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Revised Primordial Helium Abundance Based on New Atomic Data
We have derived a primordial helium abundance ofYp=0.2477+/-0.0029, based on new atomic physics computationsof the recombination coefficients of He I and of the collisionalexcitation of the H I Balmer lines together with observations andphotoionization models of metal-poor extragalactic H II regions. The newatomic data increase our previous determination of Yp by0.0086, a very significant amount. By combining our Yp resultwith the predictions made by the standard big bang nucleosynthesismodel, we find a baryon-to-photon ratio, η, in excellent agreementboth with the η-value derived by the primordial deuterium abundancevalue observed in damped Lyα systems and with the one obtainedfrom the WMAP observations.
| Progressive Star Formation in the Young SMC Cluster NGC 602
NGC 602 is a young stellar cluster located in a peripheral region of theSmall Magellanic Cloud (SMC) known as the wing. Far from the main bodyof the galaxy and abutting the Magellanic Bridge, the SMC's wing ischaracterized by low gas and stellar content. With deep optical imagingfrom the Advanced Camera for Surveys (ACS) aboard the Hubble SpaceTelescope (HST), we have discovered an extensive pre-main-sequence (PMS)population, with stellar masses in the range 0.6-3 Msolar.These low-mass PMS stars formed coevally with the central cluster about4 Myr ago. Spitzer IRAC images of the same region also reveal apopulation of young stellar objects, some of which are still embedded innebular material and most of which likely formed even more recently thanthe young stars detected with HST ACS imaging. We infer that starformation started in this region ~ 4 Myr ago with the formation of thecentral cluster and gradually propagated toward the outskirts where starformation is presently ongoing.
| Discovery of the Pre-Main-Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope Advanced Camera for Surveys Observations
We report the discovery of an extraordinary number of pre-main-sequence(PMS) stars in the vicinity of the stellar association LH 95 in theLarge Magellanic Cloud (LMC). Using the Advanced Camera for Surveys onboard the Hubble Space Telescope in wide-field mode, we obtained deephigh-resolution imaging of the main body of the association and of anearby representative LMC background field. These observations allowedus to construct the color-magnitude diagram (CMD) of the association inunprecedented detail and to decontaminate the CMD for the average LMCstellar population. The most significant result is the direct detectionof a substantial population of PMS stars and their clustering propertieswith respect to the distribution of the higher mass members of theassociation. Although LH 95 represents a rather modest star-formingregion, our photometry, with a detection limit of V<~28 mag, revealsin its vicinity more than 2500 PMS stars with masses down to ~0.3Msolar. Thus, our observations offer a new perspective on atypical LMC association: the stellar content of LH 95 is found to extendfrom bright OB stars to faint red PMS stars, suggesting a fullypopulated initial mass function from the massive blue giants down to thesubsolar mass regime.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.Research supported by the German Research Foundation (DeutscheForschungsgemeinschaft) and the German Aerospace Center (DeutscheZentrum für Luft und Raumfahrt).
| The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90
We present the results of our investigation of the intermediate-age starcluster BS 90, located in the vicinity of the H II region N66 in theSMC, observed with HST ACS. The high-resolution data provide a uniqueopportunity for a very detailed photometric study performed on one ofthe rare intermediate-age rich SMC clusters. The complete set ofobservations is centered on the association NGC 346 and contains almost100,000 stars down to V~=28 mag. In this study we focus on the northernpart of the region, which covers almost the whole stellar content of BS90. We construct its stellar surface density profile and derivestructural parameters. Isochrone fits on the CMD of the cluster resultsin an age of about 4.5 Gyr. The luminosity function is constructed andthe present-day mass function of BS 90 has been obtained using themass-luminosity relation, derived from the isochrone models. We found aslope between -1.30 and -0.95, comparable to or somewhat shallower thana typical Salpeter IMF. Examination of the radial dependence of the massfunction shows a steeper slope at larger radial distances, indicatingmass segregation in the cluster. The derived half-mass relaxation timeof 0.95 Gyr suggests that the cluster is mass segregated due to itsdynamical evolution. From the isochrone model fits we derive ametallicity for BS 90 of [Fe/H]=-0.72, which adds an important point tothe age-metallicity relation of the SMC. We discuss our findings on thisrelation in comparison to other SMC clusters.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| Star Clusters with the GTC
Star clusters are becoming very useful probes in the studies of manytopics in modern Astronomy. Actually, they are providing invaluableinformation about the origin, structure and star formation processes inextragalactic systems. Otherwise, most of our knowledge on howindividual stars are formed is derived from the analysis of stellarclusters resolved into stars. In this contribution we review the mainscientific objectives in this field, in the light of the firstgeneration instruments for the Gran Telescopio Canarias.
