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Chandra ACIS Spectroscopy of N157B: A Young Composite Supernova Remnant in a Superbubble
We present a Chandra ACIS observations of N157B, a young supernovaremnant (SNR) located in the 30 Doradus star formation region of theLarge Magellanic Cloud. This remnant contains the most energetic pulsarknown (PSR J053747.39-691020.2 E˙=4.8×1038 ergss-1), which is surrounded by a X-ray-bright nonthermal nebulathat likely represents a toroidal pulsar wind terminal shock observededge-on. Two of the eight pointlike X-ray sources detected in theobservation are shown to have near-IR and optical counterparts (within0.5" offsets), which are identified as massive stellar systems in theCloud. We confirm the nonthermal nature of the comet-shaped X-rayemission feature and show that the spectral steepening of this featureaway from the pulsar is quantitatively consistent with synchrotroncooling of shocked pulsar wind particles flowing downstream at a bulkvelocity close to the speed of light. Around the cometary nebula weunambiguously detect a spatially resolved thermal component, whichaccounts for about 1/3 of the total 0.5-10 keV flux from the remnant.This thermal component is distributed among various clumps ofmetal-enriched plasma embedded in the low surface brightnessX-ray-emitting diffuse gas. The relative metal enrichment patternsuggests that the mass of the supernova progenitor is >~20Msolar. A comparison of the X-ray data with Hubble SpaceTelescope optical images now suggests that the explosion site is closeto a dense cloud, against which a reflection shock is launched. Theinteraction between the reflected material and the nebula has likelyproduced both its cometary shape and the surrounding thermal emissionenhancement. SNR N157B is apparently expanding into the hot low-densityinterior of the surrounding superbubble formed by the young OBassociation LH 99, as revealed by Spitzer mid-infrared images. Thisscenario naturally explains the exceptionally large sizes of both thethermal and nonthermal components, as well as the lack of an outer shellof the SNR. However, the real situation in the region is likely to bemore complicated. We find that a partially round soft X-ray-emittingclump with distinct spectral properties may result from a separateoxygen-rich remnant. These results provide a rare glimpse into the SNRstructure and evolution in a region of recent star formation.
| On the X-Ray Emission from Massive Star Clusters and Their Evolving Superbubbles
Here we discuss the X-ray emission properties from the hot thermalizedplasma that results from the collisions of individual stellar winds andsupernovae ejecta within rich and compact star clusters. We propose asimple analytical way of estimating the X-ray emission generated bysuper star clusters and derive an expression that indicates how thisX-ray emission depends on the main cluster parameters. Our modelpredicts that the X-ray luminosity from the star cluster region ishighly dependent on the star cluster wind terminal speed, a quantityrelated to the temperature of the thermalized ejecta. We have alsocompared the X-ray luminosity from the super stellar cluster (SSC)plasma with the luminosity of the interstellar bubbles generated fromthe mechanical interaction of the high-velocity star cluster winds withthe interstellar medium (ISM). We found that the hard (2.0-8.0 keV)X-ray emission is usually dominated by the hotter SSC plasma, whereasthe soft (0.3-2.0 keV) component is dominated by the bubble plasma. Thisimplies that compact and massive star clusters should be detected aspointlike, hard X-ray sources embedded into extended regions of softdiffuse X-ray emission. We also compared our results with predictionsfrom the population synthesis models that take into consideration binarysystems and found that in the case of young, massive, and compact superstar clusters the X-ray emission from the thermalized star clusterplasma may be comparable to or even larger than that expected from thehigh-mass X-ray binary (HMXB) population.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/443/851
| 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.
