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Far Ultraviolet Spectroscopic Explorer Survey of Magellanic Cloud Supernova Remnants
We report the progress to date from an ongoing unbiased ultravioletsurvey of supernova remnants in the Magellanic Clouds using the FarUltraviolet Spectroscopic Explorer (FUSE) satellite. Earlier work withFUSE and other instruments has indicated that optical and/or X-raycharacteristics of supernova remnants are not always good predictors oftheir brightness in the ultraviolet. This survey is obtaining spectra ofa random large sample of Magellanic Cloud supernova remnants with abroad range of radio, optical, and X-ray properties. We proposed 39objects in the Large Magellanic Cloud and 11 objects from the SmallMagellanic Cloud, with a standard request of 10 ks per object using theFUSE 30" square aperture. To date, 39 objects have been observed in thesurvey (38 in the LMC and 1 in the SMC) and 15 have been detected, adetection rate of nearly 40%. Our survey has nearly tripled the numberof UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because ofthe diffuse source sensitivity of FUSE, upper limits on nondetectedobjects are quite sensitive in many cases, dependent on night observingfraction and whether stellar light contamination plays a role for agiven object. Estimated total luminosities in O VI, based simply onscaling the flux at the observed positions to an entire object, span abroad range from considerably brighter to many times fainter than theinferred soft X-ray luminosities, indicating that O VI can be animportant and largely unrecognized coolant in certain objects. Wecompare the optical and X-ray properties of the detected and nondetectedobjects but do not find a simple indicator for ultravioletdetectability. Nondetections may be due to clumpiness of the emission,high foreground extinction, slow shocks whose emission gets attenuatedby the Magellanic interstellar medium, or a combination of theseeffects. The characteristics of individual detected supernova remnantsare summarized in an Appendix.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| A detailed observation of a LMC supernova remnant DEM L241 with XMM-Newton
We report on an XMM-Newton observation of the supernova remnant (SNR)DEM L241 in the Large Magellanic Cloud. In the softband image, the emission shows an elongated structure, like a killifish,with a central compact source. The compact source is point-like, andnamed XMMU J053559.3-673509. The source spectrum is reproduced well by apower-law model with a photon index of Γ = 1.57 (1.51-1.62); andthe intrinsic luminosity is 2.2 ×1035~erg~s-1 in the 0.5-10.0 keV band, with anassumed distance of 50 kpc. The source has neither significant coherentpulsations in 2.0 × 10-3-8.0 Hz nor time variabilities.Its luminosity and spectrum suggest that the source might be a pulsarwind nebula (PWN) in DEM L241. The spectral feature classifies thissource as rather bright and hard PWN, which is similar to those in Kes75 and B0540-693. The elongated diffuse structure can be divided into a"Head" and "Tail", and both have soft and line-rich spectra. Theirspectra are reproduced well by a plane-parallel shock plasma (vpshock)model with a temperature of 0.3-0.4 keV, over-abundance in O and Ne, anda relative under-abundance in Fe. Such an abundance pattern and themorphology imply both that the emission is from the ejecta of the SNRand that the progenitor of DEM L241 is a very massive star, more than 20M_ȯ. This result is also supported by the existence of the centralpoint source and an OB star association, LH 88. The total thermal energyand plasma mass are ~4 × 1050 erg and ~200~M_ȯ,respectively.
| 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.
| 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.
| The Supergiant Shell LMC 2. II. Physical Properties of the 106 K Gas
LMC 2 has the highest X-ray surface brightness of all known supergiantshells in the Large Magellanic Cloud (LMC). The X-ray emission peakswithin the ionized filaments that define the shell boundary but alsoextends beyond the southern border of LMC 2 as an X-ray bright spur.ROSAT HRI images reveal the X-ray emission from LMC 2 and the spur to betruly diffuse, indicating a hot plasma origin. We have obtained ROSATPSPC and ASCA SIS spectra to study the physical conditions of the hot(>=106 K) gas interior to LMC 2 and the spur.Raymond-Smith thermal plasma model fits to the X-ray spectra,constrained by H I 21 cm emission line measurements of the columndensity, show the plasma temperature of the hot gas interior of LMC 2 tobe kT~0.1-0.7 keV and of the spur to be kT~0.1-0.5 keV. We have comparedthe physical conditions of the hot gas interior to LMC 2 with those ofother supergiant shells, superbubbles, and supernova remnants (SNRs) inthe LMC. We find that our derived electron density for the hot gasinside LMC 2 is higher than the value determined for the supergiantshell LMC 4, comparable to the value determined for the superbubble N11,and lower than the values determined for the superbubble N44 and anumber of SNRs.