| Imaging of the Stellar Population of IC 10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope
We present adaptive optics (AO) images of the central starburst regionof the dwarf irregular galaxy IC 10. The Keck II telescope laser guidestar was used to achieve near diffraction-limited performance at H andK' (Strehl ratios of 18% and 32%, respectively). The imagesare centered on the putative Wolf-Rayet (W-R) object [MAC92]24. Wecombine our AO images with F814W data from HST. By comparing theK' versus [F814W]-K' color-magnitude diagram (CMD)with theoretical isochrones, we find that the stellar population is bestrepresented by at least two bursts of star formation, one ~10 Myr agoand one much older (150-500 Myr). Young, blue stars are concentrated inthe vicinity of [MAC92]24. This population represents an OB associationwith a half-light radius of about 3 pc. We resolve the W-R object[MAC92] 24 into at least six blue stars. Four of these components havenear-IR colors and luminosities that make them robust WN starcandidates. By matching the location of C stars in the CMD with those inthe SMC we derive a distance modulus for IC 10 of about 24.5 mag and aforeground reddening of E(B-V)=0.95 mag. We find a more precise distanceby locating the tip of the giant branch in the F814W, H, andK' luminosity functions. We find a weighted mean distancemodulus of 24.48+/-0.08. The systematic error in this measurement, dueto a possible difference in the properties of the RGB populations in IC10 and the SMC, is +/-0.16 mag.
| The VLT-FLAMES survey of massive stars: surface chemical compositions of B-type stars in the Magellanic Clouds
We present an analysis of high-resolution FLAMES spectra ofapproximately 50 early B-type stars in three young clusters at differentmetallicities, NGC 6611 in the Galaxy, N 11 in the Large MagellanicCloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using thetlusty non-LTE model atmospheres code, atmospheric parameters andphotospheric abundances (C, N, O, Mg and Si) of each star have beendetermined. These results represent a significant improvement on thenumber of Magellanic Cloud B-type stars with detailed and homogeneousestimates of their atmospheric parameters and chemical compositions. Therelationships between effective temperature and spectral type arediscussed for all three metallicity regimes, with the effectivetemperature for a given spectral type increasing as one moves to a lowermetallicity regime. Additionally the difficulties in estimating themicroturbulent velocity and the anomalous values obtained, particularlyin the lowest metallicity regime, are discussed. Our chemicalcomposition estimates are compared with previous studies, both stellarand interstellar with, in general, encouraging agreement being found.Abundances in the Magellanic Clouds relative to the Galaxy are discussedand we also present our best estimates of the base-line chemicalcomposition of the LMC and SMC as derived from B-type stars.Additionally we discuss the use of nitrogen as a probe of theevolutionary history of stars, investigating the roles of rotationalmixing, mass-loss, blue loops and binarity on the observed nitrogenabundances and making comparisons with stellar evolutionary models wherepossible.Based on observations at the European Southern Observatory in programmes171.0237 and 073.0234. Tables 3-6 and Figs. 7-31 are only available inelectronic form at http://www.aanda.org
| Binary star progenitors of long gamma-ray bursts
Context: The collapsar model for long gamma-ray bursts requires arapidly rotating Wolf-Rayet star as progenitor. Aims: We test the ideaof producing rapidly rotating Wolf-Rayet stars in massive close binariesthrough mass accretion and consecutive quasi-chemically homogeneousevolution - the latter had previously been shown to provide collapsarsbelow a certain metallicity threshold. Methods: We use a 1Dhydrodynamic binary evolution code to simulate the evolution of a 16+15Mȯ binary model with an initial orbital period of 5 daysand SMC metallicity (Z=0.004). Internal differential rotation,rotationally induced mixing and magnetic fields are included in bothcomponents, as well as non-conservative mass and angular momentumtransfer, and tidal spin-orbit coupling. Results: The considered binarysystem undergoes early Case B mass transfer. The mass donor becomes ahelium star and dies as a type Ib/c supernova. The mass gainer isspun-up, and internal magnetic fields efficiently transport accretedangular momentum into the stellar core. The orbital widening preventssubsequent tidal synchronization, and the mass gainer rejuvenates andevolves quasi-chemically homogeneously thereafter. The mass donorexplodes 7 Myr before the collapse of the mass gainer. Assuming thebinary to be broken-up by the supernova kick, the potential gamma-rayburst progenitor would become a runaway star with a space velocity of 27km s-1, traveling about 200 pc during its remaining lifetime. Conclusions: .The binary channel presented here does not, as such,provide a new physical model for collapsar production, as the resultingstellar models are almost identical to quasi-chemically homogeneouslyevolving rapidly rotating single stars. However, it may provide a meansfor massive stars to obtain the required high rotation rates. Moreover,it suggests that a possibly large fraction of long gamma-ray burstsoccurs in runaway stars.
