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The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud
We have studied the optical spectra of a sample of 28 O- and earlyB-type stars in the Large Magellanic Cloud, 22 of which are associatedwith the young star forming region N11. Our observations sample thecentral associations of LH9 and LH10, and the surrounding regions.Stellar parameters are determined using an automated fitting method(Mokiem et al. 2005), which combines the stellar atmosphere codefastwind (Puls et al. 2005) with the genetic algorithm basedoptimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively.The age difference and relative distance of the associations areconsistent with a sequential star formation scenario in which stellaractivity in LH9 triggered the formation of LH10. Our sample containsfour stars of spectral type O2. From helium and hydrogen line fitting wefind the hottest three of these stars to be 49{-}54 kK (compared to45{-}46 kK for O3 stars). Detailed determination of the helium massfraction reveals that the masses of helium enriched dwarfs and giantsderived in our spectroscopic analysis are systematically lower thanthose implied by non-rotating evolutionary tracks. We interpret this asevidence for efficient rotationally enhanced mixing leading to thesurfacing of primary helium and to an increase of the stellarluminosity. This result is consistent with findings for SMC stars byMokiem et al. (2006). For bright giants and supergiants no such massdiscrepancy is found; these stars therefore appear to follow tracks ofmodestly or non-rotating objects. The set of programme stars wassufficiently large to establish the mass loss rates of OB stars in thisZ ˜ 1/2 Zȯ environment sufficiently accurate toallow for a quantitative comparison with similar objects in the Galaxyand the SMC. The mass loss properties are found to be intermediate tomassive stars in the Galaxy and SMC. Comparing the derived modified windmomenta D_mom as a function of luminosity with predictions for LMCmetallicities by Vink et al. (2001) yields good agreement in the entireluminosity range that was investigated, i.e. 5.0 < logL/Lȯ< 6.1.Appendix A is only available in electronic form at http://www.aanda.org
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Near-Infrared Observations of N11 in the Large Magellanic Cloud: Triggered Star Formation around the Periphery of LH 9
Near-infrared observations have been carried out to survey young stellarobjects in the second-largest H II region in the Large Magellanic Cloud,N11. A total area of about 700 arcmin2 is covered in the J,H, and KS bands. We selected a total of 559 OB and 127 HerbigAe/Be star candidates out of the detected sources based on theirnear-infrared colors and magnitudes. The existence of these youngstellar objects indicates that star formation activity is underway inthe whole N11 region. Many Herbig Ae/Be star candidates are distributedaround the periphery of the OB association LH 9. Spatial correlations ofthe OB and Herbig Ae/Be star candidates with the objects observed atother wavelengths (optical, radio continuum, Hα, CO, and X-ray)suggest that the birth of the young stellar populations in peripheralmolecular clouds was triggered originally by LH 9. It is likely that thetrigger for this star formation was an expanding supershell blown by theOB association. In N11 a new generation of stars would have been formedin the clouds developed from swept-up interstellar medium.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| XMM-Newton observations of the giant H II region N 11 in the LMC
Using the sensitive XMM-Newton observatory, we have observed the giant HII region N 11 in the LMC for ˜30 ks. We have detected severallarge areas of soft diffuse X-ray emission along with 37 point sources.One of the most interesting results is the possible association of afaint X-ray source with BSDL 188, a small extended object of uncertainnature.The OB associations in the field-of-view (LH9, LH10 and LH13) are alldetected with XMM-Newton, but they appear very different from oneanother. The diffuse soft X-ray emission associated with LH9 peaks nearHD 32228, a dense cluster of massive stars. The combined emission of allindividual massive stars of LH9 and of the superbubble they have createdis not sufficient to explain the high level of emission observed: hiddenSNRs, colliding-wind binaries and the numerous pre-main sequence starsof the cluster are most likely the cause of this discrepancy. Thesuperbubble may also be leaking some hot gas in the ISM since faint,soft emission can be observed to the south of the cluster. The X-rayemission from LH10 consists of three pointlike sources and a softextended emission of low intensity. The two brightest point sources areclearly associated with the fastest expanding bubbles blown by hot starsin the SW part of the cluster. The total X-ray emission from LH10 israther soft, although it presents a higher temperature than the othersoft emissions of the field. The discrepancy between the combinedemission of the stars and the observed luminosity is here less severethan for LH9 and could be explained in terms of hot gas filling thewind-blown bubbles. On the other hand, the case of LH13 is different: itdoes not harbour any extended emission and its X-ray emission could mostprobably be explained by the Sk -66°41 cluster alone.Finally, our XMM-Newton observation included simultaneous observationswith the OM camera that provide us with unique UV photometry of morethan 6000 sources and enable the discovery of the UV emission from theSNR N11L.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr /cgi-bin/qcat?J/A+A/418/841
