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Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..249F
| A radio continuum study of the Magellanic Clouds. 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.
| Classical H II regions in the Magellanic Clouds. 2: Stellar content
In this second in a series of papers on the nature of classical H IIregions in the Magellanic Clouds I investigate the properties of theunderlying stellar content of the nebulae. Particular emphasis is placedon identifying and classifying the ionizing source(s) for each H IIregion. With the exception of the LMC H II regions DEM 20 and DEM 8c, Ifind that all of the objects in this sample are ionized by more than oneO or B star. Even the faintest H II regions reflect the formation of ahandful of massive, albeit early B type, stars. Typically, one staraccounts for 60% - 70% of the ionizing photons and 2 - 5 less massivestars provide the remaining 30% - 40%. From the statistics of thehottest stars in these H II regions, and from considering all the bluestars contained within each region, the distribution of massive starswith spectral type is consistent with results found in similar galacticH II regions.
| A catalogue of binary star cluster candidates in the Large Magellanic Cloud
A photographic atlas of close pairs of star clusters in the LargeMagellanic Cloud is presented here. The criterion for inclusion ofcluster pairs in the atlas was an upper limit of 18.7 pc for theprojected separation between the centers of the clusters in each pair.Accurate coordinates for the clusters, the projected separations andestimates of the diameters and positional angles are given and some ofthe global properties of the cluster-pair population of the LMC arediscussed. It is found that the individual clusters in pairspreferentially have nearly equal sizes.
| A catalog of 255 new clusters in the Large Magellanic Cloud
A catalog of 255 newly discovered clusters has resulted from a search often fields in the LMC. Most of the clusters are faint, small, and old.The equatorial coordinates, sizes, and estimates of the B magnitudes ofthe brightest stars are given for all of the clusters. The mean diameterfor all LMC clusters in the fields is shown to be 7.7 pc. A correlationis found between size and distance from the center of the LMC, with morelarge clusters in the remote outer parts of the LMC. The total clusterpopulation of the LMC is estimated at about 4200.
| Integrated UV magnitudes of the Large Magellanic Cloud associations
UV photographs (2600 A, 350 A passband) of the LMC have been obtained bythe S183 experiment during a Skylab mission. The background is estimatedand a method for deriving the integrated fluxes is presented. Theintegrated magnitudes of about 50 associations and isocontours of theirintensities are given, along with the B and V integrated magnitudes of13 associations.
| Binary star clusters in the Large Magellanic Cloud
In a survey of the LMC cluster system, double clusters with acenter-to-center separation of less than 1.3 arcmin (18 pc) have beenidentified. It is inferred that a considerable fraction of these doubleclusters must be binaries since the calculated projection effects canaccount for only 31 of them. This inference is strongly supported by thefact that the ages available for some of the culsters of the sample (asdetermined from UBV photometry) are less than the computed times ofmerger or disruption of the binary cluster system. Furthermore, thespace distribution of these pairs indicates that these clusters belongto a very young or young population.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| 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.
| 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
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