| Confronting the Superbubble Model with X-Ray Observations of 30 Doradus C
We present an analysis of XMM-Newton observations of the superbubble 30Dor C and compare the results with the predictions from the standardwind-blown bubble model. We find that the observed X-ray spectra cannotbe fitted satisfactorily with the model alone and that there is evidencefor nonthermal X-ray emission, which is particularly important at >~4keV. The combination of the bubble model and a power law gives areasonable fit to the observed spectra. The thermal pressure and centraltemperature of the bubble are 3.3×10-11 dynescm-2 and 7.4×106 K, respectively, and weinfer that for a bubble age of t~4×106 yr the ambientdensity is n0~=38 cm-3, the mechanical luminosityis Lmech~1037 ergs s-1, and thecoefficient of thermal conductivity is ~0.05 of the Spitzer value. Thetotal unabsorbed 0.1-10 keV luminosities of the eastern and westernparts of the bubble are ~=3×1036 and~=5×1036 ergs s-1, respectively. Theunabsorbed 0.1-10 keV luminosity of the bubble model is~4×1036 ergs s-1, and so the power-lawcomponent contributes between 1/3 and 1/2 of the total unabsorbedluminosity in this energy band. The nature of the hard nonthermalemission is not clear, although recent supernovae in the bubble may beresponsible. We expect that about one or two core-collapse supernovaecould have occurred and are required to explain the enrichment of thehot gas, as evidenced by the overabundance of α-elements by afactor of >~3, compared to the mean value of ~0.5 solar for theinterstellar medium in the Large Magellanic Cloud. As in previousstudies of various superbubbles, the amount of energy currently presentin 30 Dor C is significantly less than the expected energy input fromthe enclosed massive stars over their lifetime. We speculate that asubstantial fraction of the input energy may be radiated in thefar-infrared by dust grains, which are mixed with the hot gas because ofthe thermal conduction and/or dynamic mixing.
| Thermal and Nonthermal X-Rays from the Large Magellanic Cloud Superbubble 30 Doradus C
We report on the discovery of thermal and nonthermal X-rays from theshells of the superbubble (SB) 30 Dor C in the LMC. The X-ray morphologyis a nearly circular shell with a radius of ~40 pc, which is bright onthe northern and western sides. The spectra of the shells differ fromregion to region. The southern shell shows clear emission lines and iswell fitted with a model of a thin thermal plasma (kT=0.21 keV) innonequilibrium ionization plus a power-law component. This thermalplasma is located inside of the Hα emission, which is the outeredge of the shell of the SB. The northern and western sides of the SBare dim in Hα emission but are bright in nonthermal (power-law)X-rays with a photon index of 2.1-2.9. The nonthermal X-ray shell tracesthe outer boundary of the radio shell. These features of thin thermaland nonthermal X-rays are similar to those of SN 1006, a prototype of asynchrotron X-ray shell, but the nonthermal component of 30 Dor C isabout 10 times brighter than that of SN 1006. 30 Dor C is the firstcandidate of an extragalactic SB, in which energetic electrons areaccelerating in the shell. The age is much older than that of SN 1006,and hence the particle acceleration time in this SB may be longer thanthose in normal shell-like supernova remnants. We found pointlikesources associated with some of the tight star clusters. The X-rayluminosity and spectrum are consistent with those of young clusters ofmassive stars. Pointlike sources with nonthermal spectra are also foundin the SB. These may be background objects (active galactic nuclei) orstellar remnants (neutron stars or black holes).
| OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| Wolf-Rayet binaries in the Magellanic Clouds and implications for massive-star evolution - II. Large Magellanic Cloud
We present in this second paper the results of our intensivespectroscopic campaign to search for binaries via periodicradial-velocity (RV) variations among Wolf-Rayet (WR) stars for theLarge Magellanic Cloud (LMC). We observed 61 nitrogen-rich WNE stars inthe LMC. Along with the results of Bartzakos, Moffat & Niemela onthe carbon/oxygen-rich WR stars, 2/3 of the WR population of the LMC(134 stars in total) has now been investigated for periodic RVvariability. We have also retrieved time-dependent photometric data inthe public domain from the OGLE and MACHO projects, as well as X-raydata from ROSAT and Chandra satellites, to provide additionalconstraints on the binary character. For each of our sample stars, wediscuss its observational properties: RV variations, (periodic)photometric variability, X-ray luminosity, spectral classification,abundance of hydrogen, runaway status and line-profile variations(LPVs). For the binaries we discuss additional properties, such aswind-wind collision (WWC) effects, and the orbital parameters. With thislarge sample, we discuss the global properties of the WNE population,which is expected to be the most sensitive to binary evolution withrespect to the influence of metallicity. To emphasize the relevance ofthe binary frequency test for the stellar evolution of massive stars inthe LMC, we review their observational properties and provide new andmeaningful evolutionary classes, which reconcile observational andtheoretical definitions. Finally, we draw an overall evolutionary schemefor massive-star evolution, with respect to the three main ingredientsof stellar evolution: mass, metallicity and rotation.
