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Resim Yükleyin
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Ýlgili Makaleler
Discovery of a Compact X-Ray Source in the LMC Supernova Remnant N23 with Chandra
An X-ray compact source was discovered with Chandra in a supernovaremnant (SNR) N23, located in the Large Magellanic Cloud (LMC). Thecompact source (CXOU J050552.3-680141) is seen in only the hard band(>2 keV) image of N23, while the soft-band image (< 2 keV) showsdiffuse emission of the SNR, with an extent of~60''×80''. The compact source is located atalmost the center of N23, and there is no identifiable object for thesource from previous observations at any other wavelength. The sourcespectrum is best explained by a power-law model with a photon index ofΓ=2.2+0.5-0.3 and an absorption-correctedluminosity of LX=1.0×1034 ergss-1 in the 0.5-10 keV band for a distance of 50 kpc. Neitherpulsation nor time variability of the source was detected with thisobservation with a time resolution of 3.2 s. These results correspondwith those of Hughes et al., who carried out analysis independentlyaround the same time as our work. Based on information from the best-fitpower-law model, we suggest that the source emission is most likely froma rotation-powered pulsar and/or a pulsar wind nebula. It is generallyinferred that the progenitor of N23 is a core-collapsed massive star.
| 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.
| The Σ - D relation for supernova remnants in nearby galaxies
This paper examines relations between the radio surface brightnessΣ and the diameter D (also known as Σ-D relations) for asample of extragalactic supernova remnants (SNRs) as constructed from acombination of published data and data from our own surveys. Our sampleof extragalactic SNRs is the largest ever devised for the purpose ofanalyzing Σ-D relations. The main results of this paper may besummarized as follows: (i) the empirical relations for SNRs in 10 of the11 nearby galaxies studied have the approximately trivial Σ∝D-2 form, therefore limiting their interpretation asphysically meaningful relations. In addition, these relations aresubject to selection effects rendering them even less useful. FurtherMonte Carlo simulations suggest that the effect of survey sensitivityhas the opposite effect of volume selection (e.g. Malmquist bias, avolume selection effect that shapes the Galactic sample) by tending toflatten the slopes toward a trivial relation. In this case, the trueslopes may be steeper than the observed slopes; (ii) compact M 82 SNRsappear to follow a uniquely different Σ-D relation in comparisonto the larger, older SNRs in the other 10 galaxies. Monte Carlosimulations suggest that the probability of this difference arising bychance is ≈1% to 10%, depending on what is assumed regarding theunderlying SNR population; (iii) three candidate hypernova remnants wereidentified in our sample of 11 nearby galaxies.
| 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.
| 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.
| The Global Content, Distribution, and Kinematics of Interstellar O VI in the Large Magellanic Cloud
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofinterstellar O VI absorption toward 12 early-type stars in the LargeMagellanic Cloud (LMC). The observations have a velocity resolution of<~20 km s-1 (FWHM) and clearly show O VI 1031.926 Åabsorption at LMC velocities toward all 12 stars. From theseobservations we derive column densities of interstellar O VI in thisnearby galaxy; the observed columns are in the rangelogN(OVI)=13.9-14.6, with a mean of 14.37 and a standard deviation of+/-38% (+0.14-0.21 dex). The observations probeseveral sight lines projected onto known superbubbles in the LMC, butthese show relatively little (if any) enhancement in O VI column densitycompared to sight lines toward relatively quiescent regions of the LMC.The observed LMC O VI absorption is broad, with Gaussian dispersionsσ~30-50 km s-1. This implies temperaturesT<~(2-5)×106, indicating that much of the broadeningis nonthermal because O VI has a very low abundance at such hightemperatures. The O VI absorption is typically displaced ~-30 kms-1 from the corresponding low-ionization absorptionassociated with the bulk of the LMC gas. The general properties of theLMC O VI absorption are very similar to those of the Milky Way halo. Theaverage column density of O VI and the dispersion of the individualmeasurements about the mean are identical to those measured for the haloof the Milky Way, even though the metallicity of the LMC is a factor of~2.5 lower than the Milky Way. The velocity dispersion measured for theLMC material is also consistent with recent measurements of the Galactichalo. The striking similarities in these quantities suggest that much ofthe LMC O VI may arise in a vertically extended distribution similar tothe Galactic halo. We discuss the measurements in the context of a halocomposed of radiatively cooling hot gas and/or turbulent mixing layers.If the observed O VI absorption is tracing a radiatively coolinggalactic fountain flow, the mass flow rate from one side of the LMC diskis of the order M~1 Msolar yr-1, with a mass fluxper unit area of the disk M/Ω~2×10-2Msolar yr-1 kpc-2.
| Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.
| 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.
