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Embedded Star Formation in the Eagle Nebula with Spitzer GLIMPSE
We present new Spitzer photometry of the Eagle Nebula (M16, containingthe optical cluster NGC 6611) combined with near-infrared photometryfrom 2MASS. We use dust radiative transfer models, mid-infrared andnear-infrared color-color analysis, and mid-infrared spectral indices toanalyze point-source spectral energy distributions, select candidateYSOs, and constrain their mass and evolutionary state. Comparison of thedifferent protostellar selection methods shows that mid-infrared methodsare consistent, but as has been known for some time, near-infrared-onlyanalysis misses some young objects. We reveal more than 400 protostellarcandidates, including one massive YSO that has not been previouslyhighlighted. The YSO distribution supports a picture of distributedlow-level star formation, with no strong evidence of triggered starformation in the ``pillars.'' We confirm the youth of NGC 6611 by alarge fraction of infrared excess sources and reveal a younger clusterof YSOs in the nearby molecular cloud. Analysis of the YSO clusteringproperties shows a possible imprint of the molecular cloud's Jeanslength. Multiwavelength mid-IR imaging thus allows us to analyze theprotostellar population, to measure the dust temperature and columndensity, and to relate these in a consistent picture of star formationin M16.
| An X-Ray Imaging Study of the Stellar Population in RCW 49
We present the results of a high-resolution X-ray imaging study of thestellar population in the Galactic massive star-forming region RCW 49and its central OB association Westerlund 2. We obtained a ~40 ks X-rayimage of a ~17'×17' field using the ChandraX-Ray Observatory and deep NIR images using the Infrared Survey Facilityin a concentric ~8.3'×8.3' region. Wedetected 468 X-ray sources and identified optical, NIR, and Spitzer MIRcounterparts for 379 of them. The unprecedented spatial resolution andsensitivity of the X-ray image, enhanced by optical and infrared imagingdata, yielded the following results: (1) The central OB associationWesterlund 2 is resolved for the first time in the X-ray band. X-rayemission is detected from all spectroscopically identified early-typestars in this region. (2) Most (~86%) X-ray sources with optical orinfrared identifications are cluster members in comparison with acontrol field in the Galactic plane. (3) A loose constraint (2-5 kpc)for the distance to RCW 49 is derived from the mean X-ray luminosity ofT Tauri stars. (4) The cluster X-ray population consists of low-masspre-main-sequence and early-type stars as obtained from X-ray and NIRphotometry. About 30 new OB star candidates are identified. (5) Weestimate a cluster radius of 6'-7' based on the X-ray surface numberdensity profiles. (6) A large fraction (~90%) of cluster members areidentified individually using complimentary X-ray and MIR excessemission. (7) The brightest five X-ray sources, two Wolf-Rayet stars andthree O stars, have hard thermal spectra.
| Clustered Star Formation in the Small Magellanic Cloud. A Spitzer/IRAC View of the Star-Forming Region NGC 602/N 90
We present Spitzer/IRAC photometry on the star-forming H II region N 90,related to the young stellar association NGC 602 in the Small MagellanicCloud. Our photometry revealed bright mid-infrared sources, which weclassify with the use of a scheme based on templates and models of redsources in the Milky Way, and criteria recently developed from theSpitzer Survey of the SMC for the selection of candidate Young StellarObjects (YSOs). We detected 57 sources in all four IRAC channels in a6.2'×4.8' field of view centered on N 90; 22of these sources are classified as candidate YSOs. We compare thelocations of these objects with the position of optical sources recentlyfound in the same region with high-resolution HST/ACS imaging of NGC 602by Schmalzl and coworkers, and we find that 17 candidate YSOs have oneor more optical counterparts. All of these optical sources areidentified as pre-main-sequence stars, thus indicating ongoing clusteredstar formation events in the region. The positions of the detected YSOsand their related PMS clusters give a clear picture of the current starformation in N 90, according to which the young stellar associationphotoionizes the surrounding interstellar medium, revealing the H IInebula, and triggering sequential star formation events mainly along theeastern and southern rims of the formed cavity of the parental molecularcloud.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| Fundamentalist physics: why Dark Energy is bad for astronomy
Astronomers carry out observations to explore the diverse processes andobjects which populate our Universe. High-energy physicists carry outexperiments to approach the Fundamental Theory underlying space, timeand matter. Dark Energy is a unique link between them, reflecting deepaspects of the Fundamental Theory, yet apparently accessible onlythrough astronomical observation. Large sections of the two communitieshave therefore converged in support of astronomical projects toconstrain Dark Energy. In this essay I argue that this convergence canbe damaging for astronomy. The two communities have differentmethodologies and different scientific cultures. By uncriticallyadopting the values of an alien system, astronomers risk undermining thefoundations of their own current success and endangering the futurevitality of their field. Dark Energy is undeniably an interestingproblem to tackle through astronomical observation, but it is one ofmany and not necessarily the one where significant progress is mostlikely to follow a major investment of resources.
| Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
The early solar system contained short-lived radionuclides such as60Fe (t1/2=1.5 Myr) whose most likely source was anearby supernova. Previous models of solar system formation considered asupernova shock that triggered the collapse of the Sun's nascentmolecular cloud. We advocate an alternative hypothesis, that the solarsystem's protoplanetary disk had already formed when a very close (<1pc) supernova injected radioactive material directly into the disk. Weconduct the first numerical simulations designed to answer two questionsrelated to this hypothesis: Will the disk be destroyed by such a closesupernova, and will any of the ejecta be mixed into the disk? Oursimulations demonstrate that the disk does not absorb enough momentumfrom the shock to escape the protostar to which it is bound. Only lowamounts (<1%) of mass loss occur, due to stripping byKelvin-Helmholtz instabilities across the top of the disk, which alsomix into the disk about 1% of the intercepted ejecta. These lowefficiencies of destruction and injection are due to the fact that thehigh disk pressures prevent the ejecta from penetrating far into thedisk before stalling. Injection of gas-phase ejecta is too inefficientto be consistent with the abundances of radionuclides inferred frommeteorites. On the other hand, the radionuclides found in meteoriteswould have condensed into dust grains in the supernova ejecta, and weargue that such grains will be injected directly into the disk withnearly 100% efficiency. The meteoritic abundances of the short-livedradionuclides such as 60Fe therefore are consistent withinjection of grains condensed from the ejecta of a nearby (<1 pc)supernova, into an already formed protoplanetary disk.
| Spitzer/IRAC-MIPS Survey of NGC 2244: Protostellar Disk Survival in the Vicinity of Hot Stars
We present the results from a survey of NGC 2244 from 3.6 to 24 μmwith the Spitzer Space Telescope. The 24 μm-8 μm-3.6 μm colorcomposite image of the region shows that the central cavity surroundingthe multiple O and B stars of NGC 2244 contains a large amount of cooldust visible only at 24 μm. Our survey gives a detailed look at disksurvivability within the hot-star-dominated environment in this cavity.Using mid-infrared two-color diagrams ([3.6]-[4.5] vs. [5.8]-[8.0]), weidentified 337 class II and 25 class I objects out of 1084 objectsdetected in all four of these bands with photometric uncertainty betterthan 10%. Including the 24 μm data, we found 213 class II and 20class I sources out of 279 stars also detected at this latter band. Thecenter of the class II density contours is in very good agreement withthe center of the cluster detected in the 2MASS images. We studied thedistribution of the class II sources relative to the O stars and foundthat the effect of high-mass stars on the circumstellar disks issignificant only in their immediate vicinity.
