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Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| C [III] imagery of planetary nebulae and H II regions.
Not Available
| 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.
| Near-Infrared Imaging Observations of the N159/N160 Complex in the Large Magellanic Cloud: Large Clusters of Herbig Ae/Be Stars and Sequential Cluster Formation
We have carried out deep near-infrared imaging observations of theN159/N160 star-forming region in the Large Magellanic Cloud. We observedan area of ~380 arcmin2 (~80,000 pc2 at thedistance of the LMC) in the J, H, and Ks bands. Theobservations are deep enough to detect Herbig Ae/Be stars down to ~3Msolar in the LMC. We discovered a total of 338 and 464candidate Herbig Ae/Be and OB stars, respectively, based on thenear-infrared colors and magnitudes. The Herbig Ae/Be candidatescomprise 10 clusters, the OB star candidates 13. We discovered anembedded Herbig Ae/Be cluster in the N159 East giant molecular cloud(GMC) and a Herbig Ae/Be cluster at the northeast tip of the N159 SouthGMC. Together with two neighboring H II regions, the Herbig Ae/Becluster at the tip of the N159S GMC provides a hint of the beginning ofsequential cluster formation in N159S. The spatial distributions of theHerbig Ae/Be and OB clusters, in conjunction with previously knownoptical clusters and embedded massive stars, indicate (1) sequentialcluster formation within each of the N159 and N160 star-forming regionsand (2) large-scale sequential cluster formation over the entireobserved region from N160 to N159S. Possible triggers for thelarge-scale cluster formation are the supergiant shell SGS 19 and anexpanding superbubble. Some of the Herbig Ae/Be clusters in theN159/N160 complex are significantly larger in spatial extent thanpre-main-sequence clusters of similar age in the Milky Way. Highlyturbulent gas motion in the LMC is probably responsible for forming thelarge young clusters.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| 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.
| OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| 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.
| Spectrophotometry of six high-excitation compact HII regions in the Magellanic Clouds
Series of CCD long-slit spectra have been obtained in the 3600 - 10000Å range, with the ESO 1.5m telescope, for the six brightestcompact HII regions in the LMC and SMC: N 11A, N 160 A1-A2 and N 88A, N81 and N 26A-B, respectively. For each region the spectral type of itscomplex exciting source is given. From the emission-line intensities wehave derived the gas electron density and temperature, and computed thechemical abundances of He, O, N, Ne, S, and Ar, which we compare withthe ones found for other HII regions in the Magellanic Clouds.
| The physical structure of Magellanic Cloud H II regions. II. Elemental abundances
Based on a new data set of optical and infrared spectra described inVermeij et al. (\cite{Ronald}), we analyse the gas-phase elementalabundances of a sample of H Ii regions in the Large and Small MagellanicCloud. The combined optical and infrared data set gives us access to allthe ionization stages of astrophysically important elements such assulfur and oxygen. We self-consistently determine the electrontemperatures and densities for the \elem{O}{+}, \elem{S}{++} and\elem{O}{++} ionization zones, and use these parameters in thederivation of the ionic fractions. We discuss the uncertainties on theseionic fractions. The different relations between the electrontemperatures as proposed by Garnett (\cite{Garnett}) and Thuan et al.(\cite{Thuan}) are confronted with our results. We find our electrontemperatures to be consistent with these relations, although therelation between Te,[S Iii] and Te, [O Iii] mightbe slightly steeper than predicted. We investigate the reliability ofthe Ionization Correction Factors (ICFs) used in the derivation of thefull elemental abundances of sulfur and neon. We conclude that theprescription for the ICF used to derive the sulfur abundance as given byStasińska (\cite{Stas1}) for alpha = 3 is accurate for\elem{O}{+}/O > 0.20. No conclusions could be drawn for neon.Avoiding the use of ICFs as much as possible, we then proceed to derivethe full elemental abundances. We calculate a grid of generalphotoionization models to compare our results with the ``bright-line''abundance diagnostics for oxygen (R23) and sulfur (S23(4)). Thereliability of the newly proposed S234 parameter (Oey & Shields\cite{oey}) which includes emission lines from \elem{S}{+}, \elem{S}{++}and \elem{S}{+3} is checked. We find a very good agreement between theS234 models and our analysis results. Finally, we compare the heavyelement-to-oxygen ratios of our sample objects to those of giant H Iiregions in a large sample of low-metallicity blue dwarf galaxies (Izotov& Thuan \cite{Izotov}) and with the results from Kobulnicky &Skillman (\cite{Kobul1}, \cite{Kobul2}) for the irregular galaxies NGC1569 and NGC 4214.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| 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.
| Accurate positions of H2 O masers in the Large Magellanic Cloud
Positions with subarcsecond accuracy have been measured for seven 22-GHzH2 O masers associated with Hii regions in the LargeMagellanic Cloud (LMC); two of the masers are new detections. Initialposition measurements were obtained with the 70-m antenna of theCanberra NASA Deep Space Network during a period of more than two yearsin which the antenna was used to monitor the maser emission. Thepositions were further improved using 22-GHz observations involvingthree antennas of the Australia Telescope Compact Array. The positionshave been compared with those of 1.6-GHz continuum emission and otherLMC masers (of OH and CH3 OH). The H2 O maserpositions range from within 1arcsec (270mpc) of the centre of a compactHii component to beyond the boundary of significant continuum emission.Three of the four masers located near continuum peaks are close to OHmasers. In two cases the positional agreement is better than 0.2arcsec(53mpc) in the third case the agreement is worse (0.9arcsec) but thepositions of the individual H2 O features appear to be spreadover more than 1arcsec. The velocities of the OH masers are within thespread of the H2 O velocities. The three H2 Omasers offset from continuum centres are located 3-7arcsec from opticalor infrared phenomena probably associated with very early stages of starformation; no other molecular masers are known in these directions.