| 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.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| 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.
| 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.
| Young Stellar Populations around SN 1987A
We present the first results of a study of the stellar population in aregion of 30 pc radius around SN 1987A, based on an analysis ofmultiband Hubble Space Telescope (HST) WFPC2 images. The effectivetemperature, radius and, possibly, reddening of each star weredetermined by fitting the measured broadband magnitudes to the onescalculated with model atmospheres. In particular, we have determinedeffective temperatures and bolometric luminosities for 21,995 stars, andfor a subsample of 2510 stars we also determined individual reddeningcorrections. In addition, we have identified all stars with Hαequivalent widths in excess of 8 Å, a total of 492 stars. Aninspection of the H-R diagram reveals the presence of severalgenerations of young stars, with ages between 1 and 150 Myr, superposedon a much older field population (0.6-6 Gyr). A substantial fraction ofyoung stars with ages around 12 Myr make up the stellar generationcoeval to SN 1987A progenitor. The youngest stars in the field appear tobe strong-line T Tauri stars, identified on the basis of theirconspicuous (Weq>8 Å) Hα excesses. Thisconstitute the first positive detection of low-mass (about 1-2Msolar) pre-main-sequence (PMS) stars outside the Milky Way.Their positions in the H-R diagram appear to require that star formationin the LMC occurs with accretion rates about 10 times higher than in theMilky Way, i.e., ~10-4 Msolar yr-1. SN1987A appears to belong to a loose, young cluster 12+/-2 Myr old, inwhich the slope of the present mass function is almost identical toSalpeter's, i.e., Γ=dlogN/dlogM~=-1.25 for masses above 3Msolar, but becomes much flatter for lower masses, i.e.,Γ~=-0.5. On a large scale, we find that the spatial distributionsof massive stars and low-mass PMS stars are conclusively different,indicating that different star formation processes operate for high- andlow-mass stars. This results casts doubts on the validity of an initialmass function (IMF) concept on a small scale (say, less than 10 pc).Moreover, it appears that a determination of the low-mass end IMF in theLMC requires an explicit identification of PMS stars. A preliminaryanalysis, done for the whole field as a single entity, shows that theIMF slope for the young population present over the entire region issteeper than Γ~=-1.7. Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by AURA, Inc., under NASA contract NAS5-26555.
| High-Velocity Star Formation in the Large Magellanic Cloud
Light-echo measurements show that SN 1987A is 425 pc behind the LMCdisk. It is continuing to move away from the disk at 18 kms-1. Thus, it has been suggested that SN 1987A was ejectedfrom the LMC disk. However, SN 1987A is a member of a star cluster, sothis entire cluster would have to have been ejected from the disk. Weshow that the cluster was formed in the LMC disk, with a velocityperpendicular to the disk of about 50 km s-1. Suchhigh-velocity formation of a star cluster is unusual, having no knowncounterpart in the Milky Way.
| 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.
| 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 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
| Structure and Kinematics of the Interstellar Medium in Front of SN 1987A
High-resolution (10 km s^-1) [N II] echelle spectra, sampled every 13"in a 6^'x6^' region around SN 1987A were obtained on the CTIO 4 mtelescope. The map shows a complicated velocity structure consistentwith that reported previously for the interstellar medium (ISM). Threecomponents, V_hel=265, 277, and 285 km s^-1 were identified as N157C (orthe R1170 complex: see an earlier paper of Xu, Crotts, & Kunkel).The radius of this large superbubble was found to expand at 10 km s^-1,with a lifetime of 6x10^6 yr and a total energy of 3x10^51 ergsdetermined from its radius and velocity according to superbubble theory.The V_hel=235 km s^-1 component corresponds to the near side of the 600pc giant superbubble reported earlier. This bubble is over 10^7 yearsold, and has blown out of the Large Magellanic Cloud disk. Two othercomponents, V_hel=255 and 245 km s^-1 are identified as the inner majorlight echo ring (a double-shell structure) at about 130 pc in front ofSN 1987A. There are also two high-velocity components, 300 and 313 kms^-1, which are possibly the far side of a superbubble in which SN 1987Aexploded. We also noticed that there are two components, at 279 and 301km s^-1, within 20" of SN 1987A. These structures are probably due tothe emission from the progenitor star's red supergiant wind. We findthat the time it took the SN 1987A progenitor to move to the currentlocation 300 pc behind N157C is comparable to the age of N157C as wellas that of the progenitor itself.