| From Ultracompact to Extended H II Regions. II. Cloud Gravity and Stellar Motion
The dynamical evolution of H II regions with and without stellar motionin dense, structured molecular clouds is studied. Clouds are modeled inhydrostatic equilibrium, with Gaussian central cores and external halosthat obey ρ~r-2 and ρ~r-3 power laws.Cloud gravity is included as a time-independent, external force. Stellarvelocities of 0, 2, 8, and 12 km s -1 are considered,permitting stars to move from the central core toward the edge of thecloud. Ultracompact H II regions are seen to evolve into extended H IIregions as the stars move toward lower density regions. Our mainconclusion is that ultracompact H II regions are pressure-confinedentities while they remain embedded within dense cores. The confinementcomes from either ram or ambient pressures, or a combination of both.The survival of the ultracompact regions depends on the position of thestar with respect to the core center, the stellar lifetime, and thecrossing time of the cloud core. Stars with velocities less than thecloud dispersion velocity can produce cometary ultracompact H II regionsfor 2×104 yr or more, in statistical agreement withobservations. The sequence ultracompact H II --> compact H II -->extended H II shows a variety of structures induced by variousinstabilities. Some ultracompact H II regions with a core-halomorphology could be explained by self-blocking effects, when starsovertake and ionize leading, piled-up clumps of neutral gas.
| A radiation driven implosion model for the enhanced luminosity of protostars near HII regions
Context: Molecular clouds near H II regions tend to harbor more luminousprotostars. Aims: We investigate whether a radiation-drivenimplosion mechanism enhances the luminosity of protostars near regionsof high ionizing fluxes. Methods: We performed numericalsimulations to model collapse of cores exposed to UV radiation from Ostars. We investigated the dependence of mass loss rates on the initialdensity profiles of cores and variation of UV fluxes. We derived simpleanalytic estimates of accretion rates and final masses of protostars.Results: The radiation-driven implosion mechanism can increaseaccretion rates of protostars by 1-2 orders of magnitude. On the otherhand, mass loss due to photo-evaporation is not high enough to have asignificant impact on the luminosity. The increase in accretion rateresults in luminosity 1-2 orders of magnitude higher than those ofprotostars that form without external triggering. Conclusions:Radiation-driven implosion can help explain the observed higherluminosity of protostars in molecular clouds near H II regions.
| 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.
| Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| 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
| Rotational Velocities for B0-B3 Stars in Seven Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
We present the results of a study aimed at assessing the differences inthe distribution of rotation speeds N(vsini) among young (1-15 Myr) Bstars spanning a range of masses 6 Msolar>1 Msolar pc-3)ensembles that will survive as rich, bound stellar clusters for ageswell in excess of 108 yr. Our results demonstrate (1) thatindependent of environment, the rotation rates for stars in this massrange do not change by more than 0.1 dex over ages t~1 to ~15 Myr; and(2) that stars formed in high-density regions lack the cohort of slowrotators that dominate the low-density regions and young field stars. Wesuggest that the differences in N(vsini) between low- and high-densityregions may reflect a combination of initial conditions andenvironmental effects: (1) the higher turbulent speeds that characterizemolecular gas in high-density, cluster-forming regions; and (2) thestronger UV radiation fields and high stellar densities thatcharacterize such regions. Higher turbulent speeds may lead to highertime-averaged accretion rates during the stellar assembly phase. In thecontext of stellar angular momentum regulation via ``disk-locking,''higher accretion rates lead to both higher initial angular momenta andevolution-driven increases in surface rotation rates as stars contractfrom the birth line to the zero-age main sequence (ZAMS). Stronger UVradiation fields and higher densities may lead to shorter disk lifetimesin cluster-forming regions. If so, B stars formed in dense clusters aremore likely to be ``released'' from their disks early during theirpre-main-sequence lifetimes and evolve into rapid rotators as theyconserve angular momentum and spin up in response to contraction. Bycontrast, the majority of their brethren in low-density,association-forming regions can retain their disks for much or all oftheir pre-main-sequence lifetimes, are ``locked'' by their disks torotate at constant angular speed, and lose angular momentum as theycontract toward the ZAMS, and thus arrive on the ZAMS as relativelyslowly rotating stars.
