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Discovery of the Pre-Main-Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope Advanced Camera for Surveys Observations
We report the discovery of an extraordinary number of pre-main-sequence(PMS) stars in the vicinity of the stellar association LH 95 in theLarge Magellanic Cloud (LMC). Using the Advanced Camera for Surveys onboard the Hubble Space Telescope in wide-field mode, we obtained deephigh-resolution imaging of the main body of the association and of anearby representative LMC background field. These observations allowedus to construct the color-magnitude diagram (CMD) of the association inunprecedented detail and to decontaminate the CMD for the average LMCstellar population. The most significant result is the direct detectionof a substantial population of PMS stars and their clustering propertieswith respect to the distribution of the higher mass members of theassociation. Although LH 95 represents a rather modest star-formingregion, our photometry, with a detection limit of V<~28 mag, revealsin its vicinity more than 2500 PMS stars with masses down to ~0.3Msolar. Thus, our observations offer a new perspective on atypical LMC association: the stellar content of LH 95 is found to extendfrom bright OB stars to faint red PMS stars, suggesting a fullypopulated initial mass function from the massive blue giants down to thesubsolar mass regime.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.Research supported by the German Research Foundation (DeutscheForschungsgemeinschaft) and the German Aerospace Center (DeutscheZentrum für Luft und Raumfahrt).
| The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. I. Photometry
We present a photometric study of the star-forming region NGC 346 andits surrounding field in the Small Magellanic Cloud, using data takenwith the Advanced Camera for Surveys (ACS) on board the Hubble SpaceTelescope (HST). The data set contains both short and long exposures forincreased dynamic range, and photometry was performed using the ACSmodule of the stellar photometry package DOLPHOT. We detected almost100,000 stars over a magnitude range of V~11 to V~28 mag, including allstellar types from the most massive young stars to faint lowermain-sequence and pre-main-sequence stars. We find that this region,which is characterized by a plethora of stellar systems and interestingobjects, is an outstanding example of mixed stellar populations. We takeinto account different features of the color-magnitude diagram of allthe detected stars to distinguish the two dominant stellar systems: Thestellar association NGC 346 and the old spherical star cluster BS 90.These observations provide a complete stellar sample of a field about5'×5' around the most active star-formingregion in this galaxy. Considering the importance of these data forvarious investigations in the area, we provide the full stellar catalogfrom our photometry. This paper is the first part of an ongoing study toinvestigate in detail the two dominant stellar systems in the area andtheir surrounding field.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| 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.
| The Low-Mass Initial Mass Function of the Field Population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent HST WFPC2 stellar photometry of an areain the Large Magellanic Cloud (LMC), located to the west of the bar ofthe galaxy, which accounts for the general background field of its innerdisk. The WFPC2 observations reach magnitudes as faint as V=25 mag, andthe large sample of more than 80,000 stars allows us to determine indetail the present-day mass function (PDMF) of the detectedmain-sequence stars, which is identical to the initial mass function(IMF) for masses M<~1 Msolar. The low-mass main-sequencemass function of the LMC field is found not to have a uniform slopethroughout the observed mass range; i.e., the slope does not follow asingle power law. This slope changes at about 1 Msolar tobecome more shallow for stars with smaller masses down to the lowestobserved mass of ~0.7 Msolar, giving clear indications offlattening for even smaller masses. We verified statistically that forstars with M<~1 Msolar the IMF has a slope Γ around-2, with an indicative slope Γ~=-1.4 for0.7<~M/Msolar<~0.9, while for more massive stars themain-sequence mass function becomes much steeper with Γ~=-5. Themain-sequence luminosity function (LF) of the observed field is in verygood agreement with the Galactic LF as it was previously found. Takinginto account several assumptions concerning evolutionary effects, whichshould have changed through time the stellar content of the observedfield, we reconstruct qualitatively its IMF for the whole observed massrange (0.7<~M/Msolar<~2.3), and we find that the numberof observed evolved stars is not large enough to have affectedsignificantly the form of the IMF, which thus is found almost identicalto the observed PDMF.
