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Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| SCUBA observations of the Horsehead nebula - what did the horse swallow?
We present observations taken with the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) ofthe Horsehead nebula in Orion (B33), at wavelengths of 450 and 850μm. We see bright emission from that part of the cloud associatedwith the photon-dominated region (PDR) at the `top' of the horse's head,which we label B33-SMM1. We characterize the physical parameters of theextended dust responsible for this emission, and find that B33-SMM1contains a more dense core than was previously suspected, with a mass of~2Msolar in a region of 0.31 × 0.13pc, and a peakvolume density of ~6 × 105cm-3. We comparethe SCUBA data with data from the Infrared Space Observatory (ISO) andfind that the emission at 6.75μm is offset towards the west,indicating that the mid-infrared emission is tracing the PDR while thesubmillimetre emission comes from the molecular cloud core behind thePDR. We calculate the virial balance of this core and find that it isnot gravitationally bound but is being confined by the external pressurefrom the HII region IC434, and that it will either be destroyed by theionizing radiation, or else may undergo triggered star formation.Furthermore, we find evidence for a lozenge-shaped clump in the `throat'of the horse, which is not seen in emission at shorter wavelengths. Welabel this source B33-SMM2 and find that it is brighter at submillimetrewavelengths than B33-SMM1. We calculate the physical parameters of SMM2and find it has a mass of ~4Msolar in a region 0.15 ×0.07pc, with a peak volume density of ~2 ×106cm-3 and peak column density of ~9 ×1022cm-2. SMM2 is seen in absorption in the6.75-μm ISO data, from which we obtain an independent estimate of thecolumn density in excellent agreement with that calculated from thesubmillimetre emission. We calculate the stability of this core againstcollapse and find that it is in approximate gravitational virialequilibrium. This is consistent with it being a pre-existing core inB33, possibly pre-stellar in nature, but that it may also eventuallyundergo collapse under the effects of the HII region.
| Mid-infrared source multiplicity within hot molecular cores traced by methanol masers
We present high resolution, mid-infrared (MIR) images towards three hotmolecular cores signposted by methanol maser emission: G173.49+2.42(S231, S233IR), G188.95+0.89 (S252, AFGL-5180) and G192.60-0.05(S255IR). Each of the cores was targeted with Michelle on Gemini Northusing five filters from 7.9 to 18.5 μm. We find each contains bothlarge regions of extended emission and multiple, luminous point sourceswhich, from their extremely red colours(F18.5/F7.9 >= 3), appear to be embedded youngstellar objects. The closest angular separations of the point sources inthe three regions are 0.79, 1.00 and 3.33 arcsec corresponding to linearseparations of 1700, 1800 and 6000 au, respectively. The methanol maseremission is found closest to the brightest MIR point source (within theassumed 1-arcsec pointing accuracy). Mass and luminosity estimates forthe sources range from 3 to 22 Msolar and from 50 to 40000Lsolar, respectively. Assuming the MIR sources are embeddedobjects and the observed gas mass provides the bulk of the reservoirfrom which the stars formed, it is difficult to generate the observeddistributions for the most massive cluster members from the gas in thecores using a standard form of the initial mass function.
| Self-gravity driven instabilities of interfaces in the interstellar medium
In order to understand star formation it is important to understand thedynamics of atomic and molecular clouds in the interstellar medium(ISM). Non-linear hydrodynamic flows are a key component to the ISM. Oneroute by which non-linear flows arise is the onset and evolution ofinterfacial instabilities. Interfacial instabilities act to modify theinterface between gas components at different densities andtemperatures. Such an interface may be subject to a host ofinstabilities, including the Rayleigh-Taylor, Kelvin-Helmholtz, andRichtmyer-Meshkov instabilities. Recently, a new density interfaceinstability was identified. This self-gravity interfacial instability(SGI) causes any displacement of the interface to grow on roughly afree-fall time-scale, even when the perturbation wavelength is much lessthan the Jeans length. In previous work, we used numerical simulationsto confirm the expectations of linear theory and examine the non-linearevolution of the SGI. We now continue our study by generalizing ourinitial conditions to allow the acceleration due to self-gravity to benon-zero across the interface. We also consider the behaviour of the SGIfor perturbation wavelengths near the Jeans wavelength. We conclude thatthe action of self-gravity across a density interface may play asignificant role in the ISM either by fuelling the growth of newinstabilities or modifying the evolution of existing instabilities.
| Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited
A three-dimensional smoothed particle hydrodynamics model has beenextended to study the radiation-driven implosion effect of massive starson the dynamical evolution of surrounding molecular clouds. The newelements in the upgraded code are the inclusion of Lyman continuum inthe incident radiation flux and the treatment of hydrogen ionizationprocess; the introduction of ionization heating and recombinationcooling effects; and the addition of a proper description of themagnetic and turbulent pressures to the internal pressure of themolecular cloud. This extended code not only provides a realistic modelto trace the dynamical evolution of a molecular cloud, but also can beused to model the kinematics of the ionization and shock fronts and thephotoevaporating gas surrounding the molecular cloud, which the previouscode is unable to handle.The application of this newly developed model to the structure of themiddle Eagle nebula finger suggests that the shock induced by theionizing radiation at the front side of the head precedes an ionizationfront moving towards the centre of the core, and that the core at thefingertip is at a transition stage evolving toward a state of inducedstar formation. The dynamical evolution of the velocity field of thesimulated cloud structure is discussed to illustrate the role of theself-gravity and the different cloud morphologies which appear atdifferent stages in the evolutionary process of the cloud. The motion ofthe ionization front and the evaporating gas are also investigated. Themodelled gas evaporation rate is consistent with that of other currentmodels and the density, temperature and chemical profiles are inagreement with the observed values.The relative lifetimes of different simulated cloud morphologies suggesta possible answer to the question of why more bright-rimmed clouds areobserved to possess a flat-core than an elongated-core morphology.
| A non-LTE abundance analysis of the post-AGB star ROA5701
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
| Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
| The infrared Hourglass cluster in M8*†
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.
| The maximum stellar mass, star-cluster formation and composite stellar populations
We demonstrate that the mass of the most massive star in a clustercorrelates non-trivially with the cluster mass. A simple algorithm,according to which a cluster is filled up with stars that are chosenrandomly from the standard initial mass function (IMF) but sorted withincreasing mass, yields an excellent description of the observationaldata. Algorithms based on random sampling from the IMF without sortedadding are ruled out with a confidence larger than 0.9999. A physicalexplanation of this would be that a cluster forms by more-massive starsbeing consecutively added until the resulting feedback energy sufficesto revert cloud contraction and stops further star formation. This hasimportant implications for composite populations. For example,104 clusters of mass 102Msolar will notproduce the same IMF as one cluster with a mass of106Msolar. It also supports the notion that theintegrated galaxial stellar IMF (IGIMF) should be steeper than thestellar IMF and that it should vary with the star formation rate of agalaxy.
| Radio Recombination Lines in Galactic H II Regions
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| Dynamical H II Region Evolution in Turbulent Molecular Clouds
We present numerical radiation-hydrodynamic simulations of the evolutionof H II regions formed in an inhomogeneous medium resulting fromturbulence simulations. We find that the filamentary structure of theunderlying density distribution produces a highly irregular shape forthe ionized region, in which the ionization front escapes to largedistances in some directions within 80,000 years. In other directions,on the other hand, neutral gas in the form of dense globules persistswithin 1 pc of the central star for the full duration of our simulation(400,000 years). Divergent photoablation flows from these globulesmaintain an rms velocity in the ionized gas that is close to the ionizedsound speed. Simulated images in optical emission lines showmorphologies that are in strikingly detailed agreement with thoseobserved in real H II regions.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.