| Multicolour CCD measurements of visual double and multiple stars. III
Context: Recent CCD observations were performed in the period 1998-2004for a large sample of visual double and multiple stars selected from theHipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Aims: Accurate astrometric and photometric data allowing us tocharacterise the individual components are provided. These data arecompared to Hipparcos data or to data from an older epoch to assess thenature of the observed systems. Methods: We simultaneously apply aMoffat-Lorentz profile with a similar shape to all detected componentsand adjust the profile parameters from which we obtain the relativeastrometric position (epoch, position angle, angular separation) as wellas differential multi-colour photometry (filters (B)VRI). Results: Wethus acquired recent data for 71 visual systems of which 6 are orbitalbinaries, 27 are nearby, and 30 are multiple systems. In three of thesecases, the systems remained unresolved. 23 new components were detectedand measured. Two new visual double stars of intermediate separationwere also found. The estimated accuracies in relative position are0.04° and 0.01 arcsec respectively, while those in differentialphotometry are of the order of 0.01-0.02 mag in general. Conclusions:.The nature of the association of 55 systems is evaluated. New basicbinary properties are derived for 20 bound systems. Component coloursand masses are provided for two orbital binaries.Based on observations collected at the National AstronomicalObservatory, Rozhen, and the Astronomical Observatory, Belogradchik,both operated by the Institute of Astronomy, Bulgarian Academy ofSciences. Also based on data obtained by the Hipparcos astrometrysatellite. Appendix A is only available in electronic form athttp://www.aanda.org Tables 4-6 are only available in electronic form atthe 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/464/641
| Unveiling the Cygnus OB2 stellar population with Chandra
Aims.The aim of this work is to identify the so far unknown low massstellar population of the ~2 Myr old Cygnus OB2 star forming region, andto investigate the X-ray and near-IR stellar properties of its members. Methods: We analyzed a 97.7 ks Chandra ACIS-I observation pointed at thecore of the Cygnus OB2 region. Sources were detected using the PWDetectcode and were positionally correlated with optical and near-IR catalogsfrom the literature. Source events were extracted with the Acis Extractpackage. X-ray variability was characterized through theKolmogorov-Smirnov test and spectra were fitted using absorbed thermalplasma models. Results: We detected 1003 X-ray sources. Of these, 775have near-IR counterparts and are expected to be almost all associatedwith Cygnus OB2 members. From near-IR color-color and color-magnitudediagrams we estimate a typical absorption toward Cygnus OB2 ofAv ≈ 7.0 mag. Although the region is young, very few stars(~4.4%) show disk-induced excesses in the near-IR. X-ray variability isdetected in ~13% of the sources, but this fraction increases, up to 50%,with increasing source statistics. Flares account for at least 60% ofthe variability. Despite being generally bright, all but 2 of the 26detected O- that early B-type stars are not significantly variable.Typical X-ray spectral parameters are log N_H˜ 22.25(cm-2) and kT˜ 1.35 keV with 1σ dispersion of 0.2dex and 0.4 keV, respectively. Variable and flaring sources have harderspectra with median kT= 3.3 and 3.8 keV, respectively. OB stars aretypically softer (kT ˜ 0.75 keV). X-ray luminosities range between1030 and 1031 erg s-1 for intermediate-and low-mass stars, and 2.5×1030 and between6.3×1033 erg s-1 for OB stars. Conclusions:.The Cygnus OB2 region has a very rich population of low-mass X-rayemitting stars. Circumstellar disks seem to be very scarce. X-rayvariability is related to the magnetic activity of low-mass stars(M/M_ȯ ˜0.5 to 3.0) display X-ray activity levels comparableto those of Orion Nebular Cluster (ONC) sources in the same mass range.Tables 1-3 are only available in electronic form at http://www.aanda.org
| Statistical properties of a sample of periodically variable B-type supergiants. Evidence for opacity-driven gravity-mode oscillations
