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PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC 3603
We present spatial distribution of polycyclic aromatic hydrocarbons(PAHs) and ionized gas within the Galactic giant H II region NGC 3603.Using the IRS instrument on board the Spitzer Space Telescope, we studyin particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3μm, and the [Ar II] 6.99 μm, [Ne II] 12.81 μm, [Ar III] 8.99μm, and [S IV] 10.51 μm forbidden emission lines. The observationsprobe both ionized regions and photodissociation regions. Silicateemission is detected close to the central cluster while silicateabsorption is seen further away. We find no significant variation of thePAH ionization fraction across the whole region. The emission of verysmall grains (VSGs) lies closer to the central stellar cluster thanemission of PAHs. The PAH/VSG ratio anticorrelates with the hardness ofthe interstellar radiation field suggesting a destruction mechanism ofthe molecules within the ionized gas, as shown for low-metallicitygalaxies by Madden et al.
| And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae
We report the discovery of two new circumstellar ring nebulae in thewestern Carina Nebula, and we discuss their significance in stellarevolution. The brighter of the two new objects, SBW 1, resembles alidless staring eye and encircles a B1.5 Iab supergiant. Although seenin Carina, its luminosity class and radial velocity imply a largerdistance of ~7 kpc in the far Carina arm. At that distance its size andshape are nearly identical to the equatorial ring around SN 1987A, butSBW 1's low N abundance indicates that the ring was excreted without itsstar passing through a red supergiant phase. The fainter object, SBW 2,is a more distorted ring, is N-rich, and is peculiar in that its centralstar seems to be invisible. We discuss the implications of these two newnebulae in context with other circumstellar rings such as those aroundSN 1987A, Sher 25, HD 168625, RY Scuti, WeBo 1, SuWt 2, and others. Thering bearers fall into two groups: Five rings surround hot supergiants,and it is striking that all except for the one known binary are carboncopies of the ring around SN 1987A. We propose a link between these Bsupergiant rings and B[e] supergiants, where the large spatiallyresolved rings derive from the same material that would have given riseto emission lines during the earlier B[e] phase, when it was much closerto the star. The remaining four rings surround evolved intermediate-massstars; all members of this ring fellowship are close binaries, hintingthat binary interactions govern the forging of such rings. Two-thirds ofour sample are found in or near giant H II regions. We estimate thatthere may be several thousand more dark rings in the Galaxy, but we arescarcely aware of their existence-either because they are onlyilluminated in precious few circumstances or because of selectioneffects. For intermediate-mass stars, these rings might be thepreexisting equatorial density enhancements invoked to bind the waistsof bipolar nebulae.Based in part on observations made at the Clay Telescope of the MagellanObservatory, a joint facility of the Carnegie Observatories, HarvardUniversity, the Massachusetts Institute of Technology, the University ofArizona, and the University of Michigan.
| On the Likelihood of Supernova Enrichment of Protoplanetary Disks
We estimate the likelihood of direct injection of supernova ejecta intoprotoplanetary disks using a model in which the number of stars withdisks decreases linearly with time, and clusters expand linearly withtime such that their surface density is independent of stellar number.The similarity of disk dissipation and main-sequence lifetimes impliesthat the typical supernova progenitor is very massive, ~75-100Msolar. Such massive stars are found only in clusters with>~104 members. Moreover, there is only a small regionaround a supernova within which disks can survive the blast yet beenriched to the level observed in the solar system. These two factorslimit the overall likelihood of supernova enrichment of a protoplanetarydisk to <~1%. If the presence of short-lived radionucleides inmeteorites is to be explained in this way, however, the solar systemmost likely formed in one of the largest clusters in the Galaxy, morethan 2 orders of magnitude greater than Orion, where multiple supernovaeimpacted many disks in a short period of time.