| Active Star Formation in the N11B Nebula in the Large Magellanic Cloud: A Sequential Star Formation Scenario Confirmed
The second largest H II region in the Large Magellanic Cloud, N11B hasbeen surveyed in the near-IR. We present JHKs images of theN11B nebula. These images are combined with CO (1-->0) emission-linedata and with archival New Technology Telescope and Hubble SpaceTelescope WFPC2 optical images to address the star formation activity ofthe region. IR photometry of all the IR sources detected is given. Weconfirm that a second generation of stars is currently forming in theN11B region. Our IR images show the presence of several bright IRsources that appear to be located toward the molecular cloud as seenfrom the CO emission in the area. Several of these sources show IRcolors with young stellar object characteristics, and they are primecandidates to be intermediate-mass Herbig Ae/Be stars. For the firsttime, an extragalactic methanol maser is directly associated with IRsources embedded in a molecular core. Two IR sources are found at 2"(0.5 pc) of the methanol maser reported position. Additionally, wepresent the association of the N11A compact H II region to the moleculargas, where we find that the young massive O stars have eroded a cavityin the parental molecular cloud, typical of a champagne flow. The N11region turns out to be a very good laboratory for studying theinteraction of winds, UV radiation, and molecular gas. Severalphotodissociation regions are found.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained from the archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N 11
The second-brightest star formation complex in the Large MagellanicCloud, N 11, was surveyed extensively in the J = 1-0 transition of12CO. In this paper we present maps and a cataloguecontaining the parameters of 29 individual molecular clouds in thecomplex, although more may be present. The distribution of molecular gasin the N 11 complex is highly structured. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OBstar association LH 9. In the northeastern part, a chain of molecularclouds delineates the rim of one of the so-called supergiant shells inthe LMC. There appears to be very little diffuse molecular gasin-between the individual well-defined clouds, especially in thesouthwestern ring. Most of the clouds have dimensions only slightlylarger than those of the survey beam, i.e. diameters of 25 pc or less. Asubset of the clouds mapped in J= 1-0 12CO transition wasalso observed in the J= 2-1 12CO transition, and in thecorresponding transitions of 13CO. Clouds mapped in J= 2-112CO with a two times higher angular resolution show further,clear substructure. The elements of this substructure, however, havedimensions once again comparable to those of the mapping beam. For a fewclouds, sufficient information was available to warrant an attempt atmodelling their physical parameters. They contain fairly warm(Tkin = 60-150 K) and moderately dense (nH_2 =3000 cm-3) gas. The northeastern chain of CO clouds, althoughlacking in diffuse intercloud emission, is characteristic of the morequiescent regions of the LMC, and appears to have been subject torelatively little photo-processing. The clouds forming part of thesouthwestern shell or ring, however, are almost devoid of diffuseintercloud emission, and also exhibit other characteristics of anextreme photon-dominated region (PDR).
| Bubble Nebulae around Ultraluminous X-Ray Sources
The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearbygalaxies continues to be an enigma, since their adopted isotropichigh-energy output would surpass the Eddington limit of even the mostmassive stellar black holes. Many ultraluminous X-ray sources aresurrounded by emission nebulae that show indications of both shockionization and X-ray ionization. Relatively compact X-ray ionizednebulae can be used to independently infer the luminosities, and thus toexclude possible beaming effects into our line of sight. Largerbubble-like nebulae reach several hundred parsec diameters and provideimportant information on the formation and/or mass loss history of ULX.We point out the close relationship to microquasars and the previouslyunique SS 433 system with its radio nebula W 50.