| 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.
| A ROSAT PSPC catalogue of X-ray sources in the LMC region
We analyzed more than 200 ROSAT PSPC observations in a 10 by 10 degreefield centered on the Large Magellanic Cloud (LMC) and performed between1990 and 1994 to derive a catalogue of X-ray sources. The list contains758 sources with their X-ray properties. From cross-correlations of thePSPC catalogue with the SIMBAD data base and literature searches we givelikely identifications for 144 X-ray sources based on positionalcoincidence, but taking into account X-ray properties like hardnessratios and source extent. 46 known sources are associated with supernovaremnants (SNRs) and candidates in the LMC, most of them already detectedby previous X-ray missions. Including the new candidates from\cite[Haberl & Pietsch (1999)]{HP99} based on variability studies ofthe sources in our PSPC catalogue, the number of X-ray binaries in theLMC increased to 17 and that of the supersoft sources (SSSs) to 9. Theremaining ~ 50% of the identified sources comprise mainly foregroundstars (up to 57) and background extragalactic objects (up to 15). Theoften distinguished X-ray properties of the different source types wereused for a first classification of new, unknown X-ray sources. Eight newPSPC sources are classified as SNRs from their hardness ratios and onepromising new SNR candidate with extended X-ray emission is foundfurther north than all known SNRs. Three soft X-ray sources havehardness ratios compatible to those of the known SSSs. A selection onhardness ratios and X-ray to optical flux ratio further suggests 27foreground stars and 3 AGN.
| Magellanic Cloud X-Ray Sources. III. Completion of a ROSAT Survey
This paper concludes a series of three papers presenting ROSAT HighResolution Imager (HRI) observations of unidentified Einstein andserendipitous ROSAT X-ray sources in the direction of the MagellanicClouds. Accurate positions and fluxes have been measured for thesesources. Optical photometry and spectroscopy were obtained to search foridentifications in order to determine the physical nature of thesesources. The present paper includes new data for 24 objects;identifications are given or confirmed for 30 sources. For six sources,optical finding charts showing the X-ray positions are provided. Theresults from this program are summarized, showing that the populationsof luminous X-ray sources in the Magellanic Clouds are quite differentfrom those in the Galaxy.
| 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 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.
| A radio continuum study of the Magellanic Clouds. VIII. Discrete sources common to radio and infrared surveys of the Magellanic Clouds
We compare Parkes Telescope radio surveys with the IRAS Infrared (IR)surveys of the Magellanic Clouds (MCs). We find 130 discrete sources incommon towards the Large Magellanic Cloud (LMC) with both radio and IRemission. These 130 sources are mainly H Ii regions (89) and supernovaremnants (21). For 12 of the sources we have no identification and eightare background objects. We find 38 sources in common for the SmallMagellanic Cloud (SMC). Most of these sources are intrinsic (31) to theSMC, five sources are previously known background galaxies and twosources remain ambiguous. A flux density comparison of the radio and IRsources shows very good correlation and we note that the strongestsources at both radio and IR frequencies are H Ii regions. From theradio-IR comparison we propose that some 40 new sources in the LMC and10 in the SMC are H Ii regions or SNRs. All these new sources are alsoidentified in optical surveys.