| The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud
We have studied the optical spectra of a sample of 28 O- and earlyB-type stars in the Large Magellanic Cloud, 22 of which are associatedwith the young star forming region N11. Our observations sample thecentral associations of LH9 and LH10, and the surrounding regions.Stellar parameters are determined using an automated fitting method(Mokiem et al. 2005), which combines the stellar atmosphere codefastwind (Puls et al. 2005) with the genetic algorithm basedoptimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively.The age difference and relative distance of the associations areconsistent with a sequential star formation scenario in which stellaractivity in LH9 triggered the formation of LH10. Our sample containsfour stars of spectral type O2. From helium and hydrogen line fitting wefind the hottest three of these stars to be 49{-}54 kK (compared to45{-}46 kK for O3 stars). Detailed determination of the helium massfraction reveals that the masses of helium enriched dwarfs and giantsderived in our spectroscopic analysis are systematically lower thanthose implied by non-rotating evolutionary tracks. We interpret this asevidence for efficient rotationally enhanced mixing leading to thesurfacing of primary helium and to an increase of the stellarluminosity. This result is consistent with findings for SMC stars byMokiem et al. (2006). For bright giants and supergiants no such massdiscrepancy is found; these stars therefore appear to follow tracks ofmodestly or non-rotating objects. The set of programme stars wassufficiently large to establish the mass loss rates of OB stars in thisZ ˜ 1/2 Zȯ environment sufficiently accurate toallow for a quantitative comparison with similar objects in the Galaxyand the SMC. The mass loss properties are found to be intermediate tomassive stars in the Galaxy and SMC. Comparing the derived modified windmomenta D_mom as a function of luminosity with predictions for LMCmetallicities by Vink et al. (2001) yields good agreement in the entireluminosity range that was investigated, i.e. 5.0 < logL/Lȯ< 6.1.Appendix A is only available in electronic form at http://www.aanda.org
| First Detection of Phase-dependent Colliding Wind X-Ray Emission outside the Milky Way
After having reported the detection of X-rays emitted by the peculiarsystem HD 5980, we assess here the origin of this high-energy emissionfrom additional X-ray observations obtained with XMM-Newton. Thisresearch provides the first detection of apparently periodic X-rayemission from hot gas produced by the collision of winds in an evolvedmassive binary outside the Milky Way. It also provides the first X-raymonitoring of a luminous blue variable only years after its eruption andshows that the dominant source of the X-rays is not associated with theejecta.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the US (NASA).
| Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind Clumping Does Not Depend on Ambient Metallicity
The mass-loss rates of hot, massive, luminous stars are considered adecisive parameter in shaping the evolutionary tracks of such stars andinfluencing the interstellar medium on galactic scales. The small-scalestructures (clumps) that are omnipresent in such winds may reduceempirical estimates of mass-loss rates by an evolutionarily significantfactor of >=3. So far, there has been no direct observationalevidence that wind clumping may persist at the same level inenvironments with a low ambient metallicity, where the wind-drivingopacity is reduced. Here we report the results of time-resolvedspectroscopy of three presumably single Population I Wolf-Rayet stars inthe Small Magellanic Cloud, where the ambient metallicity is ~1/5Zsolar. We detect numerous small-scale emission peaks movingoutward in the accelerating parts of the stellar winds. The generalproperties of the moving features, such as their velocity dispersions,emissivities, and average accelerations, closely match the correspondingcharacteristics of small-scale inhomogeneities in the winds of GalacticWolf-Rayet stars.