| Super Star Cluster Velocity Dispersions and Virial Masses in the M82 Nuclear Starburst
We use high-resolution near-infrared spectroscopy from Keck Observatoryto measure the stellar velocity dispersions of 19 super star clusters(SSCs) in the nuclear starburst of M82. The clusters have ages on theorder of 10 Myr, which is many times longer than the crossing timesimplied by their velocity dispersions and radii. We therefore apply thevirial theorem to derive the kinematic mass for 15 of the SSCs. The SSCshave masses of 2×105 to 4×106Msolar, with a total population mass of1.4×107 Msolar. Comparison of the loci ofthe young M82 SSCs and old Milky Way globular clusters in a plot ofradius versus velocity dispersion suggests that the SSCs are apopulation of potential globular clusters. We present the mass functionfor the SSCs and find a power-law fit with an index ofγ=-1.91+/-0.06. This result is nearly identical to the massfunction of young SSCs in the Antennae galaxies.Based on observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| The Calibration of the O/H Abundance Indicators for Extragalactic H II Regions based on O II Recombination Lines
Based on O II recombination lines we present a new calibration (called OII_{RL}) of Pagel's O_{23} indicator to determine the O/H abundanceratio in extragalactic H II regions and emission line galaxies. The OII_{RL} calibration produces O/H abundances about a factor of two higherthan those derived from the T(4363) method with t^2 = 0.00. The OII_{RL} calibration has implications for the study of differentproperties of emission line galaxies such as their metallicity, starformation rate, and initial mass function. The O II_{RL} calibrationalso affects the abundance determinations based on other O/H indicators,that include collisionally excited lines, like those known as O_3N_2,N_2, S_{23], Ar_3O_3, and S_3O_3. We argue that the controversy betweenthe T(4363) method and the photoionization models method to derive O/Hvalues is mainly due to temperature variations inside the observed H IIregions.
| Discovery of Hot Supergiant Stars near the Galactic Center
We report new results of a campaign to find Wolf-Rayet and O (WR/O)stars and high-mass X-ray binaries (HMXBs) in the Galactic center. Wesearched for candidates by cross-correlating the Two Micron All SkySurvey with a deep Chandra X-Ray Observatory catalog of point sources inthe Radio Arches region. Following up with K-band spectroscopy, we foundtwo massive stellar counterparts to CXOGC J174555.3-285126 and CXOGCJ174617.0-285131, which we classify as a broad-lined WR star of subtypeWN6b and an O Ia supergiant, respectively. Their X-ray properties aremost consistent with those of known colliding-wind binaries in theGalaxy and the Large Magellanic Cloud, although a scenario involvinglow-rate accretion onto a compact object is also possible. The O Ia starlies 4.4 pc in projection from the Quintuplet cluster and has a radialvelocity consistent with that of the Quintuplet, suggesting that thisstar might have escaped from the cluster. We also present the discoveryof a B2 Ia supergiant, which we identified as a candidate massive starusing 8 μm Spitzer maps of the Galactic center in a region near theknown massive X-ray-emitting star CXOGC J174516.1-290315. We discuss theorigin of these stars in the context of evolving stellar clusters in theGalactic center.
| Detection of extended very-high-energy γ-ray emission towards the young stellar cluster Westerlund 2
Aims. Results from γ-ray observations by the HESS telescope arrayin the direction of the young stellar cluster Westerlund 2 arepresented. Methods: Stereoscopic imaging of Cherenkov light emission ofγ-ray induced showers in the atmosphere is used to study thecelestial region around the massive Wolf-Rayet (WR) binary WR 20a.Spectral and positional analysis is performed using standard eventreconstruction techniques and parameter cuts. Results: The detection ofa new γ-ray source is reported from HESS observations in 2006.HESS J1023-575 is found to be coincident with the young stellar clusterWesterlund 2 in the well-known HII complex RCW 49. The source isdetected with a statistical significance of more than 9σ, andshows extension beyond a point-like object within the HESS point-spreadfunction. The differential γ-ray spectrum of the emission regionis measured over approximately two orders of magnitude in flux. Conclusions: The spatial coincidence between HESS J1023-575 and theyoung open cluster Westerlund 2, hosting e.g. the massive WR binary WR20a, requires one to look into a variety of potential models to accountfor the observed very-high-energy (VHE) γ-ray emission. Consideredemission scenarios include emission from the colliding wind zone of WR20a, collective stellar winds from the extraordinary ensemble of hot andmassive stars in the stellar cluster Westerlund 2, diffusive shockacceleration in the wind-blown bubble itself, and supersonic windsbreaking out into the interstellar medium (ISM). The observed sourceextension argues against a single star origin of the observed VHEemission.
