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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.
| 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.
| High-Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
We have mapped a ~5.5×5.5 pc portion of the M17 massivestar-forming region in both 850 and 450 μm dust continuum emissionusing the Submillimeter Common-User Bolometer Array (SCUBA) on the JamesClerk Maxwell Telescope (JCMT). The maps reveal more than 100 dustyclumps with deconvolved linear sizes of ~0.05-0.2 pc and masses of~0.8-120 Msolar, most of which are not associated with knownmid-infrared point sources. Fitting the clump mass function with adouble power law gives a mean power-law exponent ofαhigh=-2.4+/-0.3 for the high-mass power law,consistent with the exponent of the Salpeter stellar mass function. Weshow that a lognormal clump mass distribution with a peak at ~4Msolar produces as good a fit to the clump mass function asdoes a double power law. This 4 Msolar peak mass is wellabove the peak masses of both the stellar initial mass function and themass function of clumps in low-mass star-forming regions. Despite thedifference in intrinsic mass scale, the shape of the M17 clump massfunction appears to be consistent with the shape of the core massfunction in low-mass star-forming regions. Thus, we suggest that theclump mass function in high-mass star-forming regions may be a scaled upversion of that in low-mass regions, instead of its extension to highermasses.
| 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.
| VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions
High-quality K-band spectra of strongly reddened point sources, deeplyembedded in (ultra-)compact H II, have revealed a population of 20 youngmassive stars showing no photospheric absorption lines, but sometimesstrong Brγ emission. The Brγ equivalent widths occupy a widerange (from about 1 to over 100 Å); the line widths of 100-200 kms-1 indicate a circumstellar rather than a nebular origin.The K-band spectra exhibit one or more features commonly associated withmassive young stellar objects (YSOs) surrounded by circumstellarmaterial: a very red colour (J-K) 2, CO bandhead emission,hydrogen emission lines (sometimes doubly peaked), and Fe II and/or MgII emission lines. The large number of objects in our sample allows amore detailed definition and thorough investigation of the properties ofthe massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) themassive YSO candidates are located in a region delimited by the OBzero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e]supergiants. The massive YSO distribution in the CMD suggests that themajority of the objects are of similar spectral type as the Herbig Bestars, but some of them are young O stars. The spectral properties ofthe observed objects do not correlate with the location in the CMD. TheCO emission must come from a relatively dense ( 1010cm-3) and hot (T 2000-5000 K) region, sufficientlyshielded from the intense UV radiation field of the young massive star.The hydrogen emission is produced in an ionised medium exposed to UVradiation. The best geometrical solution is a dense and neutralcircumstellar disk causing the CO bandhead emission, and an ionisedupper layer where the hydrogen lines are produced. We present argumentsthat the circumstellar disk is more likely a remnant of the accretionprocess than the result of rapid rotation and mass loss such as inBe/B[e] stars.
| 3D spectroscopy with VLT/GIRAFFE. III. Mapping electron densities in distant galaxies
We used the moderately high spectral resolution of FLAMES/GIRAFFE (R=10000) to derive electron densities from [OII] line ratios in 6 galaxiesat z 0.55. These measurements have been done through the GIRAFFEmultiple integral field units and are the very first mapping of electrondensities in such distant objects. This allows us to confirm an outflowand identify the role of giant HII regions in galactic disks. Suchmeasurements could be used in the future to investigate the nature ofthe physical processes responsible for the high star formations ratesobserved in galaxies between z0.4 and z1.
| A SCUBA imaging survey of ultracompact HII regions. The environments of massive star formation
We present a SCUBA submillimetre (450 and 850 μm) survey of theenvironment of 105 IRAS point sources, selected from the Wood &Churchwell (1989a) and Kurtz et al. (1994) radio ultracompact (UC) Hiiregion surveys. We detected a total of 155 sub-mm clumps associated withthe IRAS point sources and identified three distinct types of object:ultracompact cm-wave sources that are not associated with any sub-mmemission (sub-mm quiet objects), sub-mm clumps that are associated withultracompact cm-wave sources (radio-loud clumps); and sub-mm clumps thatare not associated with any known ultracompact cm-wave sources(radio-quiet clumps). 90% of the sample of IRAS point sources were foundto be associated with strong sub-mm emission. We consider the sub-mmcolours, morphologies and distance-scaled fluxes of the sample of sub-mmclumps and show that the sub-mm quiet objects are unlikely to representembedded UC Hii regions unless they are located at large heliocentricdistances. Many of the 2.5 arcmin SCUBA fields contain more than onesub-mm clump, with an average number of companions (the companion clumpfraction) of 0.90. The clumps are more strongly clustered than othercandidate HMPOs and the mean clump surface density exhibits a brokenpower-law distribution with a break at 3 pc. We demonstrate that thesub-mm and cm-wave fluxes of the majority of radio-loud clumps are inexcellent agreement with the standard model of ultracompact Hii regions.We speculate on the nature of the radio-quiet sub-mm clumps and, whilstwe do not yet have sufficient data to conclude that they are in a pre-UCHii region phase, we argue that their characteristics are suggestive ofsuch a stage.