| 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.
| Massive luminous early type stars in the LMC. I. The reddening of individual stars and the LMC reddening law
In order to construct a comprehensive HRD of early type stars in theLarge Magellanic Cloud (LMC) in the first step the reddenings ofindividual stars and the LMC reddening law have been investigated. 1942LMC member stars with good U BV photometries from the Bochum photometrydata base have been first corrected individually for galactic foregroundreddening. From stars with good spectral classification the slope of theLMC internal reddening line EU-BBV was calculated for eachspectral subclass between O3 and A4. A remarkable difference to thegalactic reddening law was found. The slope of the reddening line firstdecreases for stars from O3 to B0, and then increases rapidly between B0and B3 from ~ 0.7 to ~ 1.1. For later type stars it remains higher thanfor early type stars. This effect has important consequences for allextinction corrections. We checked this using different methods. Becauseno evidence for systematically wrong classifications was found, thedifferences in the reddening slopes must be caused by the ISM of the LMCitself. Four possible causes are considered.
| The physical structure of Magellanic Cloud H II regions. I. Dataset
We present infrared and optical spectroscopic data for 11 H Ii regionsand one Supernova Remnant in the Large and Small Magellanic Cloud. Theinfrared data have been obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatoryas part of a Guaranteed Time Program on H Ii regions in Local GroupGalaxies. Aim of this project is to give a new and improved analysis ofthe physical structure of the sample H Ii regions by combining as muchspectral data as possible. A detailed account is given here of thereduction process, and the quality and reliability of the presentedfluxes are discussed. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| A CO Survey of the LMC with NANTEN: III. Formation of Stellar Clusters and Evolution of Molecular Clouds
In order to elucidate star formation in the LMC, we made a completestudy of CO clouds with NANTEN. In the present paper, we compare 55giant molecular clouds (GMCs), whose physical quantities were welldetermined, with young objects, such as young stellar clusters and HIIregions. We find that the GMCs are actively forming stars and clusters;23 and 40 are found to be associated with the clusters and the HIIregions, respectively. The clusters associated with the GMCs aresignificantly young; ~ 85% of them are younger than ~ 10 Myr. Inaddition, compact groups of the young clusters are often found at thepeak position of the GMCs, e.g., N 159 and N 44, while much loosergroups are away from the GMCs. This suggests that the clusters areformed in groups and disperse as they become old. The distributions ofthe CO, [CII], and UV indicate that the GMCs are likely to be rapidlydissipated within several Myr due to UV photons from the clusters. Wealso estimate the evolutionary time scale of the GMCs; they form starsin a few Myr after their birth, and form clusters during the next fewMyr, and are dissipated in the subsequent few Myr.
| 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.
| HST observations of the LMC compact \ion{H}{ II} region N 11A
We present a study of the LMC compact H ii region N 11A using HubbleSpace Telescope imaging observations which resolve N 11A and reveal itsunknown nebular and stellar features. The presence of a sharp ionizationfront extending over more than 4'' (1 pc) and fine structure filamentsas well as larger loops indicate an environment typical of massive starformation regions, in agreement with high [O iii]/Hβ line ratios. N11A is a young region, as deduced from its morphology, reddening, andespecially high local concentration of dust, as indicated by the Balmerdecrement map. Our observations also reveal a cluster of stars lyingtowards the central part of N 11A. Five of the stars are packed in anarea less than 2'' (0.5 pc), with the most luminous one being a mid Otype star. N 11A appears to be the most evolved compact H ii region inthe Magellanic Clouds so far studied. Based on observations with theNASA/ESA Hubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud
Using ultraviolet photometry from the Ultraviolet Imaging Telescope(UIT) combined with photometry and spectroscopy from three ground-basedoptical data sets we have analyzed the stellar content of OBassociations and field areas in and around the regions N79, N81, N83,and N94 in the Large Magellanic Cloud. In particular, we compare datafor the OB association Lucke-Hodge 2 (LH 2) to determine how stronglythe initial mass function (IMF) may depend on different photometricreductions and calibrations. Although the data sets exhibit medianphotometric differences of up to 30%, the resulting uncorrected IMFs arereasonably similar, typically Γ~-1.6 in the 5-60 Msolarmass range. However, when we correct for the background contribution offield stars, the calculated IMF flattens to Γ=-1.3+/-0.2 (similarto the Salpeter IMF slope). This change underlines the importance ofcorrecting for field star contamination in determinations of the IMF ofstar formation regions. It is possible that even in the case of anuniversal IMF, the variability of the density of background stars couldbe the dominant factor creating the differences between calculated IMFsfor OB associations. We have also combined the UIT data with the mostextensive of these ground-based optical data sets-the Magellanic CloudPhotometric Survey-to study the distribution of the candidate O-typestars in the field. We find a significant fraction, roughly half, of thecandidate O-type stars are found in field regions, far from any obviousOB associations (in accord with the 1982 suggestions of Garmany, Conti,& Chiosi for O-type stars in the solar neighborhood). These starsare greater than 2' (30 pc) from the boundaries of existing OBassociations in the region, which is a distance greater than most O-typestars with typical dispersion velocities will travel in their lifetimes.The origin of these massive field stars (either as runaways, members oflow-density star-forming regions, or examples of isolated massive starformation) will have to be determined by further observations andanalysis.
| Compact star clusters of the LMC H ii region N11 C
Based on imaging and spectroscopy obtained at the ESO NTT telescope andusing an efficient image analysis algorithm, we study the core of theLMC OB association LH 13, particularly the two compact stellar clustersSk-66deg41, and HNT in the H ii, region N 11C. We resolveSk-66deg41, into 15 components and for the first time the HNTcluster into 70 stars, and derive photometry for the members. Moreover,from medium resolution spectroscopy we determine the spectral types forsixteen stars in N 11C. We compare the color-magnitude diagrams of theclusters with that of the field stars and discuss the cluster ages. Withan age of ~ ,100 Myr, the HNT cluster appears significantly older thanthe very young (<= 5 Myr) Sk-66deg41, starburst. Wesuggest that most of the `field' O-stars in the core of N 11C haveactually been ejected from Sk-66deg41, through dynamicalinteractions in the compact cluster. The properties of theSk-66deg41, and HNT clusters suggest that we are viewingdifferent star formation regions lying at different distances along thesame line of sight. Based on observations obtained at the EuropeanSouthern Observatory, La Silla, Chile
| 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.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. I. Observations
We have used the rocketborne Wide-Field Imaging Survey Polarimeter(WISP) to image a 1.5dx4.8d area of the western side of the LargeMagellanic Cloud (LMC) at a wavelength of λ=2150 Å and aresolution of 1'x1.5′. These are the first wide-field ultravioletpolarimetric images in astronomy. We find the UV background light of theLMC to be linearly polarized at levels ranging from our sensitivitylimit of 4% to as high as ~40%. In general, the polarization in a pixelincreases as the flux decreases; the weighted mean value of polarizationacross the WISP field is 12.6%+/-2.3%. The LMC's diffuse UV background,in uncrowded areas, rises from a minimum of (5.6+/-3.1)x10-8ergs s-1 cm-2 Å-1 sr-1(23.6+/-0.5 mag arcsec-2) to (9.3+/-1.1)x10-8 ergss-1 cm-2 Å-1 sr-1(23.1+/-0.2 mag arcsec-2) in regions near the brightassociations. We use our polarization maps to investigate the geometryof the interstellar medium in the LMC and to search for evidence of asignificant contribution of scattered light from OB associations to thediffuse galactic light of the LMC. Through a statistical analysis of ourpolarization map, we identify nine regions of intense UV emission whichmay be giving rise to scattering halos in our image. We find thatstarlight from the N11 complex and the LH 15 association are thestrongest contributors to the scattered light component of the LMC'sdiffuse galactic light. This region of the northwestern LMC can bethought of as a kiloparsec-scale reflection nebula in which OB starsilluminate distant dust grains that scatter the light into our sightline. In contrast, the polarization map does not support the scatteringof light from the large B2 complex in the southern WISP field; thiseffect may be astrophysical, or it may be the result of bias in ouranalysis.
| 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.
| Ultraviolet observations of stars in the Magellanic Clouds: a historical bridge from the Sun to R 136 and beyond.