Aims.We have studied a sample of 28 periodically variable B-typesupergiants selected from the HIPPARCOS mission and 12 comparison starscovering the whole B-type spectral range. Our goal is to test if theirvariability is compatible with opacity-driven non-radialoscillations. Methods: .We have used the NLTE atmosphere codeFASTWIND to derive the atmospheric and wind parameters of the completesample through line profile fitting. We applied the method to selectedH, He, and Si line profiles, measured with the high resolution CESspectrograph attached to the ESO CAT telescope in La Silla, Chile.Results: .From the location of the stars in the (log T_eff, log g)diagram, we suggest that variability of our sample supergiants is indeeddue to the gravity modes resulting from the opacity mechanism. We findnine of the comparison stars to be periodically variable as well, andsuggest them to be new α Cyg variables. We find marginal evidenceof a correlation between the amplitude of the photometric variabilityand the wind density. We investigate the wind momentum-luminosityrelation for the whole range of B spectral type supergiants, and findthat the later types (>B5) perfectly follow the relation for Asupergiants. Additionally, we provide a new spectral type - T_effcalibration for B supergiants. Conclusions: .Our results imply thepossibility of probing internal structure models of massive stars ofspectral type B through seismic tuning of gravity modes.Figures of the spectral line fits and discussion of the individualobjects, Appendices A, B and Table 6 are only available in electronicform at http://www.aanda.org
| Capture-formed Binaries via Encounters with Massive Protostars
Most massive stars are found in the center of dense clusters and have acompanion fraction much higher than their lower mass siblings; themassive stars of the Trapezium core in Orion have ~1.5 companions each.This high multiplicity could be a consequence of formation via a capturescenario, or it could be due to fragmentation of the cores that form themassive stars. During stellar formation circumstellar disks appear to benearly ubiquitous. Their large radii compared to stellar sizes increasethe interaction radius significantly, suggesting that disk interactionswith neighboring stars could assist in capturing binary companions. Thismechanism has been studied for stars of approximately solar mass andfound to be inefficient. In this paper we present simulations ofinteractions between a 22 Msolar star-disk system and lessmassive impactors in order to study the disk-assisted capture formationof binaries in a regime suited to massive stars. The formation ofbinaries by capture is found to be much more efficient for massivecapturers. We discuss the effects of a mass-dependent velocitydispersion and mass segregation on the capture rates and consider thelong-term survival of the resulting binaries in a dense cluster.
| 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).
| Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images
The quality of modern astronomical data and the agility of currentimage-processing software enable the visualization of data in a way thatexceeds the traditional definition of an astronomical image. Twodevelopments in particular have led to a fundamental change in howastronomical images can be assembled. First, the availability ofhigh-quality multiwavelength and narrowband data allow for images thatdo not correspond to the wavelength sensitivity of the human eye,thereby introducing ambiguity in the usage and interpretation of color.Second, many image-processing software packages now use a layeringmetaphor that allows for any number of astronomical data sets to becombined into a color image. With this technique, images with as many aseight data sets have been produced. Each data set is intensity-scaledand colorized independently, creating an immense parameter space thatcan be used to assemble the image. Since such images are intended fordata visualization, scaling and color schemes must be chosen that bestillustrate the science. A practical guide is presented on how to use thelayering metaphor to generate publication-ready astronomical images fromas many data sets as desired. A methodology is also given on how to useintensity scaling, color, and composition to create contrasts in animage that highlight the scientific detail. Examples of image creationare discussed.
| Chandra Observations of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation
We present and analyze the first high-resolution X-ray images everobtained of the Eagle Nebula star-forming region. On 2001 July 30 theChandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula(M16) that includes the core of the young galactic cluster NGC 6611 andthe dark columns of dust and cold molecular gas in M16 known as the``Pillars of Creation.'' We find a total of 1101 X-ray sources in the17'×17' ACIS-I field of view. Most of theX-ray sources are low-mass pre-main-sequence or high-mass main-sequencestars in this young cluster. A handful of hard X-ray sources in thepillars are spatially coincident with deeply embedded young stellarobjects seen in high-resolution near-infrared images recently obtainedwith the VLT (McCaughrean & Andersen). In this paper, we focus onthe 40 X-ray sources in and around pillars 1-4 at the heart of the EagleNebula. None of the X-ray sources are associated with the evaporatinggaseous globules (EGGs) first observed by Hester and coworkers) in HSTWFPC2 images of M16, implying either that the EGGs do not containprotostars or that the protostars have not yet become X-ray active.Eight X-ray counts are coincident with the Herbig-Haro object HH 216,implying logLX~30.0.
