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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.
| Structure and Dynamics of Candidate O Star Bubbles in N44
Dynamical studies of superbubbles and Wolf-Rayet ring nebulae showdiscrepancies from the standard adiabatic model for windblown bubbles.We therefore study the physical properties and kinematics of threecandidate bubbles blown by single O stars to evaluate whether thesediscrepancies are also found in these simpler objects. Our samplecandidates are N44 F, N44 J, and N44 M, in the outskirts of the H IIcomplex N44 in the Large Magellanic Cloud. We have obtained ground-basedand Hubble Space Telescope emission-line images and high-dispersionechelle spectra for these objects. From the Hα luminosities andthe [O III]/Hα ratios of these nebulae, we estimate the spectraltypes of the ionizing stars to be O7 V, O9.5 V, and O9.5 V for N44 F,N44 J, and N44 M, respectively. We find that the observed expansionvelocity of 12 km s-1 for N44 F is consistent with thestellar wind luminosity expected from the central ionizing star, aspredicted by the standard bubble model. The observed upper limits forthe expansion velocities of N44 J and N44 M are also compatible with theexpected values, within the uncertainties. We also report the discoveryin N44 F of strongly defined dust columns, similar to those seen in theEagle Nebula. The photoevaporation of these dense dust features may bekinematically important and may actually govern the evolution of theshell. The inclusion of photoevaporation processes may thus underminethe apparent agreement between the observed bubble dynamics and thesimple adiabatic models.
| The HE II Emitting Nebula N44C in the Large Magellanic Cloud: Optical/Ultraviolet Spectroscopy of the Nebula and Its Ionizing Star
We present HST spectroscopy and imaging, along with new ground-basedspectroscopy and ROSAT HRI imaging, of the He II emitting nebula N44Cand its ionizing star. A GHRS spectrogram of the ionizing star yields aspectral type of about O7 for the star. The lack of P Cygni profiles forSi IV and C IV indicates that the star is not a supergiant. The nebularabundances in the ionized gas are consistent with average abundances forLMC H II regions, with the possible exception that nitrogen may beenhanced. Enrichment by a former evolved companion star is not evident.A long-slit echelle spectrogram in Hα+[N II] shows no evidence forhigh-velocity gas in N44C. This rules out high-velocity shocks as thesource of the nebular He II emission. A 108 ks ROSAT HRI image of N44Cshows no X-ray point source to a 3 σ upper limitLX<1034 ergs s-1 in the 0.1-2.0 keVband. Based on new measurements of the electron density in the He IIemitting region, we derive recombination timescales of ~20 yr forHe+2 and ~4 yr for Ne+4. If N44C is a fossil X-rayionized nebula, this places severe constraints on when the putativeX-ray source could have turned off. The presence of strong [Ne IV]emission in the nebula is puzzling if the ionizing source has turnedoff. It is possible the system is related to the Be X-ray binaries,although the O star in N44C does not show Be characteristics at thepresent time. Monitoring of X-rays and He II emission from the nebula,as well as a radial velocity study of the ionizing star, are needed tofully understand the emission line spectrum of N44C. Based in part onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.
| OB Stellar Associations in the Large Magellanic Cloud: Identification Method
We describe an objective method for the identification of stellar OBassociations in the Large Magellanic Cloud under the assumption thatthey are loose, unbound stellar systems with a young OB stellarcomponent. The method is based on star counts and spectralclassification. First we detect the areas where an enhancement of starnumber density occurs above 3 σ of the average field density inlarge regions. The boundaries at 3 σ provide the size andmorphology of the detected stellar concentrations. Further examinationat different magnitude ranges allows us to select the systems with abright stellar component within the detected areas. In the second step,star counts around the peak density of each detected stellarconcentration provide a typical value of the projected half-mass radius,in order to calculate the central density using the appropriate massfunction slope. The central density, being a crucial parameter for thebound and unbound systems, has been used as a tentative criterion forthe distinction between open clusters and associations. Finally,spectral classification from objective-prism plates provides furtherevidence for the existence of OB-type stars in these concentrations. Thefaintest magnitude at which the various systems were detected is foundto be independent of the presence or absence of gas and varies by up to4 mag. An explanation for this effect is the possible existence ofpre-main-sequence stars that are not visible in the optical region.
