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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.
| Spitzer Spectral Line Mapping of Supernova Remnants. I. Basic Data and Principal Component Analysis
We report the results of spectroscopic mapping observations carried outtoward small (1'×1') regions within thesupernova remnants W44, W28, IC 443, and 3C 391 using the InfraredSpectrograph (IRS) of the Spitzer Space Telescope. These observations,covering the 5.2-37 μm spectral region, have led to the detection ofa total of 15 fine-structure transitions of Ne+,Ne++, Si+, P+, S, S++,Cl+, Fe+, and Fe++; the S(0)-S(7) purerotational lines of molecular hydrogen; and the R(3) and R(4)transitions of hydrogen deuteride. In addition to these 25 spectrallines, the 6.2, 7.7, 8.6, 11.3, and 12.6 μm PAH emission bands werealso observed. Most of the detected line transitions have proven strongenough to map in several sources, providing a comprehensive picture ofthe relative distribution of the various line emissions observable inthe Spitzer IRS bandpass. A principal component analysis of thespectral-line maps reveals that the observed emission lines fall intofive distinct groups, each of which may exhibit a distinct spatialdistribution: (1) lines of S and H2(J>2) (2) theH2 S(0) line; (3) lines of ions with appearance potentialsless than 13.6 eV; (4) lines of ions with appearance potentials greaterthan 13.6 eV, not including S++; (5) lines of S++.Lines of group 1 likely originate in molecular material subject to aslow, nondissociative shock that is driven by the overpressure withinthe supernova remnant, and lines in groups 3-5 are associated primarilywith dissociative shock fronts with a range of (larger) shockvelocities. The H2 S(0) line shows a low-density diffuseemission component and, in some sources, a shock-excited component.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.
| Pre-main-sequence stars in the Lagoon Nebula (M8)
We report the discovery of new pre-main-sequence (PMS) stars in theLagoon Nebula (M8) at a distance of 1.25 kpc, based onintermediate-resolution spectra obtained with the Boller & Chivensspectrograph at the 6.5-m Magellan I Telescope (Las CampanasObservatory, Chile). According to the spectral types, the presence ofemission lines and the lithium λ6708 absorption line, we are ableto identify 27 classical T Tauri stars, seven weak-lined T Tauri starsand three PMS emission objects with spectral type G, which we include ina separate stellar class denominated `PMS Fe/Ge class'. Usingnear-infrared photometry either from the Two-Micron All-Sky Survey orfrom our own previous work, we derive effective temperatures andluminosities for these stars and locate them in the Hertzsprung-Russelldiagram, in order to estimate their masses and ages. We find that almostall of our sample stars are younger than 3 × 106 yr andspan over a range of masses between 0.8 and 2.5 Msolar. Across-correlation between our spectroscopic data and the X-ray sourcesdetected with the Chandra ACIS instrument is also presented.This paper includes data gathered with the 6.5-m Magellan Telescopeslocated at the Las Campanas Observatory, Chile.E-mail: julia@xeneize.dfuls.cl ‡Fellow of CONICET, Argentina. §Member of Carrera del Investigador Científico, CONICET,Argentina.
| The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| 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
| VLT/Flames observations of the star forming region NGC 6530
Context: Mechanisms regulating the evolution of pre-main sequence starscan be understood by studying stellar properties such as rotation, diskaccretion, internal mixing and binarity. To investigate such properties,we studied a sample of 332 candidate members of the massive and populousstar forming region NGC 6530. Aims: We select cluster members usingdifferent membership criteria, to study the properties of pre-mainsequence stars with or without circumstellar disks. Methods: We useintermediate resolution spectra including the Li I 6707.8 Å lineto derive radial and rotational velocities, binarity and to measure theEquivalent Width of the lithium line; these results are combined withX-ray data to study the cluster membership. Optical-IR data and Hαspectra, these latter available for a subsample of our targets, are usedto classify CTTS and WTTS and to compare the properties of stars withand without disks. Results: We find a total of 237 certain membersincluding 53 binaries. The rotational velocity distributions of starswith IR excesses are statistically different from that of stars withoutIR excesses, while the fraction of binaries with disks is significantlysmaller than that of single stars. Stars with evidence of accretion showcircumstellar disks; youth of cluster members is confirmed by thelithium abundance consistent with the initial content. Conclusions: .Asindicated by the disk-locking picture, stars with disks in general haverotational velocities lower than stars without disks. Binaries in NGC6530 seem have undergone significant disk evolution.Full Tables 1 and 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/123
| The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation
Context: . We study the stellar population of the very young cluster NGC2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only5 Myr old, has already cleared most of its inter- and circumstellardust, and with its small and uniform reddening offers a uniqueopportunity of studying its pre-main-sequence stellar population withminimal disturbance from a dense interstellar medium. Aims: .Ourmain purposes are to select cluster members down to low masses and tostudy their properties as a population (spatial properties, initial massfunction, and coronal properties). Methods: .We compare existingdeep optical photometry and Hα data with new X-ray data. We usecombined optical and X-ray criteria to select cluster members.Results: .We detect 387 X-ray sources down to log LX = 29.0(erg/s), and identify most of them (308) with star-like objects. Themajority (88%) of optically identified X-ray sources are found to bevery good candidate low-mass pre-main-sequence stars, with minimalfield-object contamination. This increases the known cluster census by asubstantial amount at low masses, with respect to previous optical/IRstudies. The fraction of stars with active accretion is found to be inthe range 5-9%. We find a significantly wider spatial distribution forlow-mass stars than for massive stars (mass segregation). We find only asmall spread around the low-mass cluster sequence in the HR diagram,indicating that star formation lasted only about 1-2 Myr. We havederived the cluster initial mass function, which appears to flatten (onthe low-mass side) at higher masses with respect to other very youngclusters. The quiescent X-ray emission of low-mass cluster stars isfound to be rather strictly correlated with the stellar bolometricluminosity: the small spread in this correlation puts an upper bound onthe amplitude of X-ray variability on time scales longer than one day(e.g., activity cycles) in such young coronal sources. We findsignificant X-ray spectral differences between low-mass stars brighterand fainter than log LX ˜ 30.3 (erg/s), respectively,with X-ray brighter stars showing hotter components (kT ˜ 2 keV),absent in fainter stars.
| The rich young cluster NGC 6530: a combined X-ray-optical-infrared study
We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530,complementing our previous studies of this cluster. We considerdifferent indicators of IR excesses, which can be taken as an indicatorof circumstellar disks and therefore of pre-main-sequence status. Weused reddening-free indices to ensure that our results are unaffected byhighly irregular, differential reddening. More than the study of the JHKbands alone (33 IR-excess stars found), we found it useful to comparevarious optical and IR colors, finding overall 333 stars with optical-IRexcess. In the field of the previously studied Chandra ACIS-Iobservation, we found 196 stars with optical-IR excess, of which 120have not been previously found in X-rays. The total number of estimatedcluster members thus becomes ≥1100. The estimated disk frequency inthe ACIS field is ~20%. By considering different optical-IR excessindices, we find only partially overlapping subsamples, corresponding todifferent characteristics of their spectral energy distributions(longer- or shorter-wavelength IR excesses, or blue-violet excess). In aregion displaced towards northwest with respect to the known clustercenter, we find an unexpected concentration of stars with optical-IRexcesses, most of which are not detected in X-rays. The spectral energydistribution of some of these objects shows more extreme excesses withrespect to most ordinary T Tauri stars, which suggests circumstellardisks with very small inner holes and high accretion rates; some objectsare best interpreted as being surrounded by reflection nebulosities, asfound in some optically detected Class I objects in nearer star-formingregions. These reflection-nebulae candidates have the lowest X-raydetection rate among all subsamples considered here. Optical-IR excessstars in the north of NGC 6530 are nearly co-spatial with asub-population of cluster stars older than the central cluster stars.This leads to the conclusion that in these northern regions of NGC 6530,far from massive cluster stars, star formation (and disk evolution) hasproceeded rather undisturbed over longer time periods than near thecluster center, where most massive stars are found, and most stars lacksubstantial disks and strong accretion.
| Pulsating pre-main sequence stars in IC 4996 and NGC 6530
Context: .Asteroseismology of pulsating pre-main sequence (PMS) starshas the potential of testing the validity of current models of PMSstructure and evolution. As a first step, a sufficiently large sample ofpulsating PMS stars has to be established, which allows to selectcandidates optimally suited for a detailed asteroseismological analysisbased on photometry from space or ground based network data. Aims:.A search for pulsating PMS members in the young open clusters IC 4996and NGC 6530 has been performed to improve the sample of known PMSpulsators. As both clusters are younger than 10 million years, allmembers with spectral types later than A0 have not reached the zero-agemain sequence yet. Hence, IC 4996 and NGC 6530 are most suitable tosearch for PMS pulsation among their A- and F-type cluster stars.Methods: .CCD time series photometry in Johnson B and V filters has beenobtained for IC 4996 and NGC 6530. The resulting light curves for 113stars in IC 4996 and 194 stars in NGC 6530 have been subject to detailedfrequency analyses. Results: .2 δ Scuti-like PMS stars havebeen discovered in IC 4996 and 6 in NGC 6530. For another PMS star ineach cluster, pulsation can only be suspected. According to the computedpulsation constants, the newly detected PMS stars seem to prefer topulsate in a similar fashion to the classical δ Scuti stars, andwith higher overtone modes.
