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Hunting Massive Stars Around the Tarantula
We have studied the N159A region which is located aproximately atα = 5^{h}40^{m}07^{s} and δ = -69°47'47''. This regionbelongs to a major complex called N159 located in the LMC at thesouthern edge of the 30 Dor Nebula. All the complex is ˜50 pcdiameter long and in its interior there are at least six HII regions(N159A, E, F, G, H, K). N159 is an extremely young complex and showscharacteristic features of active stellar fomation. The interest instudying N159 is based in the fact that it is an extragalactic (thoughnear) star forming region, in a low metallicity environment and locatedspatially close to an enormous complex of star formation like 30 Dor.These conditions make N159 an excellent place to study and learn about asubject as sequential star formation, IMF of stars at low metallicities,peculiar objects, etc. In the present work we made a spectrophotometricanalysis of a large number of N159 objects. The images used forspectroscopy were taken with the 2.5-m telescope at Las CampanasObservatory (Chile) during the nights from 26th to 28th November 2003.The images, 25 arc minutes wide, were taken with the Wide FieldReimaging CCD Camera, using masks for multiobjet spectroscopy withmedium spectral resolution.In this study aproximately 150 stars were classificated as a result ofthe analysis of 5 masks. We have found 50 O-type stars, 70 early B typestars and 30 stars of spectral type later than A (which most probablyare field stars) in a region where no spectral classification had beenobtained before.
| An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| Near-Infrared Imaging Observations of the N159/N160 Complex in the Large Magellanic Cloud: Large Clusters of Herbig Ae/Be Stars and Sequential Cluster Formation
We have carried out deep near-infrared imaging observations of theN159/N160 star-forming region in the Large Magellanic Cloud. We observedan area of ~380 arcmin2 (~80,000 pc2 at thedistance of the LMC) in the J, H, and Ks bands. Theobservations are deep enough to detect Herbig Ae/Be stars down to ~3Msolar in the LMC. We discovered a total of 338 and 464candidate Herbig Ae/Be and OB stars, respectively, based on thenear-infrared colors and magnitudes. The Herbig Ae/Be candidatescomprise 10 clusters, the OB star candidates 13. We discovered anembedded Herbig Ae/Be cluster in the N159 East giant molecular cloud(GMC) and a Herbig Ae/Be cluster at the northeast tip of the N159 SouthGMC. Together with two neighboring H II regions, the Herbig Ae/Becluster at the tip of the N159S GMC provides a hint of the beginning ofsequential cluster formation in N159S. The spatial distributions of theHerbig Ae/Be and OB clusters, in conjunction with previously knownoptical clusters and embedded massive stars, indicate (1) sequentialcluster formation within each of the N159 and N160 star-forming regionsand (2) large-scale sequential cluster formation over the entireobserved region from N160 to N159S. Possible triggers for thelarge-scale cluster formation are the supergiant shell SGS 19 and anexpanding superbubble. Some of the Herbig Ae/Be clusters in theN159/N160 complex are significantly larger in spatial extent thanpre-main-sequence clusters of similar age in the Milky Way. Highlyturbulent gas motion in the LMC is probably responsible for forming thelarge young clusters.
| 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
| 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.
| Dust Continuum Imaging of the HH 24 Region in L1630
We have mapped the 1300 and 350 μm continuum emission in the regionsurrounding the Herbig-Haro objects HH 22-HH 26 in L1630, which is partof the Orion B molecular cloud complex, using the MPIR bolometer at theIRAM 30 m telescope and the SHARC bolometer camera at the CaltechSubmillimeter Observatory (CSO). Our observations reveal the presence ofa cluster of at least nine protostellar condensations at variousevolutionary stages in the region around HH 24-HH 26. In addition to twopreviously identified class 0 protostars, two additional sources with alow 350/1300 μm flux ratio are detected. Although the lack offar-infrared fluxes precludes a definitive determination of theirnature, their low 350/1300 μm flux ratio along with the absence ofembedded infrared and radio continuum sources suggests that they arelikely to be in a very early evolutionary stage. In particular, one ofthese two sources appears quite compact in the 1300 μm image and maybe in the short-lived phase of isothermal collapse. In addition, we havedetected three millimeter continuum sources in the vicinity of the HH22-HH 23 objects, including a compact source at the origin of the HH 23optical jet. The bolometric luminosity, submillimeter-to-bolometricluminosity ratio, temperature, and mass of this source are allconsistent with those of a class 0 protostar. However, the compactnessof the source suggests that the continuum emission may be dominated byan unresolved disk, as would be expected for a more evolved protostellarsource. No evidence is found for the presence of a molecular outflowassociated with this source in our CO, 13CO, andHCO+ data obtained with the CSO.