| The Low-Mass Pre-Main-Sequence Population of the Stellar Association LH 52 in the Large Magellanic Cloud Discovered with Hubble Space Telescope WFPC2 Observations
We report on the serendipitous discovery of ~500 low-mass candidatepre-main-sequence (PMS) stars in the vicinity of the stellar associationLH 52 in the Large Magellanic Cloud. We present evidence that the redfaint sequence of these stars seen in the CMD of LH 52 from Hubble SpaceTelescope (HST) WFPC2 observations belongs only to the association andfollows almost perfectly isochrone models for PMS stars of masses downto ~0.3 Msolar. We find that this feature has a Galacticcounterpart and that the mass spectrum of the candidate PMS stars in LH52 seems to correspond to a Salpeter initial mass function with a slopeΓ~=-1.26 in the mass range 0.8-1.4 Msolar.
| High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter 12CO J=4-->3emission in the Magellanic Clouds. The survey is comprised of 156'×6' maps obtained using the AST/ROtelescope toward the molecular peaks of the Large and Small MagellanicClouds. We have used these data to constrain the physical conditions inthese objects, in particular, their molecular gas density andtemperature. We find that there are significant amounts of warmmolecular gas associated with most of these molecular peaks and thathigh molecular gas temperatures are pervasive throughout our sample. Wediscuss whether this may be due to the low metallicities and theassociated dearth of gas coolants in the Clouds and conclude that thepresent sample is insufficient to assert this effect.
| 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.
| The Initial Mass Function toward the Low-Mass End in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent Hubble Space Telescope (HST) WFPC2photometry of two areas in the Large Magellanic Cloud: the southern partof the stellar association LH 52, located on the western edge of thesupershell LMC 4, and a field between two associations, which is locatedon the southwestern edge of the shell and accounts for the generalbackground field of the galaxy. The HST WFPC2 observations reachmagnitudes as faint as V=25 mag, much deeper than have been observedearlier in stellar associations in the LMC. We determine the massfunction (MF) for main-sequence stars in the areas. Its slope in bothareas is steeper for stars with masses M<~2 Msolar(-4<~Γ<~-6) than for stars of M>~2 Msolar(-1<~Γ<~-2). Thus, as far as the field of the LMC isconcerned, the MF does not have a uniform slope throughout its observedmass range. The MF of the general field of the LMC was found previouslyto be steeper than the MF of a stellar association for massive starswith M>~5 Msolar. We conclude that this seems to also bethe case toward lower masses down to M~1 Msolar. Our dataallow us to construct the field-subtracted, incompleteness-corrected,main-sequence MF of the southwestern part of the young stellarassociation LH 52, which accounts for the initial mass function (IMF) ofthe system. Its mean slope is found to be comparable to, but moreshallow than, a typical Salpeter IMF (Γ~=-1.12+/-0.24) for massesdown to ~1 Msolar. We found indications that the IMF of theassociation probably is ``top heavy,'' owing to the large number ofintermediate-mass stars in the field of the system, while the generalLMC field is found to be responsible for the low-mass population, withM<~2 Msolar, observed in both fields. This findingsuggests that the local conditions seem to favor the formation of highermass stars in associations, and not in the background field. No evidencefor flattening of the IMF toward the low-mass regime or for a lower masscutoff in the IMF was detected in our data.
