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Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| 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
| 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.
| Arc-Shaped and Spheroidal Stellar Complexes
Complexes of young clusters and high-luminosity stars in the shape ofregular, circular arcs have been found in a number of galaxies, firstand foremost the LMC, NGC 6946, and M83. These shapes are found even instrongly inclined galaxies, suggesting that the observed arcs areprojections of partial spherical shells. Obviously, these stellar shellsmust have formed from gaseous shells swept up by some source of centralpressure and become gravitationally unstable. The power of this sourcecorresponds to several dozen supernova explosions; however, its natureremains unclear. A central cluster providing a source of O stars andsupernovae is usually absent. The presence of multiple arcs locatedclose to each other can be explained by the fall of a swarm of fragmentsor by the progenitor stars originating in a single peculiar starcluster, implying the existence of stellar objects capable of givingrise to explosions with energies an order of magnitude higher than thoseof individual supernovae. The same objects may be responsible forgamma-ray bursts. It may be that only the most massive clusters withfrequent or especially powerful supernova explosions are capable ofproducing HI supershells. Otherwise, it is impossible to explain why nosupershells have been found around numerous clusters that should becapable of producing them according to current theories. The presence ofstar clusters in shell-like structures provides extremely importantinformation about the physical conditions in and the ages of the initialgaseous shells, making stellar arcs the best available laboratory forstudies of triggered star formation.
| 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.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| UBVI imaging photometry of NGC 6231
CCD UBVI photometry in the field of the open cluster NGC 6231 wasobtained for 1060 stars down to V~ 19 mag. Memberships, reddening,distance and age of this cluster were investigated. Its lower sequencedisplays a notorious bend at V~ 13.5 mag followed by a high number offaint stars showing a large magnitude spread at constant colour. Thisdistribution of stars does not seem to be produced by a mereaccumulation of field stars seen in the direction of the cluster but bya real star excess in the zone. The evidence suggests these stars areserious candidates to be cluster members caught in their way towards theZAMS. Fitting them with pre main sequence isochrones an age spread ofabout 10 Myr is obtained. The luminosity and mass functions were foundto be flat for -7.5 < MV < 1.5 and from 3 to 80 {calMsun} respectively. Based on observations collected at theUniversity of Toronto Southern Observatory, Las Campanas, Chile. Table 1is available only in electronic form at the CDS via anonymous ftp(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Induced star formation and the origin of gamma-ray bursts
New arguments are presented in favor of the hypothesis that three giantstellar arcs and the LMC4 supershell in the Large Magellanic Cloud (LMC)are relicts of gamma-ray bursts (GRBs), whose progenitors escaped fromthe nearby old, rich star cluster NGC 1978. This may also be true of thebinary X-ray sources concentrated in this same area and the object SNRN49-SGR 0526-66, located 18' from the cluster. All these objects may begenetically related to each other and to objects such as SN 1998bw-GRB980425. The occurrence of five GRBs in the LMC over the last ~50 Myr isquite plausible. The rate of GRBs may be high, and they may be triggersfor the formation of many HI supershells and regions of active starformation.
| 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.
| CCD photometry of the young association NGC 1962-65-66-70 in the Large Magellanic Cloud.
We present CCD data in the Johnson passbands B and V of the youngassociation NGC 1962-65-66-70 in the Large Magellanic Cloud. We find theassociation to show an age spread in the range 4 to 9Myr. Some signs fora propagation of star formation from the south-eastern to the northernpart of the region can be found, but they are based on very few stars.The slope of the luminosity function is γ=0.27+/-0.02 for stars ofthe magnitude range -8
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| 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.
| Massfunctions in the Large Magellanic Cloud.
Not Available
| The OB association LH 58 in the Large Magellanic Cloud
We present charge coupled device (CCD) photometry and spectroscopy forstars in Lucke-Hodge 58, an isolated OB association in the LargeMagellanic Cloud (LMC) northwest of 30 Doradus. The photometric catalogcontains 839 stars with UBV magnitudes complete to V approximately = 19.We have obtained spectra and classified 35 stars; combined with previouspublished spectral types, we find 22 O-type stars. The earliest type isO3-4 V, and there are three Wolf-Rayet (WR) stars in the association.The slope of the initial mass function, Gamma = -1.7 +/- 0.3, is in goodagreement with other LMC associations. The presence of several evolvedsupergiants with masses about 15-25 solar mass suggests that some starformation took place as early as 10 million years ago, but the majorityof stars formed coevally within the past few million years.
| CCD photometry of the young open cluster NGC 1962-65-66-70.
Not Available
| 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.
| A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds
Not Available
| A new radio continuum survey of the Magellanic Clouds at 1.4 GHz. II - The radio morphology, and thermal and nonthermal emission of the LMC
The distribution of the radio continuum emission of the LMC is examined.The distribution of the spectral index and a reliable spectrum of theradio emission of the LMC are derived using radio continuum surveys ofthe LMC at 408, 1400, and 2300 MHz. The distribution of the spectralindex across the LMC shows a close positional agreement between flatspectrum and H-alpha emitting regions. The total nonthermal radioluminosity is derived, giving a total equipartition magnetic fieldstrength of about 6 microG. Accurate thermal radio flux densities of themost prominent LMC H II complexes are determined.
| Sequential star formation in the Magellanic Clouds
Joint stellar and nebular studies were carried out for a few adjacentstellar associations in the Magellanic Clouds. The spread in ages of WRstars of different subtypes as deduced from their different environmentsis analyzed. A similar study is briefly sketched for S Dor type stars.It is shown how a starburst propagates in the interstellar medium at ascale of 100-300 pc in about 10 to the 7th yr for three regions of theMagellanic Clouds, using both new observational data and published ones.In view of the results provided by such detailed studies, several trapsin the methods were pointed out based on integrated properties of starclusters and their associated nebulae, or their brightest stars.Finally, the moderately old age found (from their environment) for S Dortype stars and their tendency to enter into multiple systems are pointedout.
| 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.
| 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.
| 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.
| The Parkes 2700 MHz Survey (Eighth Part): Catalogue for the Declination zone -65o to -75o
Not Available
| 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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