| Hubble Space Telescope Wide Field Planetary Camera 2 Imaging of Shocks in Superbubbles
Bright X-ray emission has been detected in superbubbles in the LargeMagellanic Cloud (LMC), and it is suggested that supernova remnants(SNRs) near the inner-shell walls are responsible for this X-rayemission. To identify SNR shocks in superbubble interiors, we haveobtained Hubble Space Telescope Wide Field Planetary Camera 2emission-line images of the X-ray-bright superbubbles DEM L152 and DEML192 and the X-ray-dim superbubble DEM L106. We use these images toexamine the shell morphology and [S II]/Hα ratio variations indetail. Of these three superbubbles, DEM L152 has the highest X-raysurface brightness, the most filamentary nebular morphology, the largestexpansion velocity (~40 km s-1), and the highest [SII]/Hα ratio (0.4-0.6). Its [S II]/Hα ratio increasesoutward and peaks in sharp filaments along the periphery. DEM L192 has amoderate X-ray surface brightness, a complex but not filamentarymorphology, a moderate expansion velocity (35 km s-1), and alow [S II]/Hα ratio (~0.15). DEM L106 is not detected in X-rays.Its shell structure is amorphous and has embedded dusty features; itsexpansion velocity is less than 10 km s-1. None of the threesuperbubbles show morphological features in the shell interior that canbe identified as directly associated with SNR shocks, indicating thatthe SNR shocks have not encountered very dense material. We find thatthe [S II]/Hα ratios of X-ray-bright superbubbles are stronglydependent on the UV radiation field of the encompassed OB associations.Therefore, a tight correlation between [S II]/Hα ratio and X-raysurface brightness in superbubbles should not exist. We also find thatthe filamentary morphologies of superbubbles are associated with largeexpansion velocities and bright X-ray emission.
| 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 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.
| The Multiphase Medium in the Interstellar Complex N44
We have obtained high-resolution H I observations of N44, one of thelargest H II complexes in the Large Magellanic Cloud. The distributionand internal motions of the H I gas show dynamic effects of fast stellarwinds and supernova blasts. Numerous H I holes are detected, with themost prominent two corresponding to the optically identifiedsuperbubbles Shell 1 and Shell 2. The H I gas associated with Shell 1shows an expansion pattern similar to that of the ionized gas shell, butthe mass and kinetic energy of the H I shell are 3-7 times those of theionized gas shell. The total kinetic energy of the neutral and ionizedgas of Shell 1 is still more than a factor of 5 lower than expected in apressure-driven superbubble. It is possible that the central OBassociation was formed in a molecular cloud, and a visible superbubblewas not fully developed until the ambient molecular gas had beendissociated and cleared away. This hypothesis is supported by theexistence of a molecular cloud toward N44 and the fact that the apparentdynamic age of the superbubble Shell 1 is much shorter than the age ofits OB association LH 47. Accelerated H I gas is detected at SNR0523-679. The mass and kinetic energy in the associated H I gas are alsomuch higher than those in the ionized gas of SNR 0523-679. Studies ofinterstellar gas dynamics using ionized gas alone are clearlyinadequate; neutral gas components must be included.
| LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.
| Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud
We estimate the total predicted Lyman continuum emission rates of OBassociations for which the complete census of O star spectral typesexists. The results are compared to the observed H-alpha luminosities ofthe host H II regions. We find evidence for substantial leakage ofionizing photons from some H II regions, while others appear to beradiation-bounded. We estimate that overall for the LMC, 0-51 percent ofthe ionizing radiation escapes the local nebulae, and would be availableto ionize the diffuse, warm, ionized medium (WIM) in that galaxy. Thisrange of values is consistent with the observed 35 percent fraction ofH-alpha luminosity emitted by the WIM in the LMC, as well as thecorresponding fractions observed in other nearby galaxies. It istherefore possible that photoionization by O stars is indeed thedominant ionization mechanism for the WIM.