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| Resolving the compact H II regions in N160A with HST
Using high-resolution imaging with the Hubble Space Telescope, we studythe Large Magellanic Cloud H II region N160A and uncover severalstriking features of this complex massive star-forming site. The twocompact high excitation H II, blobs (HEBs) A1 and A2 are for the firsttime resolved and their stellar content and morphology is revealed. A1,being of higher excitation, is powered by a single massive star whosestrong wind has created a surrounding bubble. A2 harbors severalexciting stars enshrouded by large quantities of dust. The whole N160Anebula is energized by three star clusters for which we obtainphotometry and study their color-magnitude diagram. The H II region isparticularly dusty, with extinction values reaching an AV ~,2.5 mag in the visible, and it is separated from the molecular cloud byan outstanding ionization front. A previously detected infrared youngstellar object is also accurately located with respect to the H IIregion. Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withproposal #8247.
| 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.
| Detection and study of the compact HII region N26A-B in the Small Magellanic Cloud
This paper presents new imagery and spectrophotometric results for theN26 HII region in the Small Magellanic Cloud. The observations usingmonochromatic images and low-resolution spectra (3700-10 000 Å)reveal a compact and complex nebula composed of two cores A and B whereA in the region of Hβ is brighter than B by a factor ~ 5 anddistance of 2arcsec . The core A of FWHM ~ 2farcs 1 or 0.6 pc presents ahigh excitation [O III] lambda lambda5007 +4959/Hβ up to ~ 8 and ahigh reddening E(B-V) <= 0.6, while the core B is less excited buthas a higher reddening >=0.8. Each core contains one exciting source;the brighter one should be responsible for the high excitation of A. Theapparent spectral type of the two cores ranges from O7 to O9 V and thegas electron density and temperature were derived from the absorptionand emission-line intensities. The total mass of the ionized gas isevaluated at 13 Msun. The chemical abundances of He, O, N,Ne, S, and Ar were computed. These abundances seem consistent withaverage abundances for SMC HII regions, except N that appears slightlyoverabundant. N26A-B is comparable to the objects previously observed inthe LMC and SMC that we have called ``blobs''.
| 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.
| 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.
| Properties of the Young Stellar Object N 160 A-IR
We present near- and mid-infrared images of the young stellar object N160 A-IR in the Large Magellanic Cloud (LMC). For the first time, theYSO is seen in the NIR as a compact point source in the north-westernpart of the H ii region N 160 A. Furthermore, we present ISO-SWS spectrapointed on the YSO. The spectra cover the full wavelength range of SWSon board of ISO (2.5 - 45 mu m) where most of the dust features arelocated. N 160 A-IR shows the unidentified infrared bands (UIBs) andsilicate absorption on a steeply rising continuum plus fine structurelines from the H ii region. A radiative transfer model for the YSO witha luminosity of 1.4*10(6) L_ȯ being deeply embedded (A_V=60 mag) inits parental cloud reproduces the observations. The mass of the modelcloud is 50,000 M_ȯ consistent with other estimates. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) with the participation of ISAS andNASA.
| 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.
| Results of the SEST key programme: CO in the Magellanic Clouds. VII. 30 Doradus and its southern H II regions
We have mapped the (12) CO(1-0) emission from the 30 Doradus region inthe Large Magellanic Cloud with the Swedish-ESO Submillimetre Telescope(SEST). The regions investigated include the central part of the 30 Dornebula, and the southern Hii regions N 158C, N 159, and N 160. Inaddition, a few prominent CO clouds have been studied in the (2-1) and(3-2) transitions of CO. The southern area shows the strongest (12)CO(1-0) emission, a factor of 3 higher than towards the central part of30 Dor. A non-LTE analysis of the CO emission from a sample of cloudsindicate kinetic temperatures between 10 and 50 K; the highesttemperatures are found close to the 30 Dor nebula. We have estimated theCO-H2 conversion factor for the two areas, separately, underthe assumption that the virial mass, determined from CO parameters,reflects the total molecular mass. We find an unexpectedly smalldifference between the two areas. This is explained as a bias effect.Based on results collected at the European Southern Observatory, LaSilla, Chile
| The Chemical Composition of H II Regions in the Magellanic Clouds: New Calculations Using Modern Atomic Data
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| 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.
| Observations of ground-state OH in the Large Magellanic Cloud
We have carried out a series of observations of the 1665- and 1667-MHztransitions of the 2Pi3/2, J = 3/2 OH ground state towards six selectedH II regions in the Large Magellanic Cloud (IRAS 05011-6815 and MRC0510-689, 0513-694B, 0539-691, 0540-696B, 0540-697A) using the AustraliaTelescope Compact Array. The study has provided the first accuratepositions for known 1665- and 1667-MHz OH masers as well as detectingseveral new masers. The regions all contain H2O or CH3OH masers but OHmasers were detected in only four. The 1.6-GHz continuum emission wasalso imaged to investigate its spatial relationship to the associated OHmaser. Although some masers are close to compact continuum components,in other cases they are near the continuum distribution boundaries andperhaps have been created as a result of the H II region interactingwith the surrounding interstellar medium.
| Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..249F
| A radio continuum study of the Magellanic Clouds. IVa. Catalogue of radio sources in the Large Magellanic Cloud at 2.30GHz (λ=13cm).
We present a new catalogue of radio sources in the Large MagellanicCloud (LMC) based on observations at 2.30GHz with the Parkes radiotelescope. A total of 119 sources have been detected. We comparepositions and flux densities of these sources with previously publishedradio results and find no significant positional displacement or fluxdiscrepancies.
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