| The XMM-Newton view of Plaskett's star and its surroundings
XMM-Newton data of Plaskett's star (HD 47129) are used in order toanalyse its X-ray spectrum and variability and hence to derive furtherconstraints on the wind interaction in this early-type binary (O6 I +O7.5 I) system.Conventional models fail to provide a consistent fit of the EuropeanPhoton Imaging Camera (EPIC) and Reflexion Grating Spectrometer (RGS)spectra. The lines seen in the RGS spectrum have a temperature ofmaximum emissivity between 0.18 and 1.4 keV. The EPIC and RGS spectraare best fitted by a non-equilibrium model consisting of abremsstrahlung continuum at 2.2 +/- 0.1 keV and a number of independentemission lines. Our tests also suggest that an overabundance in nitrogenby a factor of ~6 might be indicated to best represent the RGS spectrum.On the other hand, a short-term variability study of the light curves ofthe system indicates that the X-ray flux of Plaskett's star did notdisplay any significant variability during our observation. This resultholds for all time-scales investigated here (from a few minutes to aboutone hour). Combining our XMM-Newton data with ROSAT archivalobservations, we find, however, a significant variability on the orbitaltime-scale. If this behaviour is indeed phase locked, it suggests aminimum in the X-ray flux when the primary star is in front. This mightbe attributed to an occultation of the colliding wind region by the bodyof the primary.Finally, 71 other X-ray sources have been detected in the field aroundPlaskett's star and most of them have a near-infrared (near-IR)counterpart with colours that are consistent with those of slightlyreddened main-sequence objects. Actually, a sizeable fraction of theX-ray sources in the EPIC images could be either foreground orbackground sources with no direct connection to HD 47129.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.E-mail: linder@astro.ulg.ac.be (NL), rauw@astro.ulg.ac.be (GR) ‡Research Associate FNRS, Belgium.
| Membership and Segregation Effects in the Young Open Cluster NGC 6530
From photographic plate data of Shanghai Astronomical Observatory with atime baseline of 87 years, proper motions and membership probabilitiesof 364 stars in the open cluster NGC 6530 region are reduced. On thebasis of membership determination, luminosity function and segregationeffect of the cluster are discussed with details. Spatial masssegregation is obviously present in NGC 6530 while there is no clearevidence for a velocity-mass (or velocity-luminosity) dependence. Theobserved spatial mass segregation for NGC 6530 might be due to acombination of initial conditions and relaxation process.
| An XMM-Newton view of the young open cluster NGC 6231. I. The catalogue
This paper is the first of a series dedicated to the X-ray properties ofthe young open cluster NGC 6231. Our data set relies on an XMM-Newtoncampaign of a nominal duration of 180 ks and reveals that NGC 6231 isvery rich in the X-ray domain too. Indeed, 610 X-ray sources aredetected in the present field of view, centered on the cluster core. Thelimiting sensitivity of our survey is approximately 6 ×10-15 erg cm-2 s-1 but clearly dependson the location in the field of view and on the source spectrum. Usingdifferent existing catalogues, over 85% of the X-ray sources could beassociated with at least one optical and/or infrared counterpart withina limited cross-correlation radius of 3´´ at maximum. Thesurface density distribution of the X-ray sources presents a slight N-Selongation. Once corrected for the spatial sensitivity variation of theEPIC instruments, the radial profile of the source surface density iswell described by a King profile with a central density of about 8sources per arcmin2 and a core radius close to 3.1 arcmin.The distribution of the X-ray sources seems closely related to theoptical source distribution. The expected number of foreground andbackground sources should represent about 9% of the detected sources,thus strongly suggesting that most of the observed X-ray emitters arephysically belonging to NGC 6231. Finally, beside a few bright but softobjects - corresponding to the early-type stars of the cluster - most ofthe sources are relatively faint (~5 × 10-15 ergcm-2 s-1) with an energy distribution peakedaround 1.0-2.0 keV.
| Research on the membership and spatial motion of the open cluster NGC6530
or stars in the region around the open cluster NGC6530, clustermemberships are calculated based on the maximum likelihood principleusing proper motions obtained from photographic plates acquired with the40 cm refraction telescope at Shanghai Astronomical Observatory.Analyses show that the membership determination is very effective with250 stars having membership probabilities p ≥ 0.9. The spatial motionof the star cluster is also determined and a simple discussion iscarried out.