| A Catalog of Midcourse Space Experiment Infrared Dark Cloud Candidates
We use 8.3 μm mid-infrared images acquired with the Midcourse SpaceExperiment satellite to identify and catalog infrared dark clouds(IRDCs) in the first and fourth quadrants of the Galactic plane. BecauseIRDCs are seen as dark extinction features against the diffuse Galacticinfrared background, we identify them by first determining a modelbackground from the 8.3 μm images and then searching for regions ofhigh decremental contrast with respect to this background. IRDCcandidates in our catalog are defined by contiguous regions bounded byclosed contours of a 2 σ decremental contrast threshold. Althoughmost of the identified IRDCs are actual cold dark clouds, some as yetunknown fraction may be spurious identifications. For largehigh-contrast clouds, we estimate the reliability to be 82%.Low-contrast clouds should have lower reliabilities. Verification of thereality of individual clouds will require additional data. We identify10,931 candidate IRDCs. For each IRDC, we also catalog cores. Thesecores, defined as localized regions with at least 40% higher extinctionthan the cloud's average extinction, are found by iteratively fittingtwo-dimensional elliptical Gaussian functions to the contrast peaks. Weidentify 12,774 cores. The catalog contains the position, angular size,orientation, area, peak contrast, peak contrast signal-to-noise, andintegrated contrast of the candidate IRDCs and their cores. Thedistribution of IRDCs closely follows the Galactic diffuse mid-infraredbackground and peaks toward prominent star-forming regions, spiral armtangents, and the so-called 5 kpc Galactic molecular ring.
| SiO J = 5-4 in the HH 211 Protostellar Jet Imaged with the Submillimeter Array
We have mapped the SiO J = 5-4 line at 217 GHz from the HH 211 molecularoutflow with the Submillimeter Array (SMA). The high-resolution map(1.6" × 0.9") shows that the SiO J = 5-4 emission comes from thecentral narrow jet along the outflow axis with a width of ~0.8" (~250AU) FWHM. The SiO jet consists of a chain of knots separated by 3"-4"(~1000 AU), and most of the SiO knots have counterparts in shockedH2 emission seen in a new, deep VLT near-infrared image ofthe outflow. A new, innermost pair of knots have been discovered at just+/-2" from the central star. The line ratio between the SiO J = 5-4 dataand the upper limits from the SiO J = 1-0 data of Chandler & Richersuggests that these knots have a temperature in excess of 300-500 K anda density of (0.5-1) × 107 cm-3. The radialvelocity measured for these knots is ~30 km s-1, comparableto the maximum velocity seen in the entire jet. The high temperature,high density, and velocity structure observed in this pair of SiO knotssuggest that they are closely related to the primary jet launched closeto the protostar.
| The Low-Mass Pre-Main-Sequence Population of the Stellar Association LH 52 in the Large Magellanic Cloud Discovered with Hubble Space Telescope WFPC2 Observations
We report on the serendipitous discovery of ~500 low-mass candidatepre-main-sequence (PMS) stars in the vicinity of the stellar associationLH 52 in the Large Magellanic Cloud. We present evidence that the redfaint sequence of these stars seen in the CMD of LH 52 from Hubble SpaceTelescope (HST) WFPC2 observations belongs only to the association andfollows almost perfectly isochrone models for PMS stars of masses downto ~0.3 Msolar. We find that this feature has a Galacticcounterpart and that the mass spectrum of the candidate PMS stars in LH52 seems to correspond to a Salpeter initial mass function with a slopeΓ~=-1.26 in the mass range 0.8-1.4 Msolar.
| Stellar Rotation: A Clue to the Origin of High-Mass Stars?