| An HI aperture synthesis mosaic of the Small Magellanic Cloud
We present the results of a survey of neutral hydrogen emission in theSmall Magellanic Cloud (SMC) with the Australia Telescope Compact Array(ATCA). The survey consists of a mosaic of 320 separate pointings of the375-m array, resulting in a resolution of 1.6 arcmin (28 pc, for adistance of 60 kpc) over a field of 20 deg^2. The rms brightnesstemperature sensitivity is 1.4 K, corresponding to an Hi column densitysensitivity of 4x10^18 cm^-2 for each velocity channel of width1.6kms^-1. The Hi distribution is complex and, on scales <~1 kpc,appears to be dominated by the effects of expanding Hi shells, which areprobably driven by the combined effects of supernovae and stellar windsfrom massive stars. The picture of the SMC that arises from the currentdata seems to challenge the earlier belief that the SMC consists of twoor more spatially separate structures with different systemicvelocities. We find that the observed multiple components are, in manycases, caused by the combined effects of the numerous shells andsupershells. Altogether, we identify six supershells (defined here asthose with radii greater than 300 pc) and 495 giant shells. For each ofthese, we measure positions, radii, velocities and expansion rates, andderive ages and kinetic energy requirements. The apparent agedistribution of shells is remarkably narrow, with a mean age of 5.4Myrand an intrinsic dispersion of 2Myr. Southern shells appear to be older,on average, by 2.5Myr. The kinetic energy of the shells is a largefraction of the gravitational binding energy of the SMC, implying thatfurther disintegration of the SMC will occur with time, and especiallyat the next close passage with the Large Magellanic Cloud (LMC) or theGalaxy, unless the SMC possesses a massive halo. Because of theirinterferometric nature, the images presented here are insensitive tostructures of size >~0 deg.6, and should not be used for derivingtotal Hi column densities.
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| The Stellar Content and Dynamics of Superbubbles in the Large Magellanic Cloud
The strong stellar winds and supernovae of the most massive starsdominate the energy transfer from the stellar to the gaseous componentof normal galaxies. This interaction is therefore a fundamental processdetermining the structure and composition of the interstellar medium(ISM), and driving star formation and galaxy evolution. Hence, theshells and bubbles that are the obvious manifestation of this phenomenonare key in understanding these processes. This work examines the stellarcontent and resulting dynamics of superbubbles primarily in the LargeMagellanic Cloud (LMC). In Oey & Massey (1994), we study tworemarkable, giant (130 pc diameter) H-alpha bubble nebulae in M33, andspectroscopically identify a single, dominant O9 star centrally locatedin each. We compare the nebular parameters with those predicted by thestandard, analytic Weaver et al (1977) model for wind-blown bubbles,taking into account the evolution of wind power from the parent stars.We show that this evolution of wind power is important in the bubbleevolution. The agreement with the model is reasonable, suggesting thatthese objects provide a compelling record of the effect that individual,normal OB stars have on the interstellar medium. The study of suchnebulae can provide insight on the evolution of the parent star and itswinds. In Oey & Massey (1995), we examine the stellar population ofan OB association, LH 47/48, which is associated with a superbubble H IIregion, DEM 152 in the N44 nebular complex of the LMC. With CCDphotometry and spectroscopy of the massive stars, we find no evidencethat an unusual stellar population gave rise to the shell morphology ofthe gas. The slope of the initial mass function (IMF), Gamma=-1.3 +-0.2, is consistent with that of other OB associations in the LMC, andthere is no significant difference in the IMF internal or external tothe