| The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands
We present the initial results from the Spitzer Survey of the SmallMagellanic Cloud (S3MC), which imaged the star-forming bodyof the SMC in all seven MIPS and IRAC wave bands. We find that theF8/F24 ratio (an estimate of PAH abundance) haslarge spatial variations and takes a wide range of values that areunrelated to metallicity but anticorrelated with 24 μm brightness andF24/F70 ratio. This suggests that photodestructionis primarily responsible for the low abundance of PAHs observed instar-forming low-metallicity galaxies. We use the S3MC imagesto compile a photometric catalog of ~400,000 mid- and far-infrared pointsources in the SMC. The sources detected at the longest wavelengths fallinto four main categories: (1) bright 5.8 μm sources with very faintoptical counterparts and very red mid-infrared colors([5.8]-[8.0]>1.2), which we identify as YSOs; (2) bright mid-infraredsources with mildly red colors (0.16<~[5.8]-[8.0]<0.6), identifiedas carbon stars; (3) bright mid-infrared sources with neutral colors andbright optical counterparts, corresponding to oxygen-rich evolved stars;and (4) unreddened early B stars (B3-O9) with a large 24 μm excess.This excess is reminiscent of debris disks and is detected in only asmall fraction of these stars (<~5%). The majority of the brightestinfrared point sources in the SMC fall into groups 1-3. We use thisphotometric information to produce a catalog of 282 bright YSOs in theSMC with a very low level of contamination (~7%).
| Past and Present Star Formation in the SMC: NGC 346 and its Neighborhood
In the quest to understand how star formation occurs and propagates inthe low-metallicity environment of the Small Magellanic Cloud (SMC), weacquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACSimages of the young and massive star-forming region NGC 346. Theseimages and their photometric analysis provide us with a snapshot of thestar formation history of the region. We find evidence for starformation extending from ~10 Gyr in the past until ~150 Myr in the fieldof the SMC. The youngest stellar population (~3+/-1 Myr) is associatedwith the NGC 346 cluster. It includes a rich component of low-masspre-main-sequence stars mainly concentrated in a number of subclustersspatially colocated with CO clumps previously detected by Rubio andcoworkers. Within our analysis uncertainties, these subclusters appearcoeval with each other. The most massive stars appear concentrated inthe central subclusters, indicating possible mass segregation. A numberof embedded clusters are also observed. This finding, combined with theoverall wealth of dust and gas, could imply that star formation is stillactive. An intermediate-age star cluster, BS 90, formed ~4.3+/-0.1 Gyrago, is also present in the region. Thus, this region of the SMC hassupported star formation with varying levels of intensity over much ofthe cosmic time.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| The Chemical Evolution of Helium
We report on measurements of the 4He abundance toward theouter Galaxy H II region S206 with the NRAO Green Bank Telescope.Observations of hydrogen and helium radio recombination lines between 8and 10 GHz were made toward the peak radio continuum position in S206.We derive 4He/H=0.08459+/-0.00088 (random)+/-0.0010 (knownsystematic), 20% lower than optical recombination line results. It isdifficult to reconcile the large discrepancy between the optical andradio values even when accounting for temperature, density, andionization structure or for optical extinction by dust. Using only M17and S206 we determine ΔY/ΔZ=1.41+/-0.62 in the Galaxy,consistent with standard chemical evolution models. High heliumabundances in the old stellar population of elliptical galaxies can helpexplain the increase in UV emission with shorter wavelength between 2000and 1200 Å, called the ``UV upturn'' or UVX. Our lower values ofΔY/ΔZ are consistent with a normal helium abundance athigher metallicity and suggest that other factors, such as a variablered giant branch mass loss with metallicity, may be important. Whencombined with 4He abundances in metal-poor galaxy H IIregions, Magellanic Cloud H II regions, and M17 that have beendetermined from optical recombination lines, including the effects oftemperature fluctuations, our radio 4He/H abundance ratio forS206 is consistent with a helium evolution of ΔY/ΔZ=1.6. Alinear extrapolation to zero metallicity predicts a 4He/Hprimordial abundance ratio about 5% lower than that given by theWilkinson Microwave Anisotropy Probe and standard big bangnucleosynthesis. The measured 4He abundances may besystematically underestimated by a few percent if clumping exists inthese H II regions.