| The chemical composition of the Galactic H II regions M8 and M17. A revision based on deep VLT echelle spectrophotometry
We present new echelle spectrophotometry of the Galactic H II regions M8and M17. The data have been taken with the VLT UVES echelle spectrographin the 3100 to 10400 Å range. We have measured the intensities of375 and 260 emission lines in M8 and M17 respectively, increasingsignificatively the number of emission lines measured in previousspectrophotometric studies of these nebulae. Most of the detected linesare permitted lines. Electron temperatures and densities have beendetermined using different diagnostics. We have derived He^+,C++, O^+ and O^{++} ionic abundances from pure recombinationlines. We have also derived abundances from collisionally excited linesfor a large number of ions of different elements. Highly consistentestimations of t^2 have been obtained by using different independentindicators; the values are moderate and very similar to those obtainedin other Galactic H II regions. We report the detection of deuteriumBalmer emission lines, up to Dɛ, in M8 and show that theirintensities are consistent with continuum fluorescence as their mainexcitation mechanism.
| A Magnetically Supported Photodissociation Region in M17
The southwestern (SW) part of the Galactic H II region M17 contains anobscured ionization front that is most easily seen at infrared and radiowavelengths. It is nearly edge-on, thus offering an excellentopportunity to study the way in which the gas changes from fully ionizedto molecular as radiation from the ionizing stars penetrates into thegas. M17 is also one of the very few H II regions for which the magneticfield strength can be measured in the photodissociation region ( PDR)that forms the interface between the ionized and molecular gas. Here wemodel an observed line of sight through the gas cloud, including theH+, H0 (PDR), and molecular layers, in a fullyself-consistent single calculation. An interesting aspect of the M17 SWbar is that the PDR is very extended. We show that the strong magneticfield that is observed to be present inevitably leads to a very deepPDR, because the structure of the neutral and molecular gas is dominatedby magnetic pressure, rather than by gas pressure, as previously hadbeen supposed. We also show that a wide variety of observed facts can beexplained if a hydrostatic geometry prevails, in which the gas pressurefrom an inner X-ray hot bubble and the outward momentum of the stellarradiation field compress the gas and its associated magnetic field inthe PDR, as has already been shown to occur in the Orion Nebula. Themagnetic field compression may also amplify the local cosmic-raydensity. The pressure in the observed magnetic field balances theoutward forces, suggesting that the observed geometry is a naturalconsequence of the formation of a star cluster within a molecular cloud.