We present the results of a study aimed at assessing whether low- andhigh-mass stars form similarly. Our approach is (1) to examine theobserved projected rotational velocities among a large sample of newlyformed stars spanning a range in mass between 0.2 and 50Msolar and (2) to search for evidence of a discontinuity inrotational properties that might indicate a difference in the stellarformation process at some characteristic mass. Our database includesrecently published values of vsini for young intermediate- and low-massstars in Orion, as well as new observations of O stars located in youngclusters and OB associations. We find that the median of the quantityvobs/vc (observed rotational speed divided byequatorial breakup velocity) is typically about 0.15 and shows noevidence of a discontinuity over the full range of stellar masses, whilethe quantity Jsini/M (derived angular momentum per unit mass) exhibits aslow, monotonic rise (J/M~M0.3) with increasing mass with noevidence of a discontinuity. We suggest that these observations are mostsimply interpreted as indicative of a single stellar formation andangular momentum regulation mechanism, one that results in rotationrates well below breakup and angular momenta per unit mass that differsystematically by no more than a factor of 3-4 over a mass rangespanning a factor of 250.
| Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| The Initial Mass Functions of Four Embedded Stellar Clusters
We present near-infrared J, H, and K images of four embedded stellarclusters in the Galaxy. We find a significant fraction ofpre-main-sequence stars present in at least one of the clusters. For theclusters dominated by main-sequence stars, we determine the initial massfunction (IMF) both by using the K luminosity function and a globalextinction correction and by deriving individual extinction correctionsfor each star based on their placement in the K versus H-Kcolor-magnitude diagram. Based on our IMFs we find a significantdiscrepancy between the mean IMF derived via the different methods,suggesting that taking individual extinctions into account is necessaryto correctly derive the IMF for an embedded cluster.
| 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.
| Molecular excitation in the Eagle nebula's fingers
Context: .The M 16 nebula is a relatively nearby Hii region, powered byO stars from the open cluster NGC 6611, which borders to a GiantMolecular Cloud. Radiation from these hot stars has sculpted columns ofdense obscuring material on a few arcmin scales. The interface betweenthese pillars and the hot ionised medium provides a textbook example ofa Photodissociation Region (PDR). Aims: .To constrain the physicalconditions of the atomic and molecular material with submillimeterspectroscopic observations. Methods: .We used the APEXsubmillimeter telescope to map a 3'×3' region in the CO J=3-2, 4-3 and 7-6 rotational lines, and a subregion in atomic carbonlines. We also observed C18O(3-2) and CO(7-6) with longerintegrations on five peaks found in the CO(3-2) map. The large scalestructure of the pillars is derived from the molecular lines' emissiondistribution. We estimate the magnitude of the velocity gradient at thetips of the pillars and use LVG modelling to constrain their densitiesand temperatures. Excitation temperatures and carbon column densitiesare derived from the atomic carbon lines. Results: .The atomiccarbon lines are optically thin and excitation temperatures are of order60 K to 100 K, well consistent with observations of other Hiiregion-molecular cloud interfaces. We derive somewhat lower temperaturesfrom the CO line ratios, of order 40 K. The Ci/CO ratio is around 0.1 atthe fingers tips.