| Single star progenitors of long gamma-ray bursts. I. Model grids and redshift dependent GRB rate
We present grids of massive star evolution models at four differentmetallicities (Z=0.004, 0.002, 0.001, 0.00001). The effects of rotationon the stellar structure and the transport of angular momentum andchemical elements through the Spruit-Tayler dynamo and rotationallyinduced instabilities are considered. After discussing uncertaintiesinvolved with the adopted physics, we elaborate the final fate ofmassive stars as a function of initial mass and spin rate, at eachconsidered metallicity. In particular, we investigate for which initialconditions long gamma-ray bursts (GRBs) are expected to be produced inthe frame of the collapsar model. Then, using an empirical spindistribution of young massive metal-poor stars and a specifiedmetallicity-dependent history of star-formation, we compute the expectedGRB rate as function of metallicity and redshift based on our stellarevolution models. The GRB production in our models is limited tometallicities of Z ⪉ 0.004, with the consequence that about 50% ofall GRBs are predicted to be found at redshifts above z = 4, with mostsupernovae occurring at redshifts below z≃ 2.2. The average GRB/SNratio predicted by our model is about 1/200 globally, and 1/1250 at lowredshift. Future strategies for testing the considered GRB progenitorscenario are briefly discussed.
| The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. I. Photometry
We present a photometric study of the star-forming region NGC 346 andits surrounding field in the Small Magellanic Cloud, using data takenwith the Advanced Camera for Surveys (ACS) on board the Hubble SpaceTelescope (HST). The data set contains both short and long exposures forincreased dynamic range, and photometry was performed using the ACSmodule of the stellar photometry package DOLPHOT. We detected almost100,000 stars over a magnitude range of V~11 to V~28 mag, including allstellar types from the most massive young stars to faint lowermain-sequence and pre-main-sequence stars. We find that this region,which is characterized by a plethora of stellar systems and interestingobjects, is an outstanding example of mixed stellar populations. We takeinto account different features of the color-magnitude diagram of allthe detected stars to distinguish the two dominant stellar systems: Thestellar association NGC 346 and the old spherical star cluster BS 90.These observations provide a complete stellar sample of a field about5'×5' around the most active star-formingregion in this galaxy. Considering the importance of these data forvarious investigations in the area, we provide the full stellar catalogfrom our photometry. This paper is the first part of an ongoing study toinvestigate in detail the two dominant stellar systems in the area andtheir surrounding field.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| Diffuse, Nonthermal X-Ray Emission from the Galactic Star Cluster Westerlund 1
We present the diffuse X-ray emission identified in Chandra observationsof the young, massive Galactic star cluster Westerlund 1. After removingpointlike X-ray sources down to a completeness limit of~2×1031 ergs s-1, we identify(3+/-1)×1034 ergs s-1 (2-8 keV) of diffuseemission. The spatial distribution of the emission can be described as aslightly elliptical Lorentzian core with a half-width at half-maximumalong the major axis of 25''+/-1'', similar to thedistribution of point sources in the cluster, plus a 5' halo of extendedemission. The spectrum of the diffuse emission is dominated by a hardcontinuum component that can be described as a kT>~3 keV thermalplasma that has a low iron abundance (<~0.3 solar) or as nonthermalemission that could be stellar light that is inverse Compton scatteredby MeV electrons. Only 5% of the flux is produced by a kT~0.7 keVplasma. The low luminosity of the thermal emission and the lack of a 6.7keV iron line suggest that <~40,000 unresolved stars with massesbetween 0.3 and 2 Msolar are present in the cluster, fewerthan previously estimated. Moreover, the flux in the diffuse emission isa factor of several lower than would be expected from a supersonicallyexpanding cluster wind, and there is no evidence for thermal remnantsproduced by supernovae. Less than 10-5 of the mechanicalluminosity of the cluster is dissipated as 2-8 keV X-rays, leaving alarge amount of energy that either is radiated at other wavelengths, isdissipated beyond the bounds of our image, or escapes into theintergalactic medium.
