187 and He IIλ3203 lines in the near-UV, which we plan to exploit in futurestudies. Our effective temperatures are compared with those recentlyobtained by Repolust, Puls & Herrero for a sample of Galactic starsusing the same techniques. We find that the Magellanic Cloud sample is3000-4000 K hotter than their Galactic counterparts for the earlythrough mid-O typess. These higher temperatures are the consequence of adecreased importance of wind emission, wind blanketing, and metal-lineblanketing at lower metallicities.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555. These observations are associatedwith programs 6417, 7739, 8633, and 9412. This paper also draws heavilyfrom data obtained from the data archive at STScI.
|Near-infrared color evolution of LMC clusters|
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
|Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC|
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
|The relation between radio flux density and ionising ultra-violet flux for HII regions and supernova remnants in the Large Magellanic Cloud|
We present a comparison between the Parkes radio surveys (Filipovic etal. 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) ofthe Large Magellanic Clouds (LMC). We have found 72 sources in common inthe LMC which are known HII regions (52) and supernova remnants (SNRs)(19). Some of these radio sources are associated with two or more UVstellar associations. A comparison of the radio flux densities andionising UV flux for HII regions shows a very good correlation, asexpected from theory. Many of the Magellanic Clouds (MCs) SNRs areembedded in HII regions, so there is also a relation between radio andUV which we attribute to the surrounding HII regions.
|The present-day chemical composition of the LMC|
High-resolution observations of five OB-type main-sequence stars in theLarge Magellanic Cloud (LMC) have been obtained with the UCLéchelle spectrograph on the 3.9-m Anglo-Australian Telescope.These spectra have been analysed using LTE model-atmosphere techniques,to derive stellar atmospheric parameters and chemical compositions. Asthese stars are located within the hydrogen burning main-sequence band,their surface abundances should reflect those of the present-dayinterstellar medium. Detailed line-by-line differential analyses havebeen undertaken relative to Galactic comparison stars. We conclude thatthere exists a general metal deficiency of -0.31+/-0.04 dex within theLMC, and find no significant abundance variations between cluster andfield stars. There is also tentative evidence to suggest a lower oxygento iron abundance ratio, and an over-deficiency of magnesium relative tothe other alpha -elements. These are discussed in terms of previousabundance analyses and models of discontinuous (or bursting) starformation within the LMC. Finally, there is some evidence to suggest agreater chemical enrichment of material within the H Ii region LH 104.
|A New Spectral Classification System for the Earliest O Stars: Definition of Type O2|
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
|Arc-Shaped and Spheroidal Stellar Complexes|
Complexes of young clusters and high-luminosity stars in the shape ofregular, circular arcs have been found in a number of galaxies, firstand foremost the LMC, NGC 6946, and M83. These shapes are found even instrongly inclined galaxies, suggesting that the observed arcs areprojections of partial spherical shells. Obviously, these stellar shellsmust have formed from gaseous shells swept up by some source of centralpressure and become gravitationally unstable. The power of this sourcecorresponds to several dozen supernova explosions; however, its natureremains unclear. A central cluster providing a source of O stars andsupernovae is usually absent. The presence of multiple arcs locatedclose to each other can be explained by the fall of a swarm of fragmentsor by the progenitor stars originating in a single peculiar starcluster, implying the existence of stellar objects capable of givingrise to explosions with energies an order of magnitude higher than thoseof individual supernovae. The same objects may be responsible forgamma-ray bursts. It may be that only the most massive clusters withfrequent or especially powerful supernova explosions are capable ofproducing HI supershells. Otherwise, it is impossible to explain why nosupershells have been found around numerous clusters that should becapable of producing them according to current theories. The presence ofstar clusters in shell-like structures provides extremely importantinformation about the physical conditions in and the ages of the initialgaseous shells, making stellar arcs the best available laboratory forstudies of triggered star formation.
|X-Rays from Superbubbles in the Large Magellanic Cloud. VI. A Sample of Thirteen Superbubbles|
We present ROSAT observations and analysis of thirteen superbubbles inthe Large Magellanic Cloud. Eleven of these observations have not beenpreviously reported. We have studied the X-ray morphology of thesuperbubbles and have extracted and analyzed their X-ray spectra.Diffuse X-ray emission is detected from each of these superbubbles, andX-ray emission is brighter than that theoretically expected for awind-blown bubble, suggesting that the X-ray emission from thesuperbubbles has been enhanced by interactions between the superbubbleshell and interior supernova remnants. We have also found significantpositive correlations between the X-ray luminosity of a superbubble andits Hα luminosity, expansion velocity, and OB star count. Further,we have found that a large fraction of the superbubbles in the sampleshow evidence of breakout regions, where hot X-ray-emitting gas extendsbeyond the Hα shell.
|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.
|Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies|
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
|Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models|
Motivated by new sounding-rocket wide-field polarimetric images of theLarge Magellanic Cloud (reported simultaneously by Cole et al.), we haveused a three-dimensional Monte Carlo radiation transfer code toinvestigate the escape of near-ultraviolet photons from young stellarassociations embedded within a disk of dusty material (i.e., a galaxy).As photons propagate through the disk, they may be scattered or absorbedby dust. Scattered photons are polarized and tracked until they escapethe dust layer, allowing them to be observed; absorbed photons heat thedust, which radiates isotropically in the far-infrared where the galaxyis optically thin. The code produces four output images: near-UV andfar-IR flux, and near-UV images in the linear Stokes parameters Q and U.From these images we construct simulated UV polarization maps of theLMC. We use these maps to place constraints on the star+dust geometry ofthe LMC and the optical properties of its dust grains. By tuning themodel input parameters to produce maps that match the observedpolarization maps, we derive information about the inclination of theLMC disk to the plane of the sky and about the scattering phase functiong. We compute a grid of models with i=28 deg, 36 deg, and 45 deg, andg=0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces theobserved polarization maps has i=36 deg+2-5 andg~0.7. Because of the low signal-to-noise in the data, we cannot placefirm constraints on the value of g. The highly inclined models do notmatch the observed centrosymmetric polarization patterns around brightOB associations or the distribution of polarization values. Our modelsapproximately reproduce the observed ultraviolet photopolarimetry of thewestern side of the LMC; however, the output images depend on many inputparameters and are nonunique. We discuss some of the limitations of themodels and outline future steps to be taken; our models make somepredictions regarding the polarization properties of diffuse lightacross the rest of the LMC.
|The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure|
LMC 2 has the brightest, most coherent filamentary structure of allknown supergiant shells in the Large Magellanic Cloud. The opticalemission-line images show active star formation regions along thewestern edge and long filaments to the east. ROSAT PSPC and HRI imagesshow bright X-ray emission from within the shell boundary, indicatingthe presence of hot gas. Counterintuitively, neither high-resolutionechelle spectra in the Hα line nor aperture synthesis H I 21 cmemission-line observations show LMC 2 to have the kinematics expected ofan expanding shell. Rather, LMC 2 appears to consist of hot gas confinedbetween H I sheets. The interior surfaces of these sheets are ionized bythe UV flux of massive stars in the star formation regions along theperiphery of LMC 2, while the heating is provided by outflows of hot gasfrom the star formation regions and by SNRs interior to LMC 2. We havecompared LMC 2 to other supergiant shells in the LMC and in more distantgalaxies. When the spatial resolution of our data are degraded, we findthat LMC 2 resembles supergiant shells observed at a distance of 4 Mpcthat have previously been interpreted as expanding shells. Therefore,great caution should be exercised in the analysis and interpretation ofthe kinematics of distant supergiant shells to prevent overestimates oftheir velocities and total kinetic energies.
|Induced star formation and the origin of gamma-ray bursts|
New arguments are presented in favor of the hypothesis that three giantstellar arcs and the LMC4 supershell in the Large Magellanic Cloud (LMC)are relicts of gamma-ray bursts (GRBs), whose progenitors escaped fromthe nearby old, rich star cluster NGC 1978. This may also be true of thebinary X-ray sources concentrated in this same area and the object SNRN49-SGR 0526-66, located 18' from the cluster. All these objects may begenetically related to each other and to objects such as SN 1998bw-GRB980425. The occurrence of five GRBs in the LMC over the last ~50 Myr isquite plausible. The rate of GRBs may be high, and they may be triggersfor the formation of many HI supershells and regions of active starformation.
|The fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud|
The catalogue provides for each of the 134 W-R stars of Population Ipresently known in the Large Magellanic Cloud, accurate equatorialcoordinates, photometric data, spectral classification, binary status,correlation with OB associations and HII regions. The miscellaneousdesignations of the stars are also listed. Although completeness is notpretended, results published during the last decade are highlighted inthe notes given for each individual star. A uniform set of findingcharts is presented. Figures 2 to 12 only in the electronic version athttp://edpsciences.com
|A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud|
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
|The OB associations LH 101 and LH 104 in the HII region N158 of the LMC|
We present photometric and spectroscopic observations of stars in theLarge Magellanic Cloud OB associations LH 101 and LH 104, located in theHII region N158, which we have also imaged. From our observations wehave constructed upper H-R diagrams for these OB associations, which wefind to consist mainly of three populations, one of 2-6 Myr for thestars inside the northern bubble (LH 104), and two populations in thesouthern HII region (LH 101), one of <= 2 Myr and the other one aged3-6 Myr. We have obtained for LH 101 a normal IMF, with a slope of Gamma= -1.29 +/- 0.20 whereas for LH 104 the IMF is flatter with a slope ofGamma = -1.05 +/- 0.12. These IMF slopes are consistent with that ofother OB associations in the LMC. Our observations reveal in the regionof LH 101 the presence of both unevolved and evolved very massive stars,whose ionizing flux is in excess of that derived from our Hβ imagesof the HII region. The north-west nebulosity in the region of LH 101thus appears to be matter bound. Based on observations obtained at ESO,La Silla, Chile.
|Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud|
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
|Ultraviolet spectral evolution of star clusters in the IUE library.|
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
|A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.|
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
|Wolf-Rayet and OF stars in the OB associations LH1O1 and LH1O4 in the LMC|
|Kinematic structure of the 30 Doradus giant H II region|
We have used the echelle CCD spectrograph on the Cerro TololoInter-American Observatory (CTIO) 4 m telescope to map the nebularvelocity field in the 30 Doradus giant H II region. The kinematics of 30Dor are very complex. The outer regions are charaterized by a smoothvelocity field, but its turbulent velocity, 30-40 km/s Full Width HalfMaximum (FWHM), is considerably higher than those in most smaller H IIregions. In the central 9 min core, multiple velocity components areobserved at most positions. The velocity field is dominated by a largenumber of expanding structures, ranging in size from 1 to 100 pc andexpansion velocities of 20-200 km/s, and often organized into largehierarchical networks. The integral of these complex expandingstructures in 30 Dor produces a surprisingly simple profile with a broadGaussian core and faint extended wings. Several fast-expanding shells,with diameters of 2-20 pc, expansion velocities of 100-300 km/s, andkinetic energies of 0.5-10 x 1050 ergs have been identified.The large fast-expanding shells and networks are coincident withextended X-ray sources and are probably associated with supernovaremnants embedded in supershells produced by the combined effects ofstellar winds and supernovae from OB associations. We have used theintensity-calibrated echelle spectra to determine the basic physical anddynamical properties of the kinematic features in 30 Dor. The expandingshells contain roughly half of the kinetic energy in the 30 Dor complex,and this energy is several times higher than the gravitational bindingenergy of the region. The energetic requirements of the gas areconsistent with the observed stellar content of 30 Dor, if the gas isaccelerated by a combination of stellar winds and supernovae.Extrapolating the current energy injection rate in the nebula over thelifetime of the OB complex suggest that 30 Dor and its vicinity willevolve into a supergiant shell as seen in the LMC and other nearbygalaxies.
|Imaging and spectroscopy of ionized shells and supershells in the Large Magellanic Cloud|
Deep H-alpha images of the Large Magellanic Cloud (LMC) have revealedthe presence of numerous supergiant (greater than 300 pc radius) andgiant shells of ionized gas. These structures are generally believed tobe the result of the action of encircled massive stars on thesurrounding interstellar medium. This paper examines the spectral andkinematic signature of this interaction through low and high dispersionspectra obtained for three supergiant and three giant shells in the LMC.One of the giant shells is an x-ray bubble embedded in the 30 Doradusnebula. The emission line ratios, including the lines (O II)lambda-3727, (O III) lambda-5007, (N II) lambda-6584, (S II)lambda-6717,31, in all but the embedded x-ray bubble, are found to beunusual compared to typical H II regions and supernova remnants in thesame galaxy. However, the emission lines and surface brightnesses ofthese structures are generally consistent with models of photoionizedgas having a very low ionization parameter due to the large distancebetween the encircled stars and the gas. Thus, emission from both thesupergiant and giant shell structures appears to be dominated byphotoionization processes. High dispersion spectra reveal that theprofiles of the ionized gas at the edges of supershells are narrow andcontain a single velocity component; spectra of the giant shells revealbroad profiles with multiple velocity components.
|UV Observations of Massive Stars in NGC 2014 and NGC 2074 in the LMC|
We report on the study of the UV characteristics of massive stars in theyoung LMC clusters NGC 2014 and NGC 2074. We obtained low-resolution IUEspectra of nine stars in NGC 2014 and six in NGC 2074. The spectraindicate that at least four of the stars in NGC 2014 are somewhatevolved O5-O7 stars with significant amounts of mass loss. The stars inNGC 2074 seem to be early B stars and show no signs of substantial massloss rates. We have also fitted the spectra with stellar atmospheremodels to derive the Teff-Lbol diagram for each cluster. Ourresults also have implications for our understanding of the interstellarreddening in the LMC.
|The detection of X-ray emission from the OB associations of the Large Magellanic Cloud|
A systematic study of the X-ray properties of OB associations in theLarge Magellanic Cloud has been carried out using data from the EinsteinObservatory. An excess of young, X-ray-bright supernova remnants isfound in the vicinity of the associations. In addition, diffuse X-rayemission is detected from over two dozen other associations;luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (thedetection threshold) to 10 to the 36th ergs/s. For several of the moreluminous examples, it is shown that emission from interstellar bubblescreated by the OB stellar winds alone is insufficient to explain theemission. It is concluded that transient heating of the bubble cavitiesby recent supernovae may be required to explain the observed X-rays andthat such a scenario is consistent with the number of X-ray-brightassociations and the expected supernova rate from the young stars theycontain.
|X-rays from superbubbles in the Large Magellanic Cloud|
Diffuse X-ray emission not associated with known supernova remnants(SNRs) are found in seven Large Magellanic Cloud H II complexesencompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34thergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30Dor, have simple ring morphologies, indicating shell structures.Modeling these as superbubbles, it is found that the X-ray luminositiesexpected from their hot interiors fall an order of magnitude below theobserved values. SNRs close to the center of a superbubble add verylittle emission, but it is calculated that off-center SNRs hitting theionized shell could explain the observed emission.
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