| Multiwavelength Analysis of the Young Open Cluster NGC 2362
We present a multiwavelength analysis of the young open cluster NGC2362; UBVRCIC CCD photometric observations,together with available data in the Chandra database, near-infrared datafrom the Two Micron All Sky Survey (2MASS), and recently publishedHα spectroscopy were used to get information about theevolutionary stage of the cluster and the main physical properties ofits stellar content. Cluster membership is estimated for everyindividual star by means of zero-age main sequence (ZAMS) and isochronefitting. The cluster is confirmed to host a rich population ofpre-main-sequence (PMS) objects and to contain a large number ofX-ray-emitting stars, which reach from the PMS members of GK spectraltype up to the most luminous OB main-sequence (MS) members. The PMScluster members show no significant age spread, and the comparison toboth PMS and post-MS isochrones suggests an homogeneous age for allcluster members. The analysis allows us to assess the validity ofcurrently used PMS evolutionary models and supports the suggestion of awell-defined positive correlation of the X-ray emission from PMS starswith their bolometric luminosity. Clear differences are found betweenthe X-ray activity properties of MS and PMS cluster members, both in therelation between X-ray luminosity and bolometric luminosity, and inspectral properties as well.
| The pre-main sequence population of NGC 6530 in M8
We report the results of an investigation of faint probable members ofthe very young open cluster NGC 6530 in M8, based on intermediateresolution spectroscopy obtained with the 6.5-m Magellan I telescope atLas Campanas Observatory. The analysis of the spectra lead us todiscover 39 new pre-main sequence stars in the region. According to thespectral types, the presence of emission lines and lithium absorptionline, we identify 30 classical T Tauri stars, 7 weak T Tauri stars andtwo Herbig Ae/Be objects. Using infrared magnitudes from 2MASS and fromour previous work and the evolutionary tracks from Palla & Stahler(1999), we estimate the masses and ages of these stars. We find thatalmost all of our sample stars are younger than 3×10^6 years andspan a range of masses between 0.8 and 2.0 Mȯ.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| 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 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.
| 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.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| The star formation region NGC 6530: Distance, ages and initial mass function
We present astrometry and BVI photometry, down to V≃22, of thevery young open cluster NGC 6530, obtained from observations taken withthe Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the Vvs. B-V and the V vs. V-I color-magnitude diagrams (CMD) show that theupper main sequence is dominated by very bright cluster stars, while,because of the high obscuration of the giant molecular cloud surroundingthe cluster, the blue envelopes of the diagrams at V≳14 are limitedto the main sequence stars at the distance of NGC 6530. This particularstructure of the NGC 6530 CMD allows us to conclude that its distance isabout d ≃ 1250 pc, significantly lower than the previousdetermination of d=1800 pc. We have positionally matched our opticalcatalog with the list of X-ray sources found in a Chandra-ACISobservation, finding a total of 828 common stars, 90% of which arepre-main sequence stars in NGC 6530. Using evolutionary tracks of\citet{sies00}, mass and age values are inferred for these stars. Themedian age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0)Mȯ, the Initial Mass Function (IMF) shows a power lawindex x=1.22±0.17, consistent with both the Salpeter index(1.35), and with the index derived for other young clusters; towardssmaller masses the IMF shows a peak and then it starts to decrease.Based on observations made with the European Southern Observatorytelescopes obtained from the ESO/ST-ECF Science Archive Facility.Full Tables 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/941
| Deepsky delights.
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| On the highly reddened members in six young galactic star clusters - a multiwavelength study
The spectral and reddening properties of 211 highly reddenedproper-motion members with V < 15 mag in six young galactic starclusters are investigated using low-resolution spectroscopic, broad-bandUBVRIJHK and mid-infrared (IR) data. We report emission features in CaIIHK and HI lines for a sample of 29 stars including 11 stars reported forthe first time and also provide either a new or more reliable spectralclass for a sample of 24 stars. CaII triplet width measurements are usedto indicate the presence of an accretion disc for a dozen stars and toindicate luminosity for a couple of stars. On the basis of spectralfeatures, near-IR excesses, dereddened colour-colour diagrams and mid-IRspectral indices we identify a group of 28 pre-main-sequence clustermembers including five highly probable Herbig Ae/Be and six classical TTauri stars. A total of 25 non-emission main-sequence (MS) stars,amounting to ~10 per cent early-type MS members, appears to showVega-like characteristics or are precursors to such a phenomenon. Thevarious membership indicators suggest that ~16 per cent of theproper-motion members are non-members. A significant fraction (>70per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC6611 show anomalous reddening with RV= 4.78 +/- 0.10, 3.54+/- 0.04, 3.87 +/- 0.05 and 3.56 +/- 0.02, respectively, indicating thepresence of grain size dust larger than that typical of the diffusemedium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 alsoshow normal behaviour while the cluster NGC 6823 appears to have normalreddening. Three highly luminous late-type giants, one in NGC 2244 andtwo in NGC 6530, appear to be members and are inpost-hydrogen-core-burning stages, suggesting a prolonged duration (~25Myr) of star formation.
| Spectroscopic binaries in southern open clusters
This is a report on an ongoing program about binaries in southern openclusters. The long-term purpose of this project is to contribute tounderstanding the formation and evolution of spectroscopic binaries,providing observational constraints that will permit tests of some ofthe current theories on binary formation in open clusters.
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