| The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure
LMC 2 has the brightest, most coherent filamentary structure of allknown supergiant shells in the Large Magellanic Cloud. The opticalemission-line images show active star formation regions along thewestern edge and long filaments to the east. ROSAT PSPC and HRI imagesshow bright X-ray emission from within the shell boundary, indicatingthe presence of hot gas. Counterintuitively, neither high-resolutionechelle spectra in the Hα line nor aperture synthesis H I 21 cmemission-line observations show LMC 2 to have the kinematics expected ofan expanding shell. Rather, LMC 2 appears to consist of hot gas confinedbetween H I sheets. The interior surfaces of these sheets are ionized bythe UV flux of massive stars in the star formation regions along theperiphery of LMC 2, while the heating is provided by outflows of hot gasfrom the star formation regions and by SNRs interior to LMC 2. We havecompared LMC 2 to other supergiant shells in the LMC and in more distantgalaxies. When the spatial resolution of our data are degraded, we findthat LMC 2 resembles supergiant shells observed at a distance of 4 Mpcthat have previously been interpreted as expanding shells. Therefore,great caution should be exercised in the analysis and interpretation ofthe kinematics of distant supergiant shells to prevent overestimates oftheir velocities and total kinetic energies.
| 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.
| Photometry of the LMC H II region N 159 A and of its stellar content. II - Young stars, gas, and dust
This paper presents an analysis of the stellar and nebular photometricobservations of the LMC H II region N 159 A and its surroundings. Wehave identified the probable exciting stars of N 159 A and of itssmaller neighbor H II regions. Fourteen O-B2 stars are present in thisfield of 1.6 arcmin x 2.5 arcsec. N 159 A itself is ionized by a closepair of stars of probable spectral types O5 to O6 V, and O7 to O8 V;these stars are separated by 1.0 arcmin and are affected by a visualextinction of 1.2 to 1.4 mag. The protostar of Jones et al. (1986),lying at the border of N 159 A, corresponds to an H-alpha emissionobject which is possibly a very compact H II region or a Herbig Ae/Bestar. The young stellar population (associated with the H II regions) issuperposed on an older population of giants, aged 10 exp 9 yr or more.The observed stellar content of N 159 A successfully accounts for thelevel of ionization of the gas as well as for this region's radiocontinuum and Balmer line emission.
| H II regions with He II emission
Recent observations reveal the existence of several Local Group H IIregions which exhibit surprisingly strong nebular H II 4686 emission,with I(4686)/I(H-beta) as high as 0.3 in some cases. The He II emissionin these objects is associated with a rather diverse set of stars, withWO stars, very early WN stars, and luminous X-ray binaries involved, andin one case what appears to be a normal O-type star. Extension of athermal X-ray spectrum to lower energies appears to satisfactorilyexplain the H II emission associated with the X-ray binaries. For theremaining objects, stellar effective temperatures of at least 60,000 Kare required to account for the observed He II emission, ifphotoionization by a hot stellar continuum is responsible. The existenceof stars hot enough to produce such emission presents an interestingproblem for stellar atmospheric models and stellar evolution and mayprovide an explanation for the He II 4686 emission commonly observed inmetal-poor extragalactic H II regions.
| He II emission in extragalactic H II regions
Spectroscopic observations confirming the presence of nebular He II4686-A emission in the SMC H II region N76 are presented, and it isdemonstrated that the He II emission associated with the WO star in IC1613 is also extended. The properties of four H II regions arediscussed. The close correlation of the emission with specific starsindicates that photoionization by the stars themselves is the excitationmechanism, and it is proposed that this may be true for those nebulaeionized by WO stars and some rare high-excitation WN stars. Theexistence of these nebulae with He II emission increases the likelihoodthat the 4686-A emission frequently observed in dwarf emission-linegalaxies is nebular in origin. This prompts the conclusion that theradiation field associated with star-forming regions can be harder thanpreviously suspected, and reopens the issue of whether photoionizationby stars in young galaxies can account for the ionization observed inQSO absorption-line systems.
| Is N44C a fossil X-ray ionized nebula?
The characteristics of the H II region N44C in the LMC are interpretedhere in terms of a fossil X-ray photoionized nebula resembling therecently discovered He III region around the luminous X-ray binary LMCX-1. The presently extinct X-ray source that has been responsible forthe high ionization in N44C could well be identical with the transientLMC X-5. From photon-counting arguments and recombination timescales ofthe interstellar plasma, it is found that less than 100 years ago theX-ray luminosity should have been comparable with that of the mostluminous X-ray binaries.
| 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.
| 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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