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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.
| Synthesis Imaging of Dense Molecular Gas in the N113 H II Region of the Large Magellanic Cloud
We present aperture synthesis imaging of dense molecular gas in theLarge Magellanic Cloud, taken with the prototype millimeter receivers ofthe Australia Telescope Compact Array (ATCA). Our observations of theN113 H II region reveal a condensation with a size of ~6" (1.5 pc) FWHM,detected strongly in the 1-0 lines of HCO+, HCN, and HNC, andweakly in C2H. Comparison of the ATCA observations withsingle-dish maps from the Mopra Telescope and sensitive spectra from theSwedish-ESO Submillimetre Telescope indicates that the condensation is amassive clump of ~104 Msolar within a larger~105 Msolar molecular cloud. The clump is centeredadjacent to a compact, obscured H II region that is part of a linearstructure of radio continuum sources extending across the molecularcloud. We suggest that the clump represents a possible site fortriggered star formation. Examining the integrated line intensities as afunction of interferometer baseline length, we find evidence fordecreasing HCO+/HCN and HCN/HNC ratios on longer baselines.These trends are consistent with a significant component of theHCO+ emission arising in an extended clump envelope and alower HCN/HNC abundance ratio in dense cores.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| 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.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| The Optical Gravitational Lensing Experiment. Catalog of Star Clusters from the Large Magellanic Cloud
We present the catalog of star clusters found in the area of about 5.8square degree in the central regions of the Large Magellanic Cloud. Itcontains data for 745 clusters. 126 of them are new objects. For eachcluster equatorial coordinates, radius, approximate number of membersand cross-identification are provided. Photometric data for all clusterspresented in the catalog and Atlas consisting of finding charts andcolor-magnitude diagrams are available electronically from the OGLEInternet archive.
| 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
| 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.
| 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.
| Near-infrared spectra and classification diagnostics of Seyfert galaxies
Observational results of a previous spectroscopic survey of Seyfertgalaxies in the near-IR are presented, and the potential for usingemission-line ratios in this spectral region as a classificationdiagnostic tool is examined. Near-IR CCD spectra, which cover thewavelength range of 7000-10,000 at a nominal resolution of about 12 A,of 15 additional Seyferts and two starburst galaxies are obtained.Relative emission-line intensities from these observations, incombination with measurements from previous studies and measurements ofnew, signal-to-noise ratio optical spectra of many of these objects, areused to study the diagnostic diagrams involving forbidden S III 9069,9531/H-alpha, forbidden O II 7320, 7330/H-alpha, forbidden S II 6716,6731/H-alpha, and forbidden O III 5007/H-beta. Comparisons are made inthese diagrams between observational data from the active galaxies andpublished measurements of H II regionlike objects, as well as withpredictions from simple one-component models calculated for the twotypes of objects.
| 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.
| Merger and disruption lifetimes of binary star clusters in the Large Magellanic Cloud
The times of merger (due to mutual gravitational interaction) anddisruption (by the tidal field of the LMC and by passing Giant MolecularClouds-GMCs) of binary star clusters in the Large Magellanic Cloud arecomputed as a function of the separation of the components. As expected,close pairs merge while wide pairs are disrupted mainly by the tidalfield, the effect of the GMCs being small. The total effect of the threeprocesses shows that the lifetime of binary clusters varies from aboutone million yr to a maximum of about 4 x 10 to the 7th yr, consistentwith the ages known for some of them.
| 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 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.
| A rediscussion of sulfur abundances in Magellanic Clouds and galactic H II regions
Near infrared spectra obtained with the ESO Reticon are presented for 23H II regions in the Galaxy and the Magellanic Clouds and for one bluecompact galaxy from which S(2+) abundances are derived. The opticalspectra of these objects, taken from the literature, are reduced, usingthe most recent atomic data. The problem of correction for unseen ionsof sulfur is reexamined, and the sulfur abundances in the sample aredetermined. Some implications on the chemical evolution of galaxies arepresented, and the possibility of deriving sulfur abundances without thehelp of (S III) measurements is discussed in an appendix. The averageS/O ratio is found to be roughly the same in all the galaxiesconsidered, close to the solar value of 2.04 x 10 to the -2 andcompatible with a production of oxygen and sulfur in massive stars.
| A survey of chemical compositions of H II regions in the Magellanic Clouds
The reported investigation had the objective to extend abundancedeterminations to a larger number of H II regions in the Small Cloud(and a few more in the Large Cloud) in connection with a study regardingthe possible occurrence of large-scale abundance gradients analogous tothose found in Sc galaxies and in the Galaxy, taking into account alsoquestions concerning the existence of a characteristic abundance patternrepresenting the entire young population of either cloud. It is foundthat the laws governing enrichment of the interstellar medium are verysimilar in the Magellanic Clouds to what they are in the outer parts ofSc galaxies, including the very massive system M101. There is no reasonto believe that any special process such as preferential escape of gashas operated to reduce the effective yield in the Magellanic Clouds.
| 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
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