| Near-IR imaging of Galactic massive clusters: Westerlund 2
Context: Most stars in the Galaxy were formed in massive clusters. Tounderstand nature's favorite mode of star formation and the initialstages of the life of most stars one needs to characterize the youngestand resolved massive clusters in the Milky Way. Unfortunately youngmassive clusters are challenging observational targets as they are rare,hence found at large distances, are still embedded in their parentalmolecular cloud, and are swamped by relatively bright nebulae. Aims: Inthis paper we propose to use deep subarcsec resolution NIR data toderive the basic parameters of the unstudied population of massivecluster Westerlund 2. Methods: We present deep JHKs images(~0.6'' seeing) and photometry of Westerlund 2. This is the mostcomplete photometric census of the cluster's population to date. Results: We detect a total of 4701, 5724, and 5397 sources in the J, H,and Ks bands respectively. By comparison with main-sequenceand pre-main-sequence model tracks, we determine an average visualextinction toward the cluster of 5.8 mag, a likely distance of 2.8 kpc,and an age of 2.0 ± 0.3 Myr for the core of the cluster. Althoughwe have the sensitivity to reach beyond the hydrogen burning limit inthe cluster, we are only complete to about 1 Mȯ due tosource confusion. We find no evidence for a top-heavy MF, and the slopeof the derived mass function is -1.20 ± 0.16. Based on theextrapolation of a field IMF, we roughly estimate the total mass of thecluster to be about 104 Mȯ. We findcompelling evidence for mass segregation in this cluster.Based on observations collected with the SOFI instrument at the NTT atthe La Silla Observatory, Chile, under the ESO program 074.C-0728(A).The tables with the photometry (Table 2) are only available inelectronic 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/466/137
| Formation of a massive protostar through disk accretion. II. SINFONI integral field spectroscopy of the M 17 silhouette disk and discovery of the associated H{2} jet
Context: Recent observational results suggest that high-mass(proto-)stars form in a similar way to low- and intermediate-mass stars,i.e., via disk accretion of gas and dust. Aims: To furthercharacterize and understand the on-going physical processes associatedwith a large circumstellar disk, seen as a dark silhouette against thebright background of the M 17 H ii region, we report and discuss newhigh angular resolution integral field spectroscopy performed in theK-band. Methods: The data were taken with the Adaptive Optics (AO)supported near-infrared integral field spectrograph SINFONI at ESO'sVery Large Telescope Yepun (VLT UT 4) as part of the scienceverification of this new instrument. Results: Based on obtained H{2} v= 1-0 S(1) and H{2} v = 1-0 S(3) emission maps, we report the discoveryof a H{2} jet, which apparently arises from the suspected protostellarsource(s) located at the very center of the disk. In addition, bothdiameter (about 4000 AU) and sub-structures of the innermost, densestpart of the flared disk are infered from Br γ, Br δ, and HeI maps. Conclusions: .Because ejection of material through ajet/outflow is always linked to accretion of gas and dust either ontothe circumstellar disk or onto the central (protostellar) source(s), thepresence of a collimated H{2} jet provides indirect but unquestionableevidence for ongoing accretion processes in the case of the M 17 disk.The high mass outflow and accretion rates of >10-5Mȯ yr-1 derived from the SINFONI data suggestthat a star of rather high mass is forming.Based on observations performed at the European Southern ObservatoryVery Large Telescope on Cerro Paranal, Chile as part of the SINFONIscience verification, program ID 60.A-9041(A). Obtained data sets arepublicly available via the ESO archivehttp://archive.eso.org/eso/eso_archive_main.html
| Searching for hidden Wolf-Rayet stars in the Galactic plane - 15 new Wolf-Rayet stars
We report the discovery of 15 previously unknown Wolf-Rayet (WR) starsfound as part of an infrared (IR) broad-band study of candidate WR starsin the Galaxy. We have derived an empirically based selection algorithmwhich has selected ~5000 WR candidate stars located within the Galacticplane drawn from the Galactic Legacy Infrared Mid-Plane SurveyExtraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR)catalogues. Spectroscopic follow-up of 184 of these reveals 11nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypesare absent from our sample and none shows evidence for circumstellardust emission. Of the candidates which are not WR stars, ~120 displayedhydrogen emission-line features in their spectra. Spectral featuressuggest that the majority of these are in fact Bsupergiants/hypergiants, ~40 of these are identified Be/B[e] candidates.Here, we present the optical spectra for six of the newly detected WRstars, and the near-IR spectra for the remaining nine of our sample.With a WR yield rate of ~7 per cent and a massive star detection rate of~65 per cent, initial results suggest that this method is one of themost successful means for locating evolved, massive stars in the Galaxy.
| Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.
| Early-type stars in the core of the young open cluster Westerlund 2
Aims.The properties of the early-type stars in the core of theWesterlund 2 cluster are examined in order toestablish a link between the cluster and the very massive Wolf-Rayetbinary WR 20a as well as the H ii complexRCW 49. Methods: Photometric monitoring as well asspectroscopic observations of Westerlund 2 are used to search for lightvariability and to establish the spectral types of the early-type starsin the cluster core. Results: The first light curves of the eclipsingbinary WR 20a in B and V filters are analysed and a distance of 8 kpc isinferred. Three additional eclipsing binaries, which are probable late Oor early B-type cluster members, are discovered, but none of the knownearly O-type stars in the cluster displays significant photometricvariability above 1% at the 1-σ level. The twelve brightest O-typestars are found to have spectral types between O3 and O6.5,significantly earlier than previously thought. Conclusions: .Thedistance of the early-type stars in Westerlund 2 is established to be inexcellent agreement with the distance of WR 20a, indicating that WR 20aactually belongs to the cluster. Our best estimate of the clusterdistance thus amounts to 8.0 ± 1.4 kpc. Despite the earlierspectral types, the currently known population of early-type stars inWesterlund 2 does not provide enough ionizing photons to account for theradio emission of the RCW 49 complex. This suggests that there mightstill exist a number of embedded early O-stars in RCW 49.Based on observations collected at the Cerro Tololo InteramericanObservatory and at the European Southern Observatory (La Silla, Chile).Tables 2, 3 and 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/981
| The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry
We present echelle spectrophotometry of the blue compact dwarf galaxyNGC 5253 obtained with the VLT UVES. We have measured the intensities ofa large number of permitted and forbidden emission lines in four zonesof the central part of the galaxy. We detect faint C II and O IIrecombination lines, the first time that these are unambiguouslydetected in a dwarf starburst galaxy. The physical conditions of theionized gas have been derived using a large number of different lineintensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fehave been determined following standard methods. C++ andO++ abundances have been derived from pure recombinationlines and are larger than those obtained from collisionally excitedlines (from 0.30 to 0.40 dex for C++ and from 0.19 to 0.28dex for O++). This result is consistent with a temperaturefluctuation parameter (t2) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized Nenrichment in certain zones of NGC 5253 and detect a possible slight Heoverabundance in the same zones. The enrichment pattern agrees with thatexpected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. Theamount of enriched material needed to produce the observed overabundanceis consistent with the mass lost by the number of W-R stars estimated inthe starbursts. We discuss the possible origin of the difference betweenabundances derived from recombination and collisionally excited lines(the so-called abundance discrepancy problem) in H II regions, findingthat a recent hypothesis based on the delayed enrichment by SN ejectainclusions seems not to explain the observed features.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 70.C-0008(A).
| Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries
We study the first ~100 Myr of the evolution of isolated star clustersinitially containing 144179 stars, including 13107 (10 per cent)primordial hard binaries. Our calculations include the effects of bothstellar and binary evolution. Gravitational interactions among the starsare computed by direct N-body integration using high-precision GRAPE-6hardware. The evolution of the core radii and central concentrations ofour simulated clusters are compared with the observed sample of young(<~100Myr) star clusters in the large Magellanic Cloud. Even thoughour simulations start with a rich population of primordial binaries,core collapse during the early phase of the cluster evolution is notprevented. Throughout the simulations, the fraction of binaries remainsroughly constant (~10 per cent). Due to the effects of mass segregationthe mass function of intermediate-mass main-sequence stars becomes asflat as α = -1.8 in the central part of the cluster (where theinitial Salpeter mass function had α = -2.35). About 6-12 per centof the neutron stars were retained in our simulations; the fraction ofretained black holes is 40-70 per cent. In each simulation about threeneutron stars become members of close binaries with a main-sequencecompanion. Such a binary will eventually become an X-ray binary, whenthe main-sequence star starts to fill its Roche lobe. Black holes arefound more frequently in binaries; in each simulated cluster we find ~11potential X-ray binaries containing a black hole. Binaries consisting oftwo white dwarfs are quite common, but few (20-30) are sufficientlyclose that they will merge within a Hubble time due to the emission ofgravitational radiation. Clusters with shorter relaxation times tend toproduce fewer merging white dwarf binaries. The white dwarf binariesthat do merge are all sufficiently massive to produce a Type Iasupernova. The densest cluster produces about twice as many bluestragglers as a field population containing the same number of binaries,and these blue stragglers are more massive, bluer and brighter than inless dense clusters.