Not Available
| The WR and O-type star population predicted by massive star evolutionary synthesis
Evolutionary calculations of massive single stars and of massive closebinaries that we use in the population number synthesis (PNS) code arepresented. Special attention is given to the assumptions/uncertaintiesinfluencing these stellar evolutionary computations (and thus the PNSresults). A description is given of the PNS model together with theinitial statistical distributions of stellar parameters needed toperform number synthesis.We focus on the population of O-type stars andWR stars in regions where star formation was continuous in time and instarburst regions. We discuss the observations that have to be explainedby the model. These observations are then compared to the PNSpredictions.We conclude that: . probably the majority of the massivestars are formed as binary components with orbital period between 1 dayand 10 yr; most of them interact. . at most 8% of the O-type stars arerunaways due to a previous supernova explosion in a binary; recentstudies of pulsar space velocities and linking the latter to the effectof asymmetrical supernova explosions, reveal that only a smallpercentage of these runaways will have a neutron star companion. . withpresent day stellar evolutionary computations, it is difficult toexplain the observed WR/O number ratio in the solar neighbourhood and inthe inner Milky Way by assuming a constant star formation rate, with orwithout binaries. The observed ratio for the Magellanic Clouds is betterreproduced. . the majority of the single WR stars may have had a binarypast. . probably merely 2-3% (and certainly less than 8%) of all WRstars have a neutron star companion. . a comparison between theoreticalprediction and observations of young starbursts is meaningful only ifbinaries and the effect of binary evolution are correctly included. Themost stringent feature is the rejuvenation caused by mass transfer.
| Ultraviolet Imaging of the Irregular Galaxy NGC 4449 with the Ultraviolet Imaging Telescope: Photometry and Recent Star Formation History
The bright Magellanic irregular galaxy NGC 4449 was observed during theAstro-2 Space Shuttle mission by the Ultraviolet Imaging Telescope(UIT), which obtained images of a 40' field centered on the galaxy intwo broad far-ultraviolet (FUV) bands centered at 1520 and 1620 Å,with 3"-5" spatial resolution. Together with Hα and Hβ fluxesfrom ground-based Fabry-Perot images, these data are analyzed in orderto explore the recent star formation history of NGC 4449. Maps of theHα-to-FUV and FUV-to-blue continuum flux ratios are presented andinterpreted using evolutionary synthesis models. Photometry is presentedboth for 22 apertures containing large OB complexes and for 57 smallapertures containing compact FUV-emitting knots. The OB complexes alongthe northern edge of the visible system have high Hα-to-FUV ratiosand thus appear to be more dominated by the current generation of starsthan are other parts of the galaxy. However, young sources do existelsewhere and are particularly conspicuous along the bar. Thesmall-aperture analysis shows three candidate regions for sequentialstar formation. Surface brightness profiles are consistent with anexponential disk in both the FUV and the optical continuum.
| 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.
| LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.
| X-Rays from Superbubbles in the Large Magellanic Cloud. V. The H II Complex N11
The giant H II complex N11 in the Large Magellanic Cloud contains OBassociations at several different stages in their life histories. Wehave obtained ROSAT PSPC and HRI X-ray observations, Curtis Schmidt CCDimages, echelle spectra in H alpha and [N II] lines, and IUEinterstellar absorption line observations of this region. The centralbubble of N11 has an X-ray luminosity a factor of only 3-7 brighter thanthat predicted for an energy-conserving superbubble, making this thefirst detection of X-ray emission from a superbubble without a strongX-ray excess. The region N11B contains an extremely young OB associationanalogous to the central association of the Carina Nebula, apparentlystill embedded in its natal molecular cloud. We find that N11B emitsdiffuse X-ray emission, probably powered by stellar winds. Finally, wecompare the tight cluster HD 32228 in N11 to R136 in 30 Dor. The latteris a strong X-ray source, while the former is not detected, showing thatstrong X-ray emission from compact objects is not a universal propertyof such tight clusters.
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