| 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.
| Correlation between the spatial distribution of circumstellar disks and massive stars in the open cluster NGC 6611. Compiled catalog and cluster parameters
Context: The observation of young stars with circumstellar diskssuggests that the disks are dissipated, starting from the inner region,by the radiation of the central star and eventually by the formation ofrocky planetesimals, over a time scale of several million years. It wasalso shown that strong UV radiation emitted by nearby massive stars canheat a circumstellar disk up to some thousand degrees, inducing thephotoevaporation of the gas. This process strongly reduces thedissipation time scale. Aims: We study whether there exists acorrelation between the spatial distribution of stars with circumstellardisks and the position of massive stars with spectral class earlier thanB5, in the open cluster NGC 6611. Methods: We created a multibandcatalog of the cluster, down to V˜ 23^m, using optical data from aWFI observation at 2.2 m of ESO in the BVI bands, the 2MASS public pointsource catalog and an archival X-ray observation made with CHANDRA/ACIS.We selected the stars with infrared excess (due to the emission of acircumstellar disk) using suitable color indices independent ofextinction, and studied their spatial distribution. Results: Thespatial distribution of the stars with K band excess (due to thepresence of a circumstellar disk) is anti correlated with that of themassive stars: the disks are more frequent at large distances from thesestars. We argue that this is in agreement with the hypothesis that thecircumstellar disks are heated by the UV radiation from the massivestars and photoevaporated.Based on observations made with the European Observatory telescopesobtained from the ESO/ST-ECF Science Archive Facility. Table 3 andAppendix are only available in electronic form at http://www.aanda.org
| Radioactive Probes of the Supernova-contaminated Solar Nebula: Evidence that the Sun Was Born in a Cluster
We construct a simple model for radioisotopic enrichment of theprotosolar nebula by injection from a nearby supernova, based on theinverse square law for ejecta dispersion. In this parameter study, thepresolar radioisotopic abundances (i.e., in solar masses) demand anearby supernova: its distance D can be no larger than 66 times theradius of the protosolar nebula, at a 90% confidence level, assuming 1Msolar of protosolar material. The relevant size of thenebula depends on its state of evolution at the time of radioactivityinjection. In one scenario, a collection of low-mass stars, includingour Sun, formed in a group or cluster with a high-mass star that endedits life as a supernova while our Sun was still a protostar, a starlesscore, or perhaps a diffuse cloud. Using recent observations ofprotostars to estimate the size of the protosolar nebula constrains thedistance of the supernova to D~0.02-1.6 pc. This supernova distancelimit is consistent with the scales of low-mass star formation aroundone or more massive stars, but it is closer than expected were the Sunformed in an isolated, solitary state. Consequently, if any presolarradioactivities originated via supernova injection, we must concludethat our Sun was a member of such a group or cluster that has sincedispersed; thus, solar system formation should be understood in thiscontext. The temporal choreography from supernova ejecta to meteoritesis important, as the modeled timescale is <=1.8 Myr. Finally, themodel does not distinguish between progenitor masses from 15 to 25Msolar, although the 20 Msolar model is somewhatpreferred.
| 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.