| X-Ray Study of Triggered Star Formation and Protostars in IC 1396N
The IC 1396N cometary globule (CG) within the large nearby H II regionIC 1396 has been observed with the ACIS detector on board the ChandraX-Ray Observatory. We detect 117 X-ray sources, of which ~50-60 arelikely members of the young open cluster Trumpler 37 dispersedthroughout the H II region, and 25 are associated with young starsformed within the globule. Infrared photometry (2MASS and Spitzer) showsthat the X-ray population is very young: 3 older Class III stars, 16classical T Tauri stars, and 6 protostars including a Class 0/I system.We infer a total T Tauri population of ~30 stars in the globule,including the undetected population, with a star formation efficiency of1%-4%. An elongated source spatial distribution with an age gradientoriented toward the exciting star is discovered in the X-ray populationof IC 1396N, supporting similar findings in other cometary globules. Thegeometric and age distribution is consistent with the radiation-drivenimplosion (RDI) model for triggered star formation in CGs by H II regionshocks. The inferred velocity of the shock front propagating into theglobule is ~0.6 km s-1. The large number of X-ray-luminousprotostars in the globule suggests either an unusually high ratio ofClass I/0 to Class II/III stars or a nonstandard initial mass functionfavoring higher mass stars by the triggering process. We find that theChandra source associated with the luminous Class 0/I protostar IRAS21391+5802 is one of the youngest stars ever detected in the X-ray band.We also establish for the first time that the X-ray absorption inprotostars arises from the local infalling envelopes rather than fromambient molecular cloud material.
| Dynamical masses of two young globular clusters in the blue compact galaxy ESO 338-IG04
We present high-resolution échelle spectroscopy, obtained withthe UVES spectrograph on ESO/VLT, of two luminous star clusters in themetal-poor blue compact galaxy ESO 338-IG04 at a distance of 37.5 Mpc.Cross-correlating with template stars, we obtain line-of-sight velocitydispersions of 33 and 17 km s-1. By combining with sizeestimates from Hubble Space Telescope images we infer dynamical massesof 1.3×107 M_ȯ and 4.0×106M_ȯ for the two clusters, making them among the most massive known.The less massive cluster is the faintest cluster for which a dynamicalmass has yet been obtained. In both clusters we detect Balmer absorptionlines which we use to estimate their ages. From the younger (~6 Myr) andmore massive cluster, we detect H ii λ4686 emission ofintermediate width, indicating the presence of very massive O-stars.Moreover, analysis of the [O iii] λ5007 and Hα emissionlines from the region near the younger cluster indicates that it isassociated with a bubble expanding at ~40 km s-1. We also seefrom the Na i D absorption lines indications of neutral gas flowstowards the younger cluster. We compare the dynamical masses with thosederived from photometry and discuss implications for the stellar initialmass function (IMF) in each cluster. Both clusters are compatible withrather normal IMFs which will favour their long-term survival andevolution into massive bona fide globular clusters.