| Detection of K_s-excess stars in the 14 Myr open cluster NGC 4755
Aims.We derive the structure, distribution of MS and PMS stars anddynamical state of the young open cluster NGC 4755. We explore thepossibility that, at the cluster age, some MS and PMS stars stillpresent infrared excesses related to dust envelopes and proto-planetarydiscs. Methods: .J, H and Ks 2MASS photometry is usedto build CMD and colour-colour diagrams, radial density profiles,luminosity and mass functions. Field-star decontamination is applied touncover the cluster's intrinsic CMD morphology and detect candidate PMSstars. Proper motions from UCAC2 are used to determine clustermembership. Results: .The radial density profile follows King'slaw with a core radius Rcore=0.7 ± 0.1 pc and alimiting radius Rlim=6.9 ± 0.1 pc. The cluster agederived from Padova isochrones is 14 ± 2 Myr. Field-stardecontamination reveals a low-MS limit at ≈1.4 M_ȯ. The core MF(χ=0.94 ± 0.16) is flatter than the halo's (χ=1.58± 0.11). NGC 4755 contains 285 candidate PMS stars of age 1{-}15 Myr, and a few evolved stars. The mass locked up in PMS, MS andevolved stars amounts to 1150 M_ȯ. Proper motions show thatK_s-excess MS and PMS stars are cluster members. K_s-excess fractions inPMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5%respectively, consistent with the cluster age. The core is deficient inPMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halobut they are scarce in the core. Conclusions: .Compared to openclusters in different dynamical states studied with similar methods, NGC4755 fits relations involving structural and dynamical parameters in theexpected locus for its age and mass. On the other hand, the flatter coreMF probably originates from primordial processes related to parentmolecular cloud fragmentation and mass segregation over 14 Myr. Starformation in NGC 4755 began ≈14 Myr ago and proceeded for about thesame length of time. Detection of K_s-excess emission in member MS starssuggests that some circumstellar dust discs survived for 10^7 yr,occurring both in some MS and PMS stars for the age and spread observedin NGC 4755.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| The youngest stellar clusters. Clusters associated with massive protostellar candidates
We report on the identification of 54 embedded clusters around 217massive protostellar candidates of which 34 clusters are new detections.The embedded clusters are identified as stellar surface densityenhancements in the 2 μm All Sky Survey (2MASS) data. Because theclusters are all associated with massive stars in their earliestevolutionary stage, the clusters should also be in an early stage ofevolution. Thus the properties of these clusters should reflectproperties associated with their formation rather than their evolution.For each cluster, we estimate the mass, the morphological type, thephotometry and extinction. The clusters in our study, by theirassociation with massive protostars and massive outflows, reinstate thenotion that massive stars begin to form after the first generation oflow mass stars have completed their accretion phase. Further, theobserved high gas densities and accretion rates at the centers of theseclusters is consistent with the hypothesis that high mass stars form bycontinuing accretion onto low mass stars.
| Extinction at 7 μm and 15 μm from the ISOGAL survey
The extinction laws at 7 μm and 15 μm are derived for more than120 sightlines in the inner Galactic plane based on the ISOGAL surveydata and the near-infrared data from DENIS and 2MASS. The tracers arethe ISOGAL point sources with [7]-[15]<0.4 which are RGB tip stars orearly AGB stars with moderate mass loss. They have well-definedintrinsic color indices (J-Ks)0, (Ks-[7])0 and (Ks-[15])0. By a linearfitting of the observed color indices Ks-[7] and Ks-[15] to the observedJ-Ks, we obtain the ratio between the E(Ks-[7]) and E(Ks-[15]) colorexcesses and E(J-Ks). We infer the selective extinctions at 7 and 15μm in terms of the near-infrared extinction in the Ks band. Thedistribution of the derived extinctions around 7 micron (A7)is well represented by a Gaussian function, with the peak at about0.47A_KS and ranging from 0.33 to0.55AKS. There is some evidence thatA_7/AKs may vary significantly depending on the line ofsight. The derived selective extinction at 15 μm suffers uncertaintymainly from the dispersion in the intrinsic color index (Ks-[15])0 whichis affected by dust emission from mass-losing AGB stars. The peak valueof A_15 is around 0.40AK_S.