| HST observations of the very young SMC ``blob'' N 88A
High-resolution Hubble Space Telescope images have allowed us for thefirst time to resolve the compact SMC ionized ``blob'' N 88A (diameter ~,3''.5 or 1 pc). This very young H ii,region, which is hatching from itsnatal molecular cloud, is heavily affected by absorbing dust associatedwith the cloud. The interstellar reddening towards N 88A is on averageAV ~ ,1.5 mag and strikingly rises to more than 3.5 mag in anarrow dust band crossing the core of the H ii,region. Such a highextinction is unprecedented for an H ii,region in the metal-poor SMC. Wepresent the photometry of some 60 stars lying towards the OB associationat the center of which lies N 88A. The exciting star(s) of N 88A is notdetected, due to the heavy extinction. The chronology of star formationis discussed for the whole region. 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. Based onobservations obtained at the European Southern Observatory, La Silla,Chile. Table 1 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/Abstract.html
| The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure
LMC 2 has the brightest, most coherent filamentary structure of allknown supergiant shells in the Large Magellanic Cloud. The opticalemission-line images show active star formation regions along thewestern edge and long filaments to the east. ROSAT PSPC and HRI imagesshow bright X-ray emission from within the shell boundary, indicatingthe presence of hot gas. Counterintuitively, neither high-resolutionechelle spectra in the Hα line nor aperture synthesis H I 21 cmemission-line observations show LMC 2 to have the kinematics expected ofan expanding shell. Rather, LMC 2 appears to consist of hot gas confinedbetween H I sheets. The interior surfaces of these sheets are ionized bythe UV flux of massive stars in the star formation regions along theperiphery of LMC 2, while the heating is provided by outflows of hot gasfrom the star formation regions and by SNRs interior to LMC 2. We havecompared LMC 2 to other supergiant shells in the LMC and in more distantgalaxies. When the spatial resolution of our data are degraded, we findthat LMC 2 resembles supergiant shells observed at a distance of 4 Mpcthat have previously been interpreted as expanding shells. Therefore,great caution should be exercised in the analysis and interpretation ofthe kinematics of distant supergiant shells to prevent overestimates oftheir velocities and total kinetic energies.
| 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
| 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.
| Morphology and Physical Structure of the Interstellar Medium
The morphology of the interstellar medium (ISM) is dependent on thewavelength at which observations are made. Morphologies at differentwavelengths reveal interstellar gas components in different physicalconditions, thus allow us to probe the physical structure of the ISM.The global structure of the ISM is determined by the gravitationaleffects of the host galaxy itself and the environment. On scales from afew parsecs to 10^3 parsecs, the physical structure of the ISM islargely determined by the energy feedback from massive stars.Interactions between massive stars and the ISM not only shape the ISMbut also produce the multiple phases of the ISM. Multi-wavelengthobservations are needed to study the structure and evolution of amulti-phase ISM. The ISM of the Large Magellanic Cloud (LMC) is used toillustrate the environmental effects and the energy feedback frommassive stars. The energy feedback is more complex than any recipe candescribe.
| 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.
| Marseille Observatory H-alpha survey of the southern Galactic Plane and Magellanic Clouds
The ionized gas regions, which are the main tracers of the spiral arms,can be used for the study and determination of the spiral structure ofour Galaxy. Towards this goal, the Marseille Observatory elaborated anddeveloped an instrument, using a scanning Fabry-Perot interferometer,particularly suited for the observation of extended objects. The surveyof the southern Galactic plane then started at the beginning of 1990.The major instrumental aim is to obtain the spectral information, andtherefore the ionized gas kinematics, in each pixel of the observedfields. Already 300 fields of 38'X38' have been observed in H-alpha witha spatial resolution of 9" X 9", covering almost the entire fourthquadrant of the Galactic plane, detecting numerous discrete HII regionsand the diffuse emission which is widely distributed. Also, theMagellanic Clouds are studied using the same instrument.
| 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.
| 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
| A catalogue of compact radio sources in and behind the Large Magellanic Cloud
We present the results of a continuum snapshot survey of a 3 deg X 4 degregion of the Large Magellanic Cloud including the area of the giantmolecular cloud and the 30 Doradus nebula. The observations have beencarried out with the Australia Telescope Compact Array (ATCA) at 1.4 and2.4 GHz. Most fields are complete to about 6 mJy peak flux density at1.4 GHz and to about 3 mJy at 2.4 GHz. The positions, peak and integralflux densities of 113 compact (< 54") sources detected at 1.4 GHz andof 70 sources (<34") detected at 2.4 GHz are presented. Positions areaccurate to about 3" and peak flux densities are accurate to about 10%or better, depending on the source position relative to the pointingcenters. 32 of the sources detected at 1.4 GHz are coincident withHα objects in the catalogue of Davies et al.; these are possiblyintrinsic to the LMC. However, we suppose that most are backgroundobjects, since the number vs. flux agrees with predictions ofextragalactic source counts from other surveys. Tables 3 and 4 are alsoavailable electronically at the CDS via ftp cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html TheAustralia Telescope is funded by the Commonwealth of Australia foroperation as a National Facility managed by CSIRO.