| The deepest Hubble Space Telescope far-ultraviolet observations in the Large Magellanic Cloud
We present a new analysis of the deepest pure-ultraviolet (UV)observations with the highest angular resolution ever performed. A setof 12 exposures with the Hubble Space Telescope (HST) WFPC2 and F160BWfilter obtained in parallel observing mode, which cover ~12arcmin2 in the Large Magellanic Cloud (LMC), north of the barand in the `general field' region of the LMC, contain stars with far-UVmonochromatic magnitudes as faint as 22 mag. The 198 detected UV sourcesrepresent an accumulated exposure of >= 2 × 104 sand reveal stars as faint as mUV~= 20 mag. We combine theseobservations with deep UBVI charge-coupled device (CCD) imaging of thesame region reaching as faint as V~= 26 mag, and reselect probableoptical counterparts for the UV sources. After a two-stagesearch-and-analysis process, we detect robust counterparts for 129stars. These are mostly upper main-sequence stars, from early B to earlyA spectral classes, with several F stars. We point out the lack of bluesupergiants, which could have been easily detected in our survey. Wemeasure a foreground extinction E(B-V) ~= 0.08 mag by Galactic dust anda surface density of star formation rate twice the average Galacticvalue. These observations indicate that relatively recent star formationtook place even off the bar of the LMC.
| Interacting supergiant shells in the Large Magellanic Cloud
We seek evidence for secondary star formation in the Large MagellanicCloud (LMC) arising in gas compressed by the interactions between HIsupergiant shells (SGS) identified from the Australia Telescope CompactArray (ATCA) survey of Kim et al., using mid-infrared (MIR) images fromthe Midcourse Space Experiment (MSX), and radio continuum images fromthe Molonglo Observatory Synthesis Telescope (MOST). We identify a densephotodissociation region shaped like a bar that lies between the rims ofLMC 4 and LMC 5 as a strong candidate for this process, and argue thatit has produced the young associations LH 52 and 53 in the densestarfield of NGC 1948 within the past few Myr. No obvious stellarcandidate matches the coincident 8.3-, 21.3-μm, and 843-MHz peaks inthis bar, although this bright spot lies between two nebulous Hαpatches. At least one other such HI-MIR bar exists in the LMC so thatthis mode of star formation is unlikely to have been a uniqueoccurrence. We have found a correlation between 8.3-μm radiance andHI column density for the LMC in general that can be predicted from theSchmidt law for spiral galaxies.
| 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.
| 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
| Three stellar associations and their field east of LMC 4 in the Large Magellanic Cloud
We present BV R photometry in an area of 20.5' x 20.5' centered on LH 95situated to the north-east of the super-bubble LMC 4. We investigate thestellar content of three stellar associations (LH 91, LH 91-I & LH95) and their surrounding fields. Our observations use the R-Hαcolour index to identify the Be star population of the region. We findthat Be stars exist in all three of the investigated associations.Within LH 95 we find a central cluster of four Be stars which stronglydetermine the HII emissivity in this area. We estimated the reddeningand the age of the systems based on isochrone fitting. The reddening wasfound to vary between E(B-V) =~ 0.15 and 0.20 mag. All systems werefound to be younger than 10 Myr, while the field is older than ~ 50 Myr.We also present the luminosity and mass functions of the systems, aswell as that of the field. It was found that the luminosity functionslope s of the field is steeper than that of the systems, which werefound to be s =~ 0.12-0.32. The MF slopes were estimated for bothsystems and field by directly counting stars between evolutionarytracks. We verify that the MF slopes of the systems are rather shallowerthan the ones of the field. The MF slopes of the systems lie in therange -1.0 < Gamma < -2.0, while those of various fields aresignificantly steeper, around Gamma =~ -4.0. LH 95 was found to beprobably under disruption. We discuss the possibility that thisassociation is in the process of dissipation.
| 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.
| A CO Survey of the LMC with NANTEN: II. Catalog of Molecular Clouds
From a 12CO (J = 1 -- 0) survey with the NANTEN telescope, wepresent a complete catalog of giant molecular clouds (GMCs) in the LargeMagellanic Cloud. In total, 107 CO clouds have been identified, 55 ofwhich were detected at more than 3 observed positions. For the 55clouds, the physical properties, such as size, line-width, virial mass,and CO luminosity, are cataloged. From a statistical analysis of thesequantities, we show that GMCs in the LMC are close to gravitationalequilibrium. A comparison with HI data indicates that most ofthe CO clouds are distributed in dense parts of HI gas, whoseHI column density is greater than 1021cm-2. It is notable that the mass ratio of themolecular-to-atomic hydrogen of the lower radial velocity component ofgas is ~ 0.2, which is a factor of 2 higher than that of thegaseous-disk component, ~ 0.1. Molecular clouds are apparently formedefficiently in the parent atomic clouds in the lower velocity component.The CO Arc, which is a few kpc scale ordered structure of CO cloudsalong the southern optical edge of the galaxy, corresponds well to thethe lower velocity component of HI gas.