| Faint open clusters with 2MASS: BH 63, Lyngå 2, Lyngå 12 and King 20
Context: .Structural and dynamical parameters of faint open clusters areprobed with quality 2MASS-photometry and analytical procedures developedfor bright clusters. Aims: .We derive fundamental parameters ofthe faint open clusters Lyngå 2, BH 63, Lyngå 12 and King20, the last three of which have no prior determinations. We also focuson the structure and dynamical state of these clusters. Methods:.J, H and Ks 2MASS photometry with errors smaller than 0.2mag are used to build CMDs, radial density profiles, colour-colourdiagrams, luminosity and mass functions. Colour-magnitude filters areused to isolate probable member stars. Field-star decontamination isapplied to Lyngå 2, Lyngå 12 and King 20. Results:.Reddening values are in the range 0.22≤E(B-V)≤1.9, with BH 63 themost reddened object. Ages of Lyngå 2, King 20, Lyngå 12 andBH 63 are ≈90, ≈200, ≈560 and ≈700 Myr, respectively. Theradial density distributions of Lyngå 12 and King 20 arewell-represented by King profiles. Lyngå 2 and BH 63 are verysmall with core and limiting radii of ≈0.12 pc and ≈1.5 pc. Yet,they fit in the small-radii tail of the open cluster size distribution.Lyngå 12 and King 20 have R_core≈0.43 pc and R_lim≈3.9 pc.Lyngå 2 and Lyngå 12 are inside the Solar circle. Totalstellar masses (extrapolating the MFs to stars with 0.08 M_ȯ) rangefrom ≈340 M_ȯ (BH 63) to ≈2300 M_ȯ (Lyngå 12).Observed masses are ~1/4 of these values. In all clusters the core massfunction is flatter than the halo's. Conclusions: .Faint openclusters can be probed with 2MASS when associated with colour-magnitudefilters and field-star decontamination. BH 63 appears to be in anadvanced dynamical state, both in the core and halo. To a lesser degreethe same applies to King 20. Marginal evidence of dynamical evolution ispresent in the cores of Lyngå 2 and Lyngå 12.
| Polarimetry in the Outskirts of NGC 6611
We present new polarimetric UBVRI observations of 25 stars in thedirection of the halo of NGC 6611, the rich stellar open clusterembedded in an ionized hydrogen complex (M16). Our plan is tocharacterize the interstellar material (ISM) associated with halo starsin order to make a comparison with the ISM dusty core characteristicsthat resulted from a previous investigation by the same authors. Of thehalo stars, 47% (8 out of 17) show indications of intrinsic polarizationin their light, similar to what was found for core stars (50%). We haveidentified the presence of nearby dust clouds located on the Local armthat produce a mean polarization of about 1%; a valueλmax=0.61+/-0.07 μm, which is slightly larger thanthat of the average ISM; and a mean direction of the polarizationvectors of θV=81.9d+/-1.8d. The ISM associated withthe halo region has λmax similar to the generalinterstellar medium (0.55+/-0.07 μm). The observed polarizations showa gradual increase from halo (Pmax=1.93%+/-0.3%) to core(Pmax=3.19%+/-0.63%). Position angles of the e-vector forboth groups are generally similar, but there exists a slight differencein mean direction between them that is within the errors. We have alsofound that the halo stars are possibly represented by Whittet & vanBreda's relationship, while in the cluster's core the dust does notfulfill the above-mentioned relationship. As a conclusion, we cannotfind any clear difference between core and halo dust characteristics,with the exception of λmax, which may suggest a changein dust size.Based on observations obtained at Complejo Astronómico ElLeoncito, operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the Universities of La Plata,Córdoba, and San Juan.
| The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611
An analysis is presented of VLT-FLAMES spectroscopy for three Galacticclusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmospherecalculations have been used to estimate effective temperatures (fromeither the helium spectrum or the silicon ionization equilibrium) andgravities (from the hydrogen spectrum). Projected rotational velocitieshave been deduced from the helium spectrum (for fast and moderaterotators) or the metal line spectrum (for slow rotators). The origin ofthe low gravity estimates for apparently near main sequence objects isdiscussed and is related to the stellar rotational velocity. Theatmospheric parameters have been used to estimate cluster distances(which are generally in good agreement with previous determinations) andthese have been used to estimate stellar luminosities and evolutionarymasses. The observed Hertzsprung-Russell diagrams are compared withtheoretical predictions and some discrepancies including differences inthe main sequence luminosities are discussed. Cluster ages have beendeduced and evidence for non-coeval star formation is found for allthree of the clusters. Projected rotational velocities for targets inthe older clusters, NGC 3293 and NGC 4755, have been found to besystematically larger than those for the field, confirming recentresults in other similar age clusters. The distribution of projectedrotational velocities are consistent with a Gaussian distribution ofintrinsic rotational velocities. For the relatively unevolved targets inthe older clusters, NGC 3293 and NGC 4755, the peak of the velocitydistribution would be 250 km s-1 with afull-width-half-maximum of approximately 180 km s-1. For NGC6611, the sample size is relatively small but implies a lower meanrotational velocity. This may be evidence for the spin-down effect dueto angular momentum loss through stellar winds, although our results areconsistent with those found for very young high mass stars. For allthree clusters we deduce present day mass functions with Γ-valuesin the range of -1.5 to -1.8, which are similar to other young stellarclusters in the Milky Way.