| FUSE Observations of HD 5980: The Wind Structure of the Eruptor
HD 5980 is a unique system containing one massive star (star A) that isapparently entering the luminous blue variable phase and an eclipsingcompanion (star B) that may have already evolved beyond this phase tobecome a Wolf-Rayet star. In this paper we present the results from FUSEobservations obtained in 1999, 2000, and 2002, and one far-UVobservation obtained by ORFEUS BEFS in 1993 shortly before the firsteruption of HD 5980. The eight phase-resolved spectra obtained by FUSEin 2002 are analyzed in the context of a wind-eclipse model. Thisanalysis shows that the wind of the eruptor obeyed a very fast velocitylaw in 2002, which is consistent with the line-driving mechanism.Large-amplitude line-profile variations on the orbital period are shownto be due to the eclipse of star B by the wind of star A, although theeclipse due to gas flowing in the direction of star B is absent. Thiscan only be explained if the wind of star A is not spherically symmetricor if the eclipsed line radiation is ``filled in'' by emissionoriginating from somewhere else in the system, e.g., in the wind-windcollision region. Except for a slightly lower wind speed, the ORFEUSBEFS spectrum is very similar to the spectrum obtained by FUSE at thesame orbital phase; there is no indication of the impending eruption.However, the trend for decreasing wind velocity suggests the occurrenceof the ``bistability'' mechanism, which in turn implies that therestructuring of the circumbinary environment caused by the transitionfrom fast, rarefied wind to slow, dense wind was observed as theeruptive event. The underlying mechanism responsible for the long-termdecrease in wind velocity that precipitated this change remains an openissue.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| The VLT-FLAMES survey of massive stars: mass loss and rotation of early-type stars in the SMC
We have studied the optical spectra of a sample of 31 O-and early B-typestars in the Small Magellanic Cloud, 21 of which are associated with theyoung massive cluster NGC 346. Stellar parameters are determined usingan automated fitting method (Mokiem et al. 2005, A&A, 441, 711),which combines the stellar atmosphere code FASTWIND (Puls et al. 2005,A&A, 435, 669) with the genetic algorithm based optimisation routinePIKAIA (Charbonneau 1995, ApJS, 101, 309). Comparison with predictionsof stellar evolution that account for stellar rotation does not resultin a unique age, though most stars are best represented by an age of 1-3Myr. The automated method allows for a detailed determination of theprojected rotational velocities. The present day v_r sin i distributionof the 21 dwarf stars in our sample is consistent with an underlyingrotational velocity (v_r) distribution that can be characterised by amean velocity of about 160 - 190 km s-1 and an effective halfwidth of 100 - 150 km s-1. The vr distributionmust include a small percentage of slowly rotating stars. If predictionsof the time evolution of the equatorial velocity for massive starswithin the environment of the SMC are correct (Maeder & Meynet 2001,A&A, 373, 555), the young age of the cluster implies that thisunderlying distribution is representative for the initial rotationalvelocity distribution. The location in the Hertzsprung-Russell diagramof the stars showing helium enrichment is in qualitative agreement withevolutionary tracks accounting for rotation, but not for those ignoringv_r. The mass loss rates of the SMC objects having luminosities of logL*/Lȯ ≳ 5.4 are in excellent agreementwith predictions by Vink et al. (2001, A&A, 369, 574). However, forlower luminosity stars the winds are too weak to determine dot{M}accurately from the optical spectrum. Three targets were classifiedas Vzstars, two of which are located close to the theoretical zero-age mainsequence. Three lower luminosity targets that were not classified as Vzstars are also found to lie near the ZAMS. We argue that this is relatedto a temperature effect inhibiting cooler from displaying the spectralfeatures required for the Vz luminosity class.
| 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.