| Mass functions and structure of the young open cluster NGC 6611
We use J, H and KS 2MASS photometry to study colour-magnitude(CMDs) and colour-colour diagrams, structure and mass distribution inthe ionizing open cluster NGC 6611. Reddening variation throughout thecluster region is taken into account followed by field-stardecontamination of the CMDs. Decontamination is also applied to derivethe density profile and luminosity functions in the core, halo andoverall (whole cluster) regions. The field-star decontamination showedthat the lower limit of the main sequence (MS) occurs at ≈5 M_ȯ.Based on the fraction of KS excess stars in the colour-colourdiagram we estimate an age of 1.3±0.3 Myr which is consistentwith the presence of a large number of pre-main sequence (PMS) stars.The distance from the Sun was estimated from known O V stars in thecluster area and the turn-on stars connecting the PMS and MS, resultingin d_ȯ=1.8±0.5 kpc. The radial density distributionincluding MS and PMS stars is fitted by a King profile with a coreradius R_core=0.70±0.08 pc. The cluster density profile mergesinto the background at a limiting radius R_lim=6.5±0.5 pc. Fromthe field-star subtracted luminosity functions we derive the massfunctions (MFs) in the form φ(m)∝ m-(1+χ). Inthe halo and through the whole cluster the MFs have slopesχ=1.52±0.13 and χ=1.45±0.12, respectively, thusslightly steeper than Salpeter's IMF. In the core the MF is flat,χ=0.62±0.16, indicating some degree of mass segregation sincethe cluster age is a factor 2 larger than the relaxation time. Becauseof the very young age of NGC 6611, part of this effect appears to berelated to the molecular cloud-fragmentation process itself. We detect362±120 PMS stars. The total observed mass including detected MS(in the range 5{-}85 M_ȯ) and PMS stars amounts to 1600 M_ȯ,thus more massive than the Trapezium cluster. Compared to older openclusters of different masses, the overall NGC 6611 fits in the relationsinvolving structural and dynamical parameters. However, the core isatypical in the sense that it looks like an old/dynamically evolvedcore. Again, part of this effect must be linked to formation processes.
| A Survey of N IV and O IV Features near 3400 Å in O2-O5 Spectra
We have conducted a survey of little-known N IV and O IV multiplets near3400 Å in an extensive sample of well-classified, very earlyO-type spectra. The initial motivation was to search for additionaluseful classification criteria for these types, but an unexpected resultis the high sensitivity of these features to evolutionary CNOprocessing. We have found a useful discriminant between O2 and latertypes in the relative strengths of the O IV multiplets, one of which issubject to selective emission in the hottest spectra; the overallstrengths of these lines also decrease between spectral types O4 and O5.More remarkable, however, are the variations in the N/O ratios amongboth individual stars and clusters. For instance, several O4 If+ spectrahave very large ratios, while main-sequence stars in the Carina Nebulagenerally have smaller values than others of the same spectral types inother regions. These effects correspond to different degrees of mixingof processed material as a function of evolutionary age and initialrotational velocities; the second effect provides significant furtherevidence that very massive stars mix while still on the main sequence.Thus, further analysis of these features will likely provide valuablediagnostics of important evolutionary parameters.
| The VLT-FLAMES survey of massive stars.
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| Deep echelle spectrophotometry of S 311, a Galactic HII region located outside the solar circle
We present echelle spectrophotometry of the Galactic HII region S 311.The data have been taken with the Very Large TelescopeUltraviolet-Visual Echelle Spectrograph in the 3100-10400 Årange.We have measured the intensities of 263 emission lines; 178 arepermitted lines of H0, D0 (deuterium),He0, C0, C+, N0,N+, O0, O+, S+,Si0, Si+, Ar0 and Fe0; someof them are produced by recombination and others mainly by fluorescence.Physical conditions have been derived using different continuum- andline-intensity ratios. We have derived He+, C++and O++ ionic abundances from pure recombination lines aswell as abundances from collisionally excited lines for a large numberof ions of different elements. We have obtained consistent estimationsof t2 applying different methods. We have found that thetemperature fluctuations paradigm is consistent with theTe(HeI) versus Te(HI) relation for HII regions, incontrast with what has been found for planetary nebulae. We report thedetection of deuterium Balmer lines up to Dδ in the blue wings ofthe hydrogen lines, whose excitation mechanism seems to be continuumfluorescence.
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