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| On the progenitor of supernova 1987 A
A previously unpublished ultralow-dispersion spectrum of Sand- uleak -69202, the stellar progenitor of SN 1987A, is presented and the uncertainpresupernova evolution of Sanduleak -69 202 is discussed.
| The Southern Galactic Plane Survey: Polarized Radio Continuum Observations and Analysis
The Southern Galactic Plane Survey (SGPS) is a radio survey in the 21 cmH I line and in 1.4 GHz full-polarization continuum, observed with theAustralia Telescope Compact Array and the Parkes 64 m single-dishtelescope. The survey spans a Galactic longitude of253deg
| The Arches Cluster Mass Function
We have analyzed H- and Ks-band images of the Arches clusterobtained using the NIRC2 instrument on Keck with the laser guide staradaptive optics (LGS AO) system. With the help of the LGS AO system, wewere able to obtain the deepest ever photometry for this cluster and itsneighborhood and derive the background-subtracted present-day massfunction down to 1.3 Msolar for the5''-9'' annulus of the cluster. We find that thepreviously reported turnover at 6 Msolar is simply due to alocal bump in the mass function (MF), and that the MF continues toincrease down to our 50% completeness limit (1.3 Msolar) witha power-law exponent of Γ= -0.91 for the mass range of1.3
| Hard X-Rays from Ultracompact H II Regions in W49A
We report the Chandra detection of hard X-ray emission from the Welchring in W49A, an organized structure of ultracompact (UC) H II regionscontaining a dozen nascent early-type stars. Two UC H II regions areassociated with hard X-ray emission in a deep Advanced CCD ImagingSpectrometer (ACIS) image exposed for ~96.7 ks. One of the two X-raysources has no near-infrared counterpart and is extended by ~5", or ~0.3pc, at a distance of ~11.4 kpc, which is spatially aligned with thecometary radio continuum emission associated with the UC H II region.The X-ray spectrum of the emission, when fit with a thermal model,indicates a heavily absorbed plasma with extinction of~5×1023 cm-2, temperature ~7 keV, and X-rayluminosity in the 3.0-8.0 keV band of ~3×1033 ergss-1. Both the luminosity and the size of the emissionresemble the extended hard emission found in UC H II regions inSagittarius B2, yet they are smaller by an order of magnitude than theemission found in massive star clusters such as NGC 3603. Threepossibilities are discussed for the cause of the hard extended emissionin the Welch ring: an ensemble of unresolved point sources, shockedinteracting winds of the young O stars, and a wind-blown bubbleinteracting with ambient cold matter.
| Subarcsecond-Resolution Radio Maps of Nearby Spiral Galaxies
We report subarcsecond-resolution Very Large Array imaging of fournearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. Ineach galaxy, compact radio continuum sources are identified in thecentral ~15''×15'' region. These compactsources are responsible for 20%-30% of the total emission from thecentral kiloparsec of the host galaxies at 2 cm but only ~5%-10% at 6cm. More than half of the compact sources appear to be H II regions. TheH II regions with rising spectra must be fairly dense(ni~104 cm-3) and are presumably veryyoung. The largest of these H II regions require the excitation of500-800 O stars within regions only a few parsecs in extent. Theseclusters approach the sizes expected for globular clusters. Thermalfree-free emission from compact sources contributes more significantlyat 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radioH II regions are found near the centers of giant molecular clouds inprojection and do not have obvious visual counterparts.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| The rich young cluster NGC 6530: a combined X-ray-optical-infrared study
We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530,complementing our previous studies of this cluster. We considerdifferent indicators of IR excesses, which can be taken as an indicatorof circumstellar disks and therefore of pre-main-sequence status. Weused reddening-free indices to ensure that our results are unaffected byhighly irregular, differential reddening. More than the study of the JHKbands alone (33 IR-excess stars found), we found it useful to comparevarious optical and IR colors, finding overall 333 stars with optical-IRexcess. In the field of the previously studied Chandra ACIS-Iobservation, we found 196 stars with optical-IR excess, of which 120have not been previously found in X-rays. The total number of estimatedcluster members thus becomes ≥1100. The estimated disk frequency inthe ACIS field is ~20%. By considering different optical-IR excessindices, we find only partially overlapping subsamples, corresponding todifferent characteristics of their spectral energy distributions(longer- or shorter-wavelength IR excesses, or blue-violet excess). In aregion displaced towards northwest with respect to the known clustercenter, we find an unexpected concentration of stars with optical-IRexcesses, most of which are not detected in X-rays. The spectral energydistribution of some of these objects shows more extreme excesses withrespect to most ordinary T Tauri stars, which suggests circumstellardisks with very small inner holes and high accretion rates; some objectsare best interpreted as being surrounded by reflection nebulosities, asfound in some optically detected Class I objects in nearer star-formingregions. These reflection-nebulae candidates have the lowest X-raydetection rate among all subsamples considered here. Optical-IR excessstars in the north of NGC 6530 are nearly co-spatial with asub-population of cluster stars older than the central cluster stars.This leads to the conclusion that in these northern regions of NGC 6530,far from massive cluster stars, star formation (and disk evolution) hasproceeded rather undisturbed over longer time periods than near thecluster center, where most massive stars are found, and most stars lacksubstantial disks and strong accretion.
| Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR140
We use hydrodynamical models of the wind-collision region in thearchetype colliding-wind system WR140 to determine the spatial andspectral distributions of the radio, X-ray, and γ-ray emissionfrom shock-accelerated electrons. Our calculations are for orbital phase0.837 when the observed radio emission is close to maximum. Using theobserved thermal X-ray emission at this phase in conjunction with theradio emission to constrain the mass-loss rates, we find that the O starmass-loss rate is consistent with the reduced estimates for O4-5supergiants by Fullerton, Massa & Prinja, and the wind-momentumratio, η = 0.02. This is independent of the opening angle deducedfrom radio very long baseline interferometry observations of the WCRthat we demonstrate fail to constrain the opening angle.We show that the turnover at ~3 GHz in the radio emission is due tofree-free absorption, since models based on the Razin effect have anunacceptably large fraction of energy in non-thermal electrons. We findthat the spectral index of the non-thermal electron energy distributionis flatter than the canonical value for diffusive shock acceleration,namely p < 2. Several mechanisms are discussed that could lead tosuch an index. Our inability to obtain fits to the radio data with p> 2 does not exclude the possibility of shock modification, butstronger evidence than that which currently exists is necessary for itssupport.Tighter constraints on p and the nature of the shocks in WR140 will beobtained from future observations at MeV and GeV energies, for which wegenerally predict lower fluxes than those in previous works. Since thehigh stellar photon fluxes prevent the acceleration of electrons beyondγ >~ 105-106, TeV emission fromcolliding-wind binary systems will provide unambiguous evidence ofpion-decay emission from accelerated ions. We finish by commenting onthe emission and physics of the multiple wind collisions in densestellar clusters, paying particular attention to the Galactic Centre.
| Non-equilibrium ionization model for stellar cluster winds and its application
We have developed a self-consistent physical model for superstellarcluster winds based on combining a 1D steady-state adiabatic windsolution and a non-equilibrium ionization calculation. Comparing withthe case of collisional ionization equilibrium, we find that thenon-equilibrium ionization effect is significant in the regime of a highratio of energy to mass input rate and manifests in a stronger softX-ray flux in the inner region of the star cluster. Implementing themodel in X-ray data analysis softwares (e.g. XSPEC) directly facilitatescomparisons with X-ray observations. Physical quantities such as themass and energy input rates of stellar winds can be estimated by fittingobserved X-ray spectra. The fitted parameters may then be compared withindependent measurements from other wavelengths. Applying our model tothe star cluster NGC 3603, we find that the wind accounts for no morethan 50 per cent of the total `diffuse' emission, and the derived massinput rate and terminal velocity are comparable to other empiricalestimates. The remaining emission most likely originate from numerouslow-mass pre-main-sequence stellar objects.
| 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).
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