| Arc-Shaped and Spheroidal Stellar Complexes
Complexes of young clusters and high-luminosity stars in the shape ofregular, circular arcs have been found in a number of galaxies, firstand foremost the LMC, NGC 6946, and M83. These shapes are found even instrongly inclined galaxies, suggesting that the observed arcs areprojections of partial spherical shells. Obviously, these stellar shellsmust have formed from gaseous shells swept up by some source of centralpressure and become gravitationally unstable. The power of this sourcecorresponds to several dozen supernova explosions; however, its natureremains unclear. A central cluster providing a source of O stars andsupernovae is usually absent. The presence of multiple arcs locatedclose to each other can be explained by the fall of a swarm of fragmentsor by the progenitor stars originating in a single peculiar starcluster, implying the existence of stellar objects capable of givingrise to explosions with energies an order of magnitude higher than thoseof individual supernovae. The same objects may be responsible forgamma-ray bursts. It may be that only the most massive clusters withfrequent or especially powerful supernova explosions are capable ofproducing HI supershells. Otherwise, it is impossible to explain why nosupershells have been found around numerous clusters that should becapable of producing them according to current theories. The presence ofstar clusters in shell-like structures provides extremely importantinformation about the physical conditions in and the ages of the initialgaseous shells, making stellar arcs the best available laboratory forstudies of triggered star formation.
| The open cluster Havlen-Moffat No. 1 revisited
A deep CCD UBVRI photometric survey combined with UBVRI polarimetricobservations of 21 bright stars was carried out in the region of theopen cluster Havlen-Moffat No. 1. Our data reveal that the extinctionlaw in this cluster is variable and that six cluster stars show veryhigh polarisation values (>4%), probably because of the presence of anearby small dust cloud. The cluster is at a distance of d = 3300 pc, itis 2-4 Myr old and the initial mass function of its most massive stars(M > 3 Msun ) has a flat slope of x~ 0.7. As an additionalresult, it was possible to reconcile the absolute magnitudes of the twoWN7-type members using the R-values valid in the regions where they arelocated. Based on observations collected at the University of TorontoSouthern Observatory, Las Campanas, Chile, and the ComplejoAstronómico El Leoncito, (CASLEO), Argentina, operated underagreement between the Consejo Nacional de InvestigacionesCientíficas y Técnicas de la República Argentinaand the National Universities of La Plata, Córdoba and San Juan.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/371/908.
| 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.
| Differences in the fractions of Be stars in galaxies
The number ratios Be/(B+Be) of Be to B-type stars in young, well studiedclusters of the Galaxy, the LMC and SMC are examined. In order todisentangle age and metallicity effects we choose clusters in the sameage interval and for which reliable photometric and spectroscopic dataare available. Number counts are made for various magnitude intervals,and the results are found to be stable with respect to this choice. Inthe magnitude interval MV = -5 to -1.4 (i.e. O9 to B3) weobtained a ratio Be/(B+Be) = 0.11, 0.19, 0.23, 0.39 for 21 clusterslocated in the interior of the Galaxy, the exterior of the Galaxy, theLMC and the SMC, respectively. Various hypotheses for these differencesare examined. An interesting possibility is that the average rotation isfaster at low metallicities as a result of star formation processes. Themuch higher relative N-enrichment found by Venn et al. (\cite{vencar})in A-type supergiants of the SMC, compared to galactic supergiants, alsostrongly supports the presence of more rotational mixing at lowmetallicities. We discuss whether high rotational mixing may be thesource of primary nitrogen in the early chemical evolution of galaxies.
| Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
We have analyzed both high-resolution optical absorption-line spectraand UV spectra obtained with IUE of the LMC SN 1987A, in order todetermine abundances and physical conditions in the various neutralinterstellar clouds along the line of sight to the supernova (SN). Wehave used a flat-fielding procedure to enhance the signal-to-noiseratios (S/Ns) and the reliability of weak features in the UV spectra andhave modeled the UV line profiles using the component structure derivedfrom the higher resolution, high-S/N optical spectra of Ca II and Na I.Fits to the Ca II, Ca I, and Na I absorption-line profiles reveal (atleast) 46 components, at velocities -24 km s^-1<~v_solar<~296 kms^-1, which can be associated with the 10 component groups discerniblein the lower resolution UV spectra. From the UV spectra, we determinedcomponent-group column densities for C I, Mg I, Mg II, Al II, Si II, PII, Cl I, Ti II, Cr II, Mn II, Fe II, Ni II, and Zn II-with 1 sigmauncertainties less than 0.1 dex in many cases. These are the mostextensive and accurate abundances yet measured for the neutral ISM inthe LMC. The component velocities, the patterns of relative elementalabundances [X/Zn] and [X/Fe], and various diagnostic ratios have beenused to estimate the locations and physical characteristics [N(H), T,n_e] of these component groups. (Systematic differences among thediagnostic ratios make the derived physical properties somewhatuncertain, however.) The components at low velocities (5 kms^-1<~v<~23 km s^-1) have relative abundances and values for thediagnostic ratios very similar to those found for warm, diffuse Galacticdisk clouds and likely are due to a mixture of warm and cool gas in theGalactic disk. The components at velocities 56 km s^-1<~v<~90 kms^-1 are due to a mixture of warm and cool gas, apparently withnegligible depletions, in the Galactic halo. The twointermediate-velocity component groups (109 km s^-1<~v<~140 kms^-1 and 155 km s^-1<~v<~176 km s^-1) both have relativeabundances similar to those found for Galactic halo clouds. These warm(T>~4500 K), partially ionized clouds are probably located in theGalactic halo and in the LMC, respectively. The components at velocities191 km s^-1<~v<~225 km s^-1 also have relative abundances similarto those in the halo clouds but are likely due to gas in the LMC,perhaps very close to the SN. The component groups at 238 kms^-1<~v<~255 km s^-1 and 265 km s^-1<~v<~270 km s^-1 areprobably located on opposite sides of the main LMC component group (atvelocities 275 km s^-1<~v<~296 km s^-1) (using absorption-linedata for several other adjacent lines of sight and the structureinferred from SN light-echo observations). Although the relativeabundances and diagnostic ratios for those three LMC groups are similarto those found for warm, low-density Galactic disk clouds, the widths ofindividual components seen in very high resolution spectra of Na I and KI imply that T is generally less than about 1500 K. Higher N(Na I)/N(CaII) ratios, the presence of CH, and the C I fine structure levelpopulations suggest that the main LMC group contains both cool and warmgas. For the LMC components, the total N(H) estimated from the observedrelative abundances and inferred depletions is consistent with the valueobtained from Lyalpha absorption toward the neighboring star Sk -69deg203, after accounting for differences in reddening and for an overallsubsolar metallicity of 0.2-0.3 dex for the LMC ISM. Since the relativeabundance patterns determined for stars and gaseous nebulae in both theSMC and the LMC appear to be similar to the solar pattern (for theelements whose interstellar abundances we have considered), thesimilarities in relative gas-phase interstellar abundances in our Galaxyand in the Magellanic Clouds suggest that the dust depletions followsimilar patterns as well-despite differences in metallicity anddust-to-gas ratio among the three galaxies. These local relativeabundance/depletion patterns may thus be used to infer total (gas+dust)abundances for QSO absorption-line systems at various redshifts.