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| High velocity motions inside the HII region N 103 of the Large Magellanic Cloud.
We have observed the HII region N 103 of the Large Magellanic Cloud witha scanning Fabry-Perot interferometer at Hα and [OIII]5007wavelengths. The kinematics of this field shows high velocity motions.We discuss their origin: Supernova explosion or particularly strongstellar winds. By calculating the energy input inside the gas, we showthat it is unlikely that the high velocity motions are due to thestellar winds of the embbeded stars. Then the nebula N 103 is linked totwo supernova remnants of different ages. The oldest one can berepresented by a bubble, 152pc wide, seen projected against the HIIregion, and probably lying at the edge of the HII region. The excitingstars of the nebula are actually members of the LMC cluster NGC 1850B;they provide a photon flux large enough to ionize the quiet part of thegas.
| Magnetically Uplifted Clumps in Cooling Flow Clusters
We study the dynamics of magnetic flux tubes in cooling flow clusters.The cluster magnetic fields are assumed to be confined within these longtubes. The tubes are assumed to be deformed into an initial U shape, andwe show that the magnetic tension forces can uplift the lower segmentsof the tubes to the height of the "tube feet," or even higher. Formagnetic flux tubes having magnetic pressure of ~> 10% of the thermalpressure, radii of 5 kpc, and with a distance of 25 kpc between the feetof the tubes, the magnetic tension can lift dense, and therefore cool,material up to ~10 kpc above the feet. Thus, if the feet are anchored atr = 25 kpc, the magnetic tension can uplift material from r ~ 10 kpc outto r ~ 35 kpc, where r is the distance from the cluster center. Wepropose that such a process may explain X-ray filaments and clumps that.have been found recently in the central regions of several cooling flowclusters. We discuss the effects which should be included in futurecalculations in order to make the model more realistic and test itagainst observations.
| 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.
| 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.
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| 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.
| Gas motions in and origin of the supergiant shell LMC4.
IUE high-dispersion spectra of stars in the supergiant shell LMC4havebeen used to derive velocities and column densities of absorbing gasclouds. The HI 21-cm profiles for the LMC4region from the LMC data ofRohlfs et al. (1984) have been analysed and interpreted in cloudcomponents. Combining the absorption and emission line data, a detailedpicture emerges of the location along the line of sight of the variousgas components and of the stars. LMC4 has a systemic velocity of285km/s. A pocket of gas in the center of LMC4is found to be receding at~325km/s while over the entire area we find gas near 260km/s. These twocomponents of gas differ in various ways. We argue that the gas at325km/s most likely belongs to a shell breakup. The gas at 260km/sprobably does not belong to the LMC4but lies in front of the supershell.There is no evidence for a systematic radial expansion of the gas fromthe geometric center of LMC4. Also, there is gas near 220km/s found inseveral of the IUE spectra. Given that the velocity differs by 65km/sfrom that of the main body of LMC4and the large spatial extent wepropose this gas cloud to be in the Milky Way halo. Our results arecompared with several theoretical scenarios for superbubble structures.We conclude that the origin of LMC4 is best explained by a process ofself-propagating star formation. Our data are in agreement withexpansion velocities around 10km/s.
| 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.
| Results of the ESO / SEST Key Programme on Co/ in the Magellanic Clouds - Part One - a Survey of Co/ in the Large Magellanic Cloud and the Small Magellanic Cloud
As the first part of the ESO-Swedish SEST Key Programme on CO in theMagellanic Clouds, we have observed ^12^CO J = 1-0 towards 92 positionsin the LMC and 42 positions in the SMC. In the SMC we searched foremission from H II regions, dark clouds and IRAS infrared sources. Thegenerally negative detection rate of non-IRAS sources in the SMC led toan LMC source selection based on the IRAS results. In both galaxies, COwas detected towards the majority of sources observed. We also observed^13^C0 J = 1-0 towards the brighter ^12^CO sources in the LMC (37) andSMC (9). Compared to the strength of CO lines observed in the Milky WayGalaxy with identical linear resolutions, velocity-integrated COemission is weaker by at least a factor of three in the LMC sources andan order of magnitude in the SMC sources. The mean velocity-integratedisotopic intensity ratio I_12_/I_13_ is 12.5 in the LMC and about 15 inthe SMC. Individual ratios range from 8.5 to 20. These isotopicintensity ratios are two to three times higher than those found inGalactic molecular clouds.
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