| A radio continuum study of the Magellanic Clouds. VII. Discrete radio sources in the Magellanic Clouds
We present a study of discrete radio sources in the Magellanic Clouds(MCs) using the latest large-scale radio surveys made with the Parkesradio telescope between 1.4 and 8.55 GHz. These surveys achieved highersensitivity then previous surveys done with the Parkes telescope and sothe number of discrete radio sources detected towards the MCs hasincreased by factor of five. Also, we have obtained improved positions,flux densities and radio spectral indices for all of these sources. Atotal of 483 sources towards the Large Magellanic Cloud (LMC) and 224towards the Small Magellanic Cloud (SMC) have been detected at at leastone radio frequency. Most of the MC's sources have been classified inone of three groups: SNRs, H Ii regions or background sources accordingto classification criteria established here. In total, 209 discreteradio sources in the LMC and the 37 sources in the SMC are classifiedhere to be either H Ii regions or SNRs. We investigate their luminosityfunctions as well as the statistics of background sources behind theMCs. Also, we examine the distribution of SNRs and H Ii regions in theMCs. Tables 5 and 6 are only available electronically at the CDS via ftp130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| A radio continuum study of the Magellanic Clouds. VI. Discrete sources common to radio and X-ray surveys of the Magellanic Clouds
By comparing Parkes telescope radio surveys with the X-ray ROSAT All-SkySurvey (RASS) we have found 71 discrete sources of both radio and X-rayemission in the Large Magellanic Cloud (LMC). These 71 sources aremainly supernova remnants (SNRs) and SNR candidates (36), and backgroundsources (27). For six of the sources we have no proposed identificationand the other two are Hii regions. A source-intensity comparison of theradio and X-ray sources shows very little correlation, but we note thatthe strongest SNRs at both radio and X-ray frequencies are young SNRsfrom Population I. Six new LMC SNR candidates are proposed. From theradio flux density of the SNRs we have estimated the SNR birth rate tobe one every 100 (+/-20) yr and the star-formation rate (SFR) to be 0.7(+/-0.2) M_ȯyr(-1) . A similar comparison was undertaken for theSmall Magellanic Cloud (SMC), but instead of the RASS we used a rosterof pointed observations made with the ROSAT Position SensitiveProportional Counter (PSPC). This comparison resulted in 27 sources incommon between the Parkes radio and ROSAT PSPC surveys. Two new SMCsources are proposed for SNR candidates. The SMC SNR birth rate wasestimated to be one every 350 (+/-70) yr and the SFR was estimated to be0.15 (+/-0.05) M_ȯyr(-1) . Tables 2 and 3 are also availableelectronically at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http: //cdsweb.u-strasbg.fr/Abstract.html
| Kinematics of the very young nebula N59 at the edge of the supershell LMC4
The dynamics of the nebula N59 (B053540-6736), located at the boundaryof the supershell LMC4 in the Large Magellanic Cloud, is studied using ascanning Fabry-Perot interferometer. It is shown that the nebulae NGC2032 and 2035, which form the bright core of the H II region N59A(B053530-6736), belong to a single H II region which looks divided dueto the presence of a heavy dust lane. This bright core presents anexpansion motion of 24 km s(-1) . The kinetic energy involved in thismotion is of about 1.5 x 10(49) erg. This value is compatible eitherwith a supernova explosion origin or with a formation by the winds ofinterior massive stars. Since no clear traces of a SN explosion havebeen found in this nebula and since the stellar content of N59A(B053530-6736) is rich in blue stars, we conclude that these stars,mainly the very massive star HDE 269810 (R122), and probably other starshidden by the dust lane, are sufficient in providing the wind power todrive the expansion motion. The dust lane seems to be mixed in with thenebular gas and the stars, suggesting a site where star formation maystill take place. An extended shell, probably ionized by the star R122,has been detected at the same velocities as the slab, at blueshiftedvelocity, seen in the foreground absorption. The star R122 contributesto the high excitation of the faint diffuse gas, and perhaps of somefarther nebulae. To the East, the SNR 0536-676 remains as a trace of theexistence of another massive star which had already exploded. Thekinematics of N59B (B053610-6736) which contains the SNR 0536-676, isalso studied, corroborating the results of previous studies. Based onobservations collected at the European Southern Observatory
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| 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.
| LMC stellar X-ray sources observed with ROSAT. 1: X-ray data and search for optical counterparts
Observations of Einstein Large Magellanic Cloud (LMC) X-ray pointsources have been made with ROSAT's High-Resolution Imager to obtainaccurate positions from which to search for optical counterparts. Thispaper is the first in a series reporting results of the ROSATobservations and subsequent optical observations. It includes the X-raypositions and fluxes, information about variability, optical findingcharts for each source, a list of identified counterparts, andinformation about candidates which have been observed spectroscopicallyin each of the fields. Sixteen point sources were measured at a greaterthan 3 sigma level, while 15 other sources were either extended or lesssignificant detections. About 50% of the sources are serendipitousdetections (not found in previous surveys). More than half of the X-raysources are variable. Sixteen of the sources have been opticallyidentified or confirmed: six with foreground cool stars, four withSeyfert galaxies, two with supernova remnants (SNR) in the LMC, and fourwith peculiar hot LMC stars. Presumably the latter are all binaries,although only one (CAL 83) has been previously studied in detail.