| Be stars in and around young clusters in the Magellanic Clouds
We present the results of a search for Be stars in six fields centeredon the young clusters NGC 330 and NGC 346 in the SMC, and NGC 1818, NGC1948, NGC 2004 and NGC 2100 in the LMC. Be stars were identified bydifferencing R band and narrow-band Hα CCD images. Ourcomparatively large images provide substantial Be star populations bothwithin the clusters and in their surrounding fields. Magnitudes,positions and finding charts are given for the 224 Be stars found. Thefraction of Be stars to normal B stars within each cluster is found tovary significantly although the average ratio is similar to the averageBe to B star ratio found in the Galaxy. In some clusters, the Be starpopulation is weighted to magnitudes near the main sequence turn-off.The Be stars are redder in V-I than normal main-sequence stars ofsimilar magnitude and the redness increases with increasing Hαemission strength. Tables 3 to 8 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Far-Ultraviolet Imaging of the Field Star Population in the Large Magellanic Cloud with the Hubble Space Telescope
We present an analysis of the deepest pure UV observations with thehighest angular resolution ever performed, a set of 12 exposures withthe Hubble Space Telescope (HST) Wide Field Planetary Camera 2 andF160BW filter obtained in parallel observing mode, which covers ~12arcmin^2 in the Large Magellanic Cloud (LMC), north of the bar, and inthe ``general field'' regime of the LMC. The 341 independentmeasurements of 198 objects represent an accumulated exposure of>=2x10^4 s and reveal stars as faint as m_UV~=22 mag. Theobservations show that about two-thirds of the UV emission from the LMCis emitted by our HST-detected UV stars in the field, that is, not inclusters or associations. We identified optical counterparts in theRoyal Observatory Edinburgh/Naval Research Laboratory photometriccatalog for about one-third of the objects. The results are used todiscuss the nature of these UV sources, to estimate the diffuse UVemission from the LMC as a prototype of dwarf galaxies, and to evaluatethe contamination by field stars of UV observations of globular and openclusters in the LMC. We find that the projected density of UV stars inthe general field of the LMC is a few times higher than in the Galacticdisk close to the Sun. Combining our data with observations by theUltraviolet Imaging Telescope allows us to define the stellar UVluminosity function from m_UV=8-18 mag and to confirm that the fieldregions in the LMC have been forming stars at a steady rate during thelast 1 Gyr, with an initial mass function close to the Salpeter law.
| Young clusters in the Magellanics Clouds.
Not Available
| 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.
| 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.
| Magellanic Cloud Cepheids - Abundances
We have determined parameters and abundances for three SMC and nine LMCCepheids that have not been previously studied spectroscopically formetallicity, as well as three SMC and one LMC Cepheids observedpreviously by Luck and Lambert. This work nearly doubles the number ofCepheids with spectroscopic metallicities in the SMC and triples thenumber in the LMC. For Galactic reference stars we have determined theabundances of 11 supergiants of spectral type F0 to K1. These abundancesare used herein to examine the dispersion in cosmic (Fe/H) values foundin LMC Cepheids by Luck and Lambert. The Magellanic Cloud Cepheids tendto be longer-period Cepheids, and thus have physical gravities that are1.0 dex and lower. Spectroscopic gravities are even lower. Fortunately,the (Fe/H) content as determined from Fe I is not strongly dependent ongravity and is thus well determined. We combine our abundances withother published high-resolution spectroscopic results, deriving a mean(Fe/H) for the SMC of -0.68. The total range seen in the (Fe/H) ratiosderived herein is -0.84 to -0.65. This result is in agreement withprevious studies showing a small intrinsic range in the metallicities inthe SMC. For the LMC, from the Cepheids studied here we obtain -0.30,with a range of -0.55 to -0.19. Combining all available data, we obtaina mean (Fe/H) of -0.34. From this analysis, there is little evidence fora significant metallicity dispersion in the LMC.
| Bow-shock induced star formation in the LMC?
