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Публикации по объекту
Near-Infrared Observations of N11 in the Large Magellanic Cloud: Triggered Star Formation around the Periphery of LH 9
Near-infrared observations have been carried out to survey young stellarobjects in the second-largest H II region in the Large Magellanic Cloud,N11. A total area of about 700 arcmin2 is covered in the J,H, and KS bands. We selected a total of 559 OB and 127 HerbigAe/Be star candidates out of the detected sources based on theirnear-infrared colors and magnitudes. The existence of these youngstellar objects indicates that star formation activity is underway inthe whole N11 region. Many Herbig Ae/Be star candidates are distributedaround the periphery of the OB association LH 9. Spatial correlations ofthe OB and Herbig Ae/Be star candidates with the objects observed atother wavelengths (optical, radio continuum, Hα, CO, and X-ray)suggest that the birth of the young stellar populations in peripheralmolecular clouds was triggered originally by LH 9. It is likely that thetrigger for this star formation was an expanding supershell blown by theOB association. In N11 a new generation of stars would have been formedin the clouds developed from swept-up interstellar medium.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N 11
The second-brightest star formation complex in the Large MagellanicCloud, N 11, was surveyed extensively in the J = 1-0 transition of12CO. In this paper we present maps and a cataloguecontaining the parameters of 29 individual molecular clouds in thecomplex, although more may be present. The distribution of molecular gasin the N 11 complex is highly structured. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OBstar association LH 9. In the northeastern part, a chain of molecularclouds delineates the rim of one of the so-called supergiant shells inthe LMC. There appears to be very little diffuse molecular gasin-between the individual well-defined clouds, especially in thesouthwestern ring. Most of the clouds have dimensions only slightlylarger than those of the survey beam, i.e. diameters of 25 pc or less. Asubset of the clouds mapped in J= 1-0 12CO transition wasalso observed in the J= 2-1 12CO transition, and in thecorresponding transitions of 13CO. Clouds mapped in J= 2-112CO with a two times higher angular resolution show further,clear substructure. The elements of this substructure, however, havedimensions once again comparable to those of the mapping beam. For a fewclouds, sufficient information was available to warrant an attempt atmodelling their physical parameters. They contain fairly warm(Tkin = 60-150 K) and moderately dense (nH_2 =3000 cm-3) gas. The northeastern chain of CO clouds, althoughlacking in diffuse intercloud emission, is characteristic of the morequiescent regions of the LMC, and appears to have been subject torelatively little photo-processing. The clouds forming part of thesouthwestern shell or ring, however, are almost devoid of diffuseintercloud emission, and also exhibit other characteristics of anextreme photon-dominated region (PDR).
| The physical structure of Magellanic Cloud H II regions. I. Dataset
We present infrared and optical spectroscopic data for 11 H Ii regionsand one Supernova Remnant in the Large and Small Magellanic Cloud. Theinfrared data have been obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatoryas part of a Guaranteed Time Program on H Ii regions in Local GroupGalaxies. Aim of this project is to give a new and improved analysis ofthe physical structure of the sample H Ii regions by combining as muchspectral data as possible. A detailed account is given here of thereduction process, and the quality and reliability of the presentedfluxes are discussed. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| 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.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
| A radio continuum study of the Magellanic Clouds. VIII. Discrete sources common to radio and infrared surveys of the Magellanic Clouds
We compare Parkes Telescope radio surveys with the IRAS Infrared (IR)surveys of the Magellanic Clouds (MCs). We find 130 discrete sources incommon towards the Large Magellanic Cloud (LMC) with both radio and IRemission. These 130 sources are mainly H Ii regions (89) and supernovaremnants (21). For 12 of the sources we have no identification and eightare background objects. We find 38 sources in common for the SmallMagellanic Cloud (SMC). Most of these sources are intrinsic (31) to theSMC, five sources are previously known background galaxies and twosources remain ambiguous. A flux density comparison of the radio and IRsources shows very good correlation and we note that the strongestsources at both radio and IR frequencies are H Ii regions. From theradio-IR comparison we propose that some 40 new sources in the LMC and10 in the SMC are H Ii regions or SNRs. All these new sources are alsoidentified in optical surveys.
| A radio continuum study of the Magellanic Clouds. VII. Discrete radio sources in the Magellanic Clouds
We present a study of discrete radio sources in the Magellanic Clouds(MCs) using the latest large-scale radio surveys made with the Parkesradio telescope between 1.4 and 8.55 GHz. These surveys achieved highersensitivity then previous surveys done with the Parkes telescope and sothe number of discrete radio sources detected towards the MCs hasincreased by factor of five. Also, we have obtained improved positions,flux densities and radio spectral indices for all of these sources. Atotal of 483 sources towards the Large Magellanic Cloud (LMC) and 224towards the Small Magellanic Cloud (SMC) have been detected at at leastone radio frequency. Most of the MC's sources have been classified inone of three groups: SNRs, H Ii regions or background sources accordingto classification criteria established here. In total, 209 discreteradio sources in the LMC and the 37 sources in the SMC are classifiedhere to be either H Ii regions or SNRs. We investigate their luminosityfunctions as well as the statistics of background sources behind theMCs. Also, we examine the distribution of SNRs and H Ii regions in theMCs. Tables 5 and 6 are only available electronically at the CDS via ftp130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Formation of the nebular complex N11 in the Large Magellanic Cloud.
N 11, the second largest nebula of the LMC, is formed of a large bubblesurrounded by 9 bright nebulae and filaments. We have observed thisnebular complex with a scanning Fabry-Perot interferometer at Hαand [OIII] 5007 wavelengths. The kinematics of this field agrees withthe results of the stellar content and of the molecular studies, andshows that such a structure can be the consequence of a sequential starformation. The elements of this result are deduced from the energeticinput inside the ionized gas by the stellar winds of the associationsLH9 and LH10. The evaluated dynamical lifespan of small nebular entitiesassociated with the more massive stars of LH10, are found significantlyshorter than for the gas ionized by the association LH9. Thereforesequential star formation can be triggered at the shock boundaries ofthe nebula excited by an OB association in a time scale shorter thanpreviously found, and of at most a few 10^6^yrs. It thus leads to theevidence for a new class of giant bubble, designated as "ring of HIIregions" bubble with several distinctive characteristics. It isdiscussed that the bubbles of this type are formed by sequential starformation over time scales shorter than previously found for somesuperbubbles.
| 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 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.
| 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 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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