| An atlas of the interstellar environment of Wolf-Rayet stars in the Magellanic clouds
We have made a complete study of the interstellar environment around theWolf-Rayet stars in the Magellanic Clouds. We present, in the form of anatlas, the results of a complete imaging survey in Hα and of anextensive survey in the (O III) alpha 5007 emission line. As a result ofthis survey, we have more than doubled the total number of ring nebulaeknown. These include cases of both rings of stellar ejecta and ringnebulae resulting from the sweeping up of the surrounding interstellarmedium. We find that 34% of WN3-WN4 stars, 36% of late WN types, and 26%of WC4-WC5 stars are associated with a ring nebula of some kind. Thesefigures are very similar to the percentage of Wolf-Rayet stars havingring nebulae in the solar neighborhood. The size distribution of ringnebulae is also similar. From the fact that the majority of Wolf-Rayetstars do not show ring nebulae, it is clear that mass loss in earlierphases of evolution, and the collective effects of the energy input fromthe clusters of OB stars with which the Wolf-Rayet stars are frequentlyassociated, has profoundly modified the preexisting circumstellarenvironment. However, on the basis of statistics, we cannot exclude thepossibility that all Wolf-Rayet stars have possessed a ring nebula atsome stage in their evolution.
| 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.
| The stellar content of the Large Magellanic Cloud II region N 59 A
We present UBV photometry of the stellar cluster associated with N59A, adusty H II region in the LMC. N59A's main detected source of ionizationis an O5V (or possibly earlier type) star with a visual extinction of1.2 mag. N59A also contains fifteen O-B3 stars that may contribute tothe ionization; these stars are affected by greatly differing amounts ofextinction. However, the observed stellar content of N59A cannotcompletely account for the ionization of the gas and the heating of theassociated dust. Some early massive star(s) still probably lieundetected in the core of (or behind) the central absorbing cloud. Inaddition to this young population associated with the H II region, wedetect an underlying older population of giant stars. We have alsodetected one Galactic star, and a few supergiant candidates. Theseresults are discussed in terms of the initial mass function.
| The detection of X-rays from the hot interstellar medium of the Large Magellanic Cloud
A comprehensive reanalysis is presented of the Einstein IPC data base onthe LMC, and a number of new algorithms are used to improve thereliability of the point source detection in this crowded region and toproduce the first large-scale map of diffuse emission free from theeffects of solar scattered X-rays and cosmic-ray particles. Algorithmsdescribed in detail include a technique to decontaminate fieldscontaining solar scattered flux, a mechanism for obtaining aspectrum-weighted vignetting function, and a source excision andsmoothing algorithm which results in a diffuse map of uniformstatistical quality. A catalog of discrete X-rays sources in thedirection of the LMC is presented which contains 33 new sources andeliminates a number of spurious and/or marginal directions from previouslists. A possible detection of the long-sought shadowing effect on thecosmic X-ray background produced by cold LMC gas is reported.
| 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.
| Dust in emission nebulae of the LMC derived from photometric reddening of stars
VBLUW photometric observations of stars in emission nebulae of the LMCare reported. The luminosities and extinctions of the stars are derived.Agreement is found between the average photometric extinctions of thenebulae and the extinctions derived from the Balmer line decrementmeasured by Caplan and Deharveng (1985 and 1986). The photometricextinctions are shown in the CO map of the LMC (Cohen et al., 1988).
| Vacuum ultraviolet images of the Large Magellanic Cloud
Linearized, absolutely calibrated VUV images of the LMC with aresolution of about 50 arcsec are presented. The images were made by asounding rocket payload in two bandpasses with effective wavelengths forhot stars near 1500 A and 1930 A. The flux in each bandpass is measuredfor the associations in the list of Lucke and Hodge (1970). The resultsare discussed and their relationship to the overall characteristics ofstar formation in the LMC are discussed. A simple model for propagatingstar formation in the LMC is presented whose results closely resemblethe distribution of associations revealed by the VUV images.
| Properties of supernova remnants at known distances. I - Surface brightness and radio spectral index
The observational properties of 84 SNRs are studied in three spectraldomains. Radio data at 1 GHz, X-ray data (0.15-4.5 keV), and opticaldata (forbidden S II lines) for the SNRs located in Magellanic clouds, M31, and M 33 are compared. The composite surface brightness-diameterrelation is analyzed. The dependence of radio spectral index on diameterand apparent ambient density and the relationship between radio surfacebrightness and electron density are examined. The comparison revealsthat there is a correlation between radio and X-ray surface brightnessof SNRs.
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