supershell. The inferred stellar ionizing flux is consistent withthe observed nebular H-alpha flux. We do find evidence for propagatingstar formation: the H-R diagram suggests an age of >~ 10 Myr for thepopulation interior to the bubble with more recent, >~ 5 Myr, starformation on the exterior. Using the detailed data on the stellarpopulation, we compare a numerical form of the Weaver et al. (1977)evolutionary model for wind-driven bubbles with the observed shellkinematics, finding a substantial discrepancy: the observed shell radiusis too small, and/or expansion velocity too large to be explained withthis version of the model. We discuss possible explanations for theinconsistency. Oey (1996a) presents $UBV$ photometry of the stellarpopulations associated with 7 superbubble nebulae and 5 classical H IIregions in the LMC. Although the nebular morphology of the superbubblesappears to be substantially evolved compared to the classical nebulae,the color-magnitude diagrams do not reveal any noticeable correlationbetween the resident stellar population and nebular morphology. Oey(1996b) examines the stellar population enclosed within 6 of thesuperbubbles and compare these clusters with previously studied OBassociations in classical H II regions. The H-R diagrams, constructedwith spectral classifications of the most massive stars, do not revealany systematic differences between OB associations resident withinsuperbubbles and classical nebulae: the main-sequence turnoffs showstars as massive and luminous as those in classical H II regions.Assuming the superbubble structures result from the stellar winds and/orsupernovae of the associations, the similarity of the stellarpopulations to those of classical H II regions implies that the shellformation timescale is somewhat shorter than the cluster evolutionarytimescale for these objects. The stellar winds and/or supernovae of theone or two most massive stars must therefore dominate the formation ofthe superbubbles. The star-forming events for the superbubbleassociations are also no more extended in duration than that of other OBassociations. Finally, the IMF slopes are not systematically differentfrom those previously found. Since the OB associations withinsuperbubbles appear normal, the shell structures must be the result ofnormal OB stellar influences. I also present a few spectrograms ofinteresting massive stars, including S149, a probable new B[e]supergiant. Based on the stellar populations observed within this sampleof LMC superbubbles, Oey (1996c) uses the numerical version of thestandard, pressure-driven bubble model (Oey & Massey 1995) toinvestigate the shell dynamics. The results fall into two distinctcategories corresponding to a subset of objects for which the observedexpansion velocity is too large for the observed shell radius("high-velocity" superbubbles), and a subset of objects which appearsmore dynamically consistent with the model ("low-velocitysuperbubbles"). Both subsets of objects imply an overestimate in theshell growth rate equivalent to an overestimate in input power by up toan order of magnitude. The high-velocity objects exhibit X-ray evidenceof supernova activity, suggesting that the dynamical discrepancy is dueto acceleration by SNR impacts. (SECTION: Dissertation Summaries)
| UBV Photometry of OB Associations within Superbubbles of the Large Magellanic Cloud
This work presents UBV photometry of the stellar populations associatedwith seven superbubble nebulae and five classical H II regions in theLarge Magellanic Cloud. Although the nebular morphology of thesuperbubbles appears to be substantially evolved compared to theclassical nebulae, the color-magnitude diagrams do not reveal anynoticeable correlation between the resident stellar population andnebular morphology. The photometry presented here will be used in aforthcoming paper to examine further the stellar content and dynamics ofthese superbubbles.
| Morphology and stellar content of complexes in the LMC.
