|Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar|
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.
|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.
|Lyman Continuum Extinction by Dust in H II Regions of Galaxies|
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.
|A 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.
|Near-infrared observations of galaxies in Pisces-Perseus. II. Extinction effects and disk opacity|
We study the correlations with inclination of H-band disk and bulgestructural parameters and I-H colour profiles for a sample of 154 spiralgalaxies, in order to detect possible effects due to internal extinctionby dust. The selection of the sample assures that galaxies at differentinclinations are not intrinsically different, so that the observedcorrelations represent the real behaviour of the parameter considered.All the parameters are derived from a bi-dimensional fitting of thegalaxy image. We find that extinction, though small at near infraredwavelengths, is sufficient to produce observable effects. In particularthe observed increase of the average disk scalelength and the reddeningof the disk I-H colour at high inclinations are clear signatures of thepresence of dust. The total H-band disk luminosity depends little oninclination; on the other hand significant corrections to the face-onaspect are derived for the H-band central disk brightness and the diskscalelength. The bulge parameters exhibit little or no dependence oninclination. Simulations carried out with a simple model for aninternally-extincted galaxy show that these results imply a centralH-band optical depth between 0.3 and 0.5.
|Two serendipitous low-mass LMC clusters discovered with HST1|
We present V and I photometry of two open clusters in the LMC down toV~26. The clusters were imaged with the Wide Field and Planetary Camera2 (WFPC2) on board the Hubble Space Telescope (HST), as part of theMedium Deep Survey Key Project. Both are low-luminosity (M_V~-3.5),low-mass (M~10^3 Msolar) systems. The chance discovery of these twoclusters in two parallel WFPC2 fields suggests a significantincompleteness in the LMC cluster census near the bar. One of theclusters is roughly elliptical and compact, with a steep light profile,a central surface brightness mu_V(0)~20.2 mag arcsec^-2, a half-lightradius r_hl~0.9 pc (total visual major diameter D~3 pc) and an estimatedmass M~1500 Msolar. From the colour-magnitude diagram and isochrone fitswe estimate its age as tau~(2-5)x10^8 yr. Its mass function has a fittedslope of Gamma=Deltalogphi(M)/DeltalogM=-1.8+/-0.7 in the range probed(0.9<~M/Msolar<~4.5). The other cluster is more irregular andsparse, having shallower density and surface brightness profiles. Weobtain Gamma=-1.2+/-0.4, and estimate its mass as M~400 Msolar. Aderived upper limit for its age is tau<~5x10^8 yr. Both clusters havemass functions with slopes similar to that of R136, a massive LMCcluster, for which HST results indicate Gamma~-1.2. They also seem to berelaxed in their cores and well contained in their tidal radii.
|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.
|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.
|A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.|
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
|A Search for Methanol Masers in the Magellanic Clouds|
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
|The initial mass function for massive stars in the Magellanic Clouds. 1: UBV photometry and color-magnitude diagrams for 14 OB associations|
UBV charge coupled device (CCD) photometry has been obtained for 14 OBassociations in the Magellanic Clouds using the University of Toronto's0.6 m telescope and the Carnegie Institution of Washington's 1.0 mreflector, both on Las Campanas, Chile. The data are presented and usedto construct color-magnitude diagrams for the purposes of investigatingthe massive-star content of the associations.
|A comparison of far infrared and H-alpha emission of H-II regions in the Magellanic Clouds|
From a comparison of the IRAS and smoothed H-alpha maps of theMagellanic Clouds, it was found that H-II regions with core-halostructure usually have higher F(60 microns)/F(H-alpha) ratios andprobably emit more in the far infrared than do extended low-density H-IIregions. This is consistent with the idea that the far infrared emissionis mainly produced by dust within H-II regions.
|Bar star clusters in the LMC - Formation history from UBV integrated photometry|
The sample of star clusters in the LMC Bar region with integrated UBVphotometry was enlarged by approximately a factor four, totaling 129objects. The (B-V) histogram gap between blue and red clustersdisappears with this deeper sample. Age groups in terms of equivalentSWB types were derived and their spatial distribution studied. Clustersyounger than t about 200 Myr are not homogeneously distributed throughthe bar. In particular a strong star forming event at t about 100 Myrwas detected in the eastern part of the Bar, consisting of a compactgrouping of seven coeval clusters around NGC 2058 and NGC 2065. Also, 11close pairs and two trios are analyzed, and the colors indicate thatonly four pairs are clearly not coeval.
|A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds|
|Extinction and reddening of H II regions in the Large Magellanic Cloud|
Absolute H-alpha and H-beta of H II regions in the Large MagellanicCloud are used in conjunction with radio continuum, 21-cm line, andstellar UBV data to study the dust associated with these regions.Various hypotheses concerning the optical properties and the spatialdistribution of the dust are examined, and the predicted characteristicsare compared with the observations. Uniform interstellar extinctioncannot explain the data. The following picture which is consistent withthe observations is suggested: some of the extinction is due to clumpeddust well outside of the emission zone, and the rest is caused by dustlocated closer to the ionized gas where scattering effects areimportant. This latter dust probably lies mostly just outside theboundary of the H II region, but some of the dust may be internal.
|Absolute H-alpha and H-beta photometry of LMC H II regions|
Absolute photoelectric H-alpha and H-beta photometry of 51 H II regionsand filaments in the LMC has been done. In this paper, theinstrumentation is described, the reduction procedures are explained,and the results are presented in the form of a catalog. A correspondingatlas is provided, showing photographs from the ESO red survey on whichobserved radio continuum contours and position indicators aresuperposed.
|The Identifications of HDE Objects with Large Magellanic Cloud Clusters and Nebulae|
|Large Magellanic Cloud sources at 3.4-cm wavelength|
Selected regions of the Large Magellanic Cloud have been surveyed atwavelength 3.4 cm with a 2.5 arcmin half-power beamwidth. Improvedspectral indices are given for 35 sources in the LMC area. Contour mapsof the 30 Doradus nebula and the four important sources to the south ofit are presented.
|The nebular complexes of the large and small Magellanic Clouds|
Long exposures of the complexes of ionized hydrogen in both the LMC andSMC have been taken with the 48-in. SRC Schmidt camera through a H-alpha+ forbidden NII interference filter of 100-A bandwidth. These plates andidentifying charts are presented in a form in which little informationis lost. A catalog of many individual emission regions in both thesegalaxies is also compiled. The relationships between the nebulositiesand OB associations as well as between 21-cm neutral hydrogen emissionand continuum radio emission are discussed, and a number ofsupernova-remnant candidates are listed for further study.
|The radio continuum of the Large Magellanic Cloud. I. The Sources at 6 cm wavelength|
|A catalogue of stellar associations in the Large Magellanic Cloud.|
Abstract image available at:http://adsabs.harvard.edu/abs/1970AJ.....75..171L
|Catalogues of Hα-EMISSION Stars and Nebulae in the Magellanic Clouds.|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1956ApJS....2..315H&db_key=AST