| Far Ultraviolet Spectroscopic Explorer Survey of Magellanic Cloud Supernova Remnants
We report the progress to date from an ongoing unbiased ultravioletsurvey of supernova remnants in the Magellanic Clouds using the FarUltraviolet Spectroscopic Explorer (FUSE) satellite. Earlier work withFUSE and other instruments has indicated that optical and/or X-raycharacteristics of supernova remnants are not always good predictors oftheir brightness in the ultraviolet. This survey is obtaining spectra ofa random large sample of Magellanic Cloud supernova remnants with abroad range of radio, optical, and X-ray properties. We proposed 39objects in the Large Magellanic Cloud and 11 objects from the SmallMagellanic Cloud, with a standard request of 10 ks per object using theFUSE 30" square aperture. To date, 39 objects have been observed in thesurvey (38 in the LMC and 1 in the SMC) and 15 have been detected, adetection rate of nearly 40%. Our survey has nearly tripled the numberof UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because ofthe diffuse source sensitivity of FUSE, upper limits on nondetectedobjects are quite sensitive in many cases, dependent on night observingfraction and whether stellar light contamination plays a role for agiven object. Estimated total luminosities in O VI, based simply onscaling the flux at the observed positions to an entire object, span abroad range from considerably brighter to many times fainter than theinferred soft X-ray luminosities, indicating that O VI can be animportant and largely unrecognized coolant in certain objects. Wecompare the optical and X-ray properties of the detected and nondetectedobjects but do not find a simple indicator for ultravioletdetectability. Nondetections may be due to clumpiness of the emission,high foreground extinction, slow shocks whose emission gets attenuatedby the Magellanic interstellar medium, or a combination of theseeffects. The characteristics of individual detected supernova remnantsare summarized in an Appendix.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Primordial Helium Abundance: A Reanalysis of the Izotov-Thuan Spectroscopic Sample
A reanalysis is made for the helium abundance determination for theIzotov-Thuan spectroscopic sample of extragalactic H II regions. We findthat the effect of underlying stellar absorption of the He I lines,which is more important for metal-poor systems, affects significantlythe inferred primordial helium abundance Yp obtained in thezero-metallicity limit and the slope of linear extrapolation, dY/dZ.This brings Yp from 0.234+/-0.004 to 0.250+/-0.004 anddY/dZ=4.7+/-1.0 to 1.1+/-1.4. Conservatively, this indicates theimportance of the proper understanding of underlying stellar absorptionfor accurate determinations of the primordial helium abundance to theerror of δYp~=0.002-0.004.
| Hα line profiles for a sample of supergiant HII regions. I. The main spectral component
We present an analysis of the Hα emission line profiles of asample of giant extragalactic HII regions, selected from among thebrightest and most isolated in a group of spiral galaxies for which wehave photometric and spectroscopic data: NGC 157, NGC 3631, NGC 6764,NGC 3344, NGC 4321, NGC 5364, NGC 5055, NGC 5985, NGC 7479. Our studyconfirms that the majority of the line profiles are composed of abright, main component and two fainter, high velocity components that wedenominate wings. Here, we analyze the kinematics of the principalcomponents, finding a relation between the Hα luminosity,L_Hα, and the turbulent velocity dispersion, σ_nt. A linearfit to the relation between these quantities yields log L_Hα =(35.6 ± 0.4) + (2.87 ± 0.2)log σ_nt, in agreementwith previous studies. We compute the mass of each HII region using boththe virial theorem and the Hα luminosity, confirming that, thoughthese estimates do not coincide exactly, they are comparable within theuncertainties and consequently that the HII regions in our sample areapproximately virialized.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| Age distribution of young clusters and field stars in the Small Magellanic Cloud
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
| An ATCA radio-continuum study of the Small Magellanic Cloud - IV. A multifrequency analysis of the N66 region
Traditional identification of supernova remnants (SNRs) include the useof radio spectral index, optical spectral studies (including strong[SII], [NII], [OI], [OII] and [OIII] lines) and X-rayco-identifications. Each of these can have significant limitationswithin the context of a particular SNR candidate and new identificationmethods are continually sought. In this paper, we explore subtractiontechniques by Ye, Turtle and Kennicutt to remove thermal emissionestimated from Hα flux from radio-continuum images. The remainingnon-thermal emission allows the identification of SNRs embedded withinthese HII regions. Subtraction images of the N66 region in the SmallMagellanic Cloud (SMC) using Hα wide-field optical CCD images fromthe Curtis Schmidt Telescope and the recent Australia Telescope CompactArray (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640MHz) dataare presented as an example. These show three SNRs (B0057 - 724, B0056 -724 and B0056 - 725) separated from their surrounding HII radioemission. 2.3-m dual-beam spectrograph long-slit spectra from selectedregions within N66 suggest the presence of an additional SNR with noradio or X-ray emission. Radio spectral index, [SII]/Hα ratio andarchived Chandra images of N66 combine to give a more coherent pictureof this region, confirming B0057 - 724 as an SNR. The N66 nebula complexis divided into 10 components, composed separately of these SNRs and HIIregions.