| 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.
| Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Aims.We derive accurate parameters related to the CMD, structure anddynamical state of M 52 and NGC 3960, whose fields are affected bydifferential reddening. Previous works estimated their ages in theranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods: .J, Hand Ks 2MASS photometry with errors <0.2 mag is used tobuild CMDs, radial density profiles, luminosity and mass functions, andcorrect for differential reddening. Field-star decontamination isapplied to uncover the cluster's intrinsic CMD morphology, andcolour-magnitude filters are used to isolate stars with high probabilityof being cluster members. Results: .The differential-reddeningcorrected radial density profile of M 52 follows King's law with coreand limiting radii of R_core =0.91 ± 0.14 pc and R_lim =8.0± 1.0 pc. NGC 3960 presents an excess of the stellar density overKing's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ±0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 areR_tidal=13.1 ± 2.2 pc and R_tidal=10.7 ± 3.7 pc. Clusterages of M 52 and NGC 3960 derived with Padova isochrones are constrainedto 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF(χ_core=0.89 ± 0.12) is flatter than the halo's(χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up inMS/evolved stars in M 52 is ~1200 M_ȯ, and the total mass(extrapolated to 0.08M_ȯ) is ~3800 M_ȯ. The total mass in NGC3960 is ~1300 M_ȯ. Conclusions: .Compared to open clusters indifferent dynamical states studied with similar methods, the core andoverall parameters of M 52 are consistent with an open cluster moremassive than 1000 M_ȯ and ~60 Myr old, with some mass segregationin the inner region. The core of NGC 3960 is in an advanced dynamicalstate with strong mass segregation in the core/halo region, while thesomewhat flat overall MF (χ≈ 1.07) suggests low-mass starevaporation. The excess stellar density in the core may suggestpost-core collapse. The dynamical evolution of NGC 3960 may have beenaccelerated by the tidal Galactic field, since it lies ≈0.5 kpcinside the Solar circle.
| Formation of Pillars at the Boundaries between H II Regions and Molecular Clouds
We investigate numerically the hydrodynamic instability of an ionizationfront (IF) accelerating into a molecular cloud, with imposed initialperturbations of different amplitudes. When the initial amplitude issmall, the imposed perturbation is completely stabilized and does notgrow. When the initial perturbation amplitude is large enough, roughly,the ratio of the initial amplitude to wavelength is greater than 0.02,portions of the IF temporarily separate from the molecular cloudsurface, locally decreasing the ablation pressure. This causes theappearance of a large, warm H I region and triggers nonlinear dynamicsof the IF. The local difference of the ablation pressure andacceleration enhances the appearance and growth of a multimodeperturbation. The stabilization usually seen at the IF in the linearregime does not work due to the mismatch of the modes of theperturbations at the cloud surface and of the density in the H II regionabove the cloud surface. Molecular pillars are observed in the latestages of the large amplitude perturbation case. The velocity gradientin the pillars is in reasonably good agreement with that observed in theEagle Nebula. The initial perturbation is imposed in three differentways: in density, in incident photon number flux, and in the surfaceshape. All cases show both stabilization for a small initialperturbation and large growth of the second harmonic by increasingamplitude of the initial perturbation above a critical value.
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