| Ultraviolet Imaging Telescope Observations of the Magellanic Clouds
We present wide-field far-ultraviolet (FUV; 1300-1800 Å) images ofthe Large and Small Magellanic Clouds (LMC, SMC). These data wereobtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1(1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the imagesprovide an extensive FUV mosaic of the SMC and contain numerous regionsin the LMC, covering a wide range of stellar densities and current starformation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OBassociations are completely or partially included in the observedfields. FUV data can identify the hottest OB stars more easily than canoptical photometry, and these stars dominate the ionizing flux, which iscorrelated to the observed Hα flux of the associated H ii regions.Of the H ii regions in the catalog of Davies, Elliott, & Meaburn(DEM), the UIT fields completely or partially include 102 DEM regions inthe LMC and 74 DEM regions in the SMC. We present a catalog of FUVmagnitudes derived from point-spread function photometry for 37,333stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMCwere presented recently by Cornett et al.), with a completeness limit ofm_UV ~ 15 mag and a detection limit of m_UV ~ 17.5. The averageuncertainty in the photometry is ~0.1 mag. The full catalog withastrometric positions, photometry, and other information is alsoavailable from publicly accessible astronomical data archives. We dividethe catalog into field stars and stars that are in DEM regions. Weanalyze each of these two sets of stars independently, comparing thecomposite UV luminosity function of our data with UV magnitudes derivedfrom stellar evolution and atmosphere models in order to derive theunderlying stellar formation parameters. We find a most probable initialmass function (IMF) slope for the LMC field stars of Gamma = -1.80 +/-0.09. The statistical significance of this single slope for the LMCfield stars is extremely high, though we also find some evidence for afield star IMF slope of Gamma ~ -1.4, roughly equal to the Salpeterslope. However, in the case of the stars in the DEM regions (the starsin all the regions were analyzed together as a single group), we findthree IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and-2.0. No typical age for the field stars is found in our data for timeperiods up to a continuous star formation age of 500 Myr, which is themaximum age consistent with the completeness limit magnitude of thecatalog's luminosity function. The best age for the collection ofcluster stars was found to be t_0 = 3.4 +/- 1.9 Myr; this is consistentwith the age expected for a collection of OB stars from many differentclusters.
| 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.
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..249F
| 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.
| 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.
| A Three-dimensional Study Using Light Echoes of the Structure of the Interstellar Medium in Front of SN 1987A
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...451..806X
| UIT and Optical Imagery of Large Magellanic Cloud Associations LH 52 and LH 53: Ages and Initial Mass Function Slopes
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...446..622H
| 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.
| The three-dimensional structure of the ISM in front of supernova 1987A
The expanding light echoes surrounding SN 1987A have been studiedphotographically over a period exceeding 3yr, sufficiently long that aview of the three-dimensional nature of the reflecting material can nowbe derived. We present two different projections of the dustdistribution. We find a number of discrete clouds, a linear formationand two prominent sheets of material, one of which is clearly splitwhile the other has significant depth. We find an association with theHII region N157C, but not with the supergiant shell that forms part ofthe N157C complex, which we show to lie behind the supernova. Althoughwe still prefer to assign the major echoes to the front and rear facesof a single, uncatalogued supergiant shell, we cannot preclude theexistence of two foreground supergiant shells, one inside the other andboth containing the supernova. We discuss the association of the echoeswith absorption features and recommend further observations that canclarify the structure of this region of the LMC.
| Resolution of massive compact clusters in the 30 Doradus periphery with the Hubble Space Telescope
Hubble Space Telescope Wide Field/Planetary Camera UBV images of threemassive, compact multiple systems with the SNR 30 Dor B/NGC 2060 and 30Dor C/NGC 2044 are discussed and illustrated. In two cases, WN+OBobjects have been resolved into additional components to thosepreviously known from ground-based observations, substantially reducingthe luminosities of the WN stars and rendering them currentlyunidentified; in the third case, the components of a B+Kcomposite-spectrum object have been clearly identified. The results areof significance for evolutionary interpretations of these massive starsand for determinations of the upper IMF in extragalactic systems.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
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