Three LMC stellar aggregates and two LMC stellar complexes locatedinside the constellations Shapley I, IV, IX, and X have been examined inorder to study their morphology and properties. Star counts wereperformed on excellent quality film copies of direct plates taken withthe 1.2m U.K. Schmidt telescope. They have been used for derivingisodensity contour mapping of the four studied regions. Low dispersionobjective prism plates taken with the same telescope were also used toclassify the spectra of the stars down to M_V_~0.0mag. Combination ofthe two sets of data was used to define the boundaries of these regions,their age, the density and the spatial distribution of their OB stars.It is therefore found that the bright and massive OB type stars are thepredominant stellar component of the four studied regions. They areembedded in a fainter and less massive stellar component within theboundaries of a region, revealed by the isopleths, where the limitingdetection magnitude is down to M_V_~1.5mag. Thus it appears that thestellar content of the complexes and aggregates is made not only by thestars as massive as ~40Msun_, but they also contain lowermass stars, at least ~3Msun_. The spatial distribution ofearly type stars (down to M_V_~0.0mag) shows a gradient which reveals aregion coinciding with the one, defined by the isopleths, where fainterstars are also located. For the near future, we plan to study whetherthe gradient of the radial distribution of the early type starsrepresents the mass distribution of the molecular cloud from which thesestellar structures are formed, or it is due to sequential star formationprocess and/or expansion because of the high stellar winds of the verymassive stars.
| 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.
| Triggered Star Formation and the Dynamics of a Superbubble in the LMC: The OB Association LH 47/48 in DEM 152
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...452..210O
| Massfunctions in the Large Magellanic Cloud.
Not Available
| Stellar associations in the Large Magellanic Cloud. I. LH 15, LH 47, 48, 49, LH 52, 53, LH 83, LH 91, 95.
Five LMC stellar groups of stellar associations around LH 15, 47, 52,83, 91 have been examined in order to define their boundaries, theirstellar content and their dynamical behaviour. Spectral classificationof individual stars from objective prism spectra and star count indirect plates taken with the 1.2m U.K. Schmidt Telescope in Australiahave provided the observational material for this investigation.Isodensity contour maps and the distribution of spectral types reveals15 associations (some are newly detected) that contain sometimes apartfrom the OB type stars, faint A type stars and late type supergiantsgiving evidence for a continuous star formation. Their central massdensity indicates that all of them are unbound systems with disruptiontimes not longer than their age derived from their stellar population'sevolutionary stage.
| Supernova remnants in the Large Magellanic Cloud
Twelve supernova remnants in the Large Magellanic Cloud have now beenobserved with the Australia Telescope. These were all imaged in totalintensity and where possible the linear polarization was also mapped. Inmany respects this survey is similar to previous single-dishobservations of Galactic supernova remnants, and comparisons are madewith these results. Preliminary images are shown for several sources.The survey is continuing with additional array configurations and atother wavelengths.
| 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 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.
| The expansive motions of the giant shells in the N44 (DEM 150, 151, 152) interstellar complex in the Large Magellanic Cloud
Eechelle observations of H-alpha profiles reveal that the near-side ofthe primary 70 x 50 pc diameter giant shell in the N44 complex (Shell 1)is expanding towards the observer at 46 km/s and the far-side isexpanding away at 33 km/s with respect to 307 km/s heliocentric radialvelocity. Two components with heliocentric radial velocities at 287 and307 km/s are found in the Na I interstellar absorption line profiles inthe light of stars in the Ob association, LH47, enclosed by Shell 1.These components are shown to originate in the extensive clouds of theLMC gas out of which the N44 complex formed. Four independent methodsare used to show that a Lyman photon rate from equivalent to 10 + or - 5O5-type supergiant stars is needed to radiatively ionize the N44complex. If the supernova rate in LH 47 is proportional to the Lymanflux to give 280 + or - 140 explosions in 3.5 x 10 to the 6th yr, theShell 1 must be a momentum-conserving snowplough.
| 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.
| A dynamical approach to the study of the bright young associations in the Large Magellanic Cloud
A model is presented and discussed in order to investigate how thedynamical properties of a young globular cluster depend on the detailsof the process of formation. Several parameters are constrained byapplying the model to some massive associations. Values of a number ofobservables are obtained. The approach provided in the paper shows how,qualitatively, the evolution of a newly formed object, after the gascloud expulsion, is affected by the features of the physical environmentwherein the formation process takes place.
| 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.
| Studies of massive stars in the Magellanic Clouds. I - New spectral classifications of OB types in the LMC
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986AJ.....92...48C&db_key=AST
| Age determination of extragalactic H II regions
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
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