| Discovery of a Population of Pre-Main-Sequence Stars in NGC 346 from Deep Hubble Space Telescope ACS Images
We report the discovery of a rich population of low-mass stars in theyoung, massive NGC 346 star-forming region in the Small Magellanic Cloudfrom deep V, I, and Hα images taken with the Hubble SpaceTelescope Advanced Camera for Surveys. These stars have likely formedtogether with the NGC 346 cluster, ~=3-5 Myr ago. Their magnitudes andcolors are those of pre-main-sequence stars in the mass range 0.6-3Msolar, mostly concentrated in the main cluster, but withsecondary subclusters spread over a region across ~45 pc. Thesesubclusters appear to be spatially coincident with previously knownknots of molecular gas identified in ground-based and Infrared SpaceObservatory observations. We show that NGC 346 is a complex region,being shaped by its massive stars, and the observations presented hererepresent a key step toward the understanding of how star formationoccurred and has progressed in this low-metallicity environment.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Fundamental Properties of O-Type Stars
We present a comprehensive analysis of high-resolution, far-ultravioletHST STIS, FUSE, and optical spectra of 18 O stars in the SmallMagellanic Cloud. Our analysis is based on the OSTAR2002 grid of NLTEmetal-line-blanketed model atmospheres calculated with our code TLUSTY.We systematically explore and present the sensitivity of various UV andoptical lines to different stellar parameters. We have obtainedconsistent fits of the UV and the optical spectrum to derive theeffective temperature, surface gravity, surface composition, andmicroturbulent velocity of each star. Stellar radii, masses, andluminosities follow directly. For stars of the same spectral subtype, wefind a general good agreement between effective temperaturedeterminations obtained with TLUSTY, CMFGEN, and FASTWIND models, whichare all lower than the standard Teff calibration of O stars.We propose a new calibration between the spectral type and effectivetemperature based on our results from UV metal lines, as well as opticalhydrogen and helium lines. The lower effective temperatures translateinto ionizing luminosities that are smaller by a factor of 3 compared toluminosities inferred from previous standard calibrations. The chemicalcomposition analysis reveals that the surface of about 80% of theprogram stars is moderately to strongly enriched in nitrogen, whileshowing the original helium, carbon, and oxygen abundances. Our resultssupport the new stellar evolution models that predict that the surfaceof fast rotating stars becomes nitrogen-rich during the main-sequencephase because of rotationally induced mixing. Enrichment factors are,however, larger than predicted by stellar evolution models. Most starsexhibit the ``mass discrepancy'' problem, which we interpret as a resultof fast rotation that lowers the measured effective gravity. Nitrogenenrichment and low spectroscopic masses are therefore two manifestationsof fast rotation. Our study thus emphasizes the importance of rotationin our understanding of the properties of massive stars and provides aframework for investigating populations of low-metallicity massive starsat low and high redshifts.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-2655. Also based on observations made with theNASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer, which is operatedfor NASA by Johns Hopkins University under NASA contract NAS5-32985.
| The Low-Mass Pre-Main-Sequence Population of the Stellar Association LH 52 in the Large Magellanic Cloud Discovered with Hubble Space Telescope WFPC2 Observations
We report on the serendipitous discovery of ~500 low-mass candidatepre-main-sequence (PMS) stars in the vicinity of the stellar associationLH 52 in the Large Magellanic Cloud. We present evidence that the redfaint sequence of these stars seen in the CMD of LH 52 from Hubble SpaceTelescope (HST) WFPC2 observations belongs only to the association andfollows almost perfectly isochrone models for PMS stars of masses downto ~0.3 Msolar. We find that this feature has a Galacticcounterpart and that the mass spectrum of the candidate PMS stars in LH52 seems to correspond to a Salpeter initial mass function with a slopeΓ~=-1.26 in the mass range 0.8-1.4 Msolar.
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