The structure of supergiant shells, in particular of LMC 4, is hard toexplain with stochastic self-propagating star formation. A series ofsupergiant structures lies along the outer edge of the LMC and form asequence increasing clockwise in age. We have considered the rotation ofthe LMC and its motion through the halo of the Milky Way and proposethat these structures find their origin in star formation induced in thebow-shock formed at the leading edge of the LMC. Due to the rotation ofthe LMC these structures then move aside.
| H_2 and its relation to CO in the LMC and other magellanic irregular galaxies
H_2 column densities towards CO clouds in the LMC and SMC are estimatedfrom their far-infrared surface brightness and HI column density. Thenewly derived H_2 column densities imply N(H_{2)}/I(CO) conversionfactors (in units of 10(21) mol cm(-2) (K km s(-1) )(-1) ) X_LMC =1.3+/-0.2 and X_SMC = 12+/-2. LMC and SMC contain total (warm) H_2masses of 1.0+/-0.3 x 10(8) Msun and 0.75+/-0.25 x 10(8)Msun respectively. Local H_2/HI mass ratios similar to thosein LMC and SMC are found in the magellanic irregulars NGC 55, 1569,4214, 4449 and 6822 and in the extragalactic HII region complexes NGC604, 595 and 5461 in M 33 and M 101 respectively. In these HII regionsand in NGC 4449, we find X = 1-2; in NGC 55, 4214 and 6822 X = 3-6 againin units of 10(21) mol cm(-2) (K km s(-1) )(-1) . The post-starburstgalaxy NGC 1569 has a very high value similar to that of the SMC. TheCO-H_2 conversion factor X is found to depend on both the ambientradiation field intensity per nucleon {sigma _FIR/{N_H and metallicity[O]/[H]: log X ~ 0.9+/-0.1 logfrac {sigma _FIR{N_H - 3.5+/-0.2log([O])/([H]). Neglecting dependency on radiation field, a reasonableapproximation is also provided by log X ~ -2.7+/-0.3 log([O])/([H]).Milky Way values are consistent with these relations. This result isinterpreted as the consequence of selective photodissociation of COsubjected to high radiation field energy densities and poor(self)shielding in low-metallicity environments, and especially thepreferential destruction of diffuse CO in `interclump' gas. Althoughlocally H_2 may be the dominant ISM-component, the average global H_2/HImass ratio is 0.2+/-0.04 and the average H_2 gas mass fraction is0.12+/-0.02. Magellanic irregulars have warm molecular gas fractionsvery similar to those of our Galaxy, whereas other global properties(mass, luminosity, metallicity, CO luminosity) are very different.
| No stellar age gradient inside supergiant shell LMC4
The youngest stellar populations of a 'J'-shaped region inside thesupergiant shell (SGS) LMC4 have been analysed with CCD photometry in B,V passbands. This region consists of 2 coherent strips, one from theeast to the west reaching about 400;pc across the OB superassociationLH77 and another extending about 850;pc from south to north. Thestandard photometric methods yield 25 colour-magnitude diagrams (CMDs)which were used for age determination of the youngest star population byisochrone fitting. The resultant ages lie in the range from 9;Myr to16;Myr without correlation with the distance to the LMC4 centre. Wetherefore conclude that there must have been one triggering event forstar formation inside this great LMC SGS with a diameter of 1.4;kpc. Weconstruct the luminosity function and the mass function of five regionsconsisting of 5 fields to ensure that projection effects don't mask theresults. The slopes lie in the expected range (gamma in [0.22;0.41] andGamma in [-1.3;-2.4] respectively). The greatest values of the slopeoccur in the north, which is caused by the absence of a young,number-dominating star population. We have calculated the rate withwhich supernovae (SNe) have exploded in LMC4, based on the finding thatall stars are essentially coeval. A total of 5--7*10(3) supernovae hasdumped the energy of 10(54.5) ;erg over the past 10;Myr into LMC4, infact enough to tear the original star-forming cloud apart in the timespan between 5 and 8;Myr after the starformation burst. We conclude thatLMC4 can have been formed without a contribution from stochasticself-propagating star formation (SSPSF), although the ring of youngassociations and HiI\ regions around the edge have been triggered by theevents inside LMC4. Based on observations collected at the EuropeanSouthern Observatory (ESO), La Silla, Chile.
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