| 10 MK Gas in M17 and the Rosette Nebula: X-Ray Flows in Galactic H II Regions
We present the first high spatial resolution X-ray images of twohigh-mass star forming regions, the Omega Nebula (M17) and the RosetteNebula (NGC 2237-2246), obtained with the Chandra X-Ray ObservatoryAdvanced CCD Imaging Spectrometer instrument. The massive clusterspowering these H II regions are resolved at the arcsecond level intomore than 900 (M17) and 300 (Rosette) stellar sources similar to thoseseen in closer young stellar clusters. However, we also detect softdiffuse X-ray emission on parsec scales that is spatially and spectrallydistinct from the point-source population. The diffuse emission hasluminosity LX~=3.4×1033 ergs s-1in M17 with plasma energy components at kT~=0.13 and ~=0.6 keV (1.5 and7 MK), while in Rosette it has LX~=6×1032ergs s-1 with plasma energy components at kT~=0.06 and ~=0.8keV (0.7 and 9 MK). This extended emission most likely arises from thefast O star winds thermalized either by wind-wind collisions or by atermination shock against the surrounding media. We establish that onlya small portion of the wind energy and mass appears in the observeddiffuse X-ray plasma; in these blister H II regions, we suspect thatmost of it flows without cooling into the low-density interstellarmedium. These data provide compelling observational evidence that strongwind shocks are present in H II regions.
| Search for massive protostar candidates in the southern hemisphere. II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellarcandidates we have observed in various tracers a sample of 235 sourcesselected from the IRAS Point Source Catalog, mostly with δ <-30 degr, with the SEST antenna at millimeter wavelengths. The samplecontains 142 Low sources and 93 High, which are believed to be indifferent evolutionary stages. Both sub-samples have been studied indetail by comparing their physical properties and morphologies. Massivedust clumps have been detected in all but 8 regions, with usually morethan one clump per region. The dust emission shows a variety of complexmorphologies, sometimes with multiple clumps forming filaments orclusters. The mean clump has a linear size of 0.5 pc, a mass of 320M_ȯ for a dust temperature Td = 30 K, an H2 density of9.5×105 cm-3, and a surface density of 0.4 gcm-2. The median values are 0.4 pc, 102 M_ȯ,4×104 cm-3, and 0.14 g cm-2,respectively. The mean value of the luminosity-to-mass ratio, L/M≃ 99 L_ȯ/M_ȯ, suggests that the sources are in a young,pre-ultracompact Hii phase. We have compared the millimeter continuummaps with images of the mid-IR MSX emission, and have discovered 95massive millimeter clumps non-MSX emitters, either diffuse orpoint-like, that are potential prestellar or precluster cores. Thephysical properties of these clumps are similar to those of the others,apart from the mass that is 3 times lower than for clumps with MSXcounterpart. Such a difference could be due to the potential prestellarclumps having a lower dust temperature. The mass spectrum of the clumpswith masses above M 100 M_ȯ is best fitted with a power-lawdN/dM propto M-α with α = 2.1, consistent withthe Salpeter (1955) stellar IMF, with α=2.35. On the other hand,the mass function of clumps with masses 10 M_ȯ M 120M_ȯ is better fitted with a power law of slope α = 1.5, moreconsistent with the mass function of molecular clouds derived from gasobservations.
| Recent developments on studies of open clusters
Studies on open clusters are of great significance for astronomy andastrophysics, for which distinct developments have been made recently inboth observational and theoretical respects. In this paper, currentresearch of open cluster is reviewed, and membership determination,basic parameters determination, evolution of cluster and clustermembers, structure and dynamics of open cluster are also described indetail.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| Spectroscopic orbits of two short-period early-type binaries using two-dimensional cross-correlations
We apply the two-dimensional cross-correlation technique TODCOR toderive spectroscopic orbits for the two B-type double-linedspectroscopic binaries HD 66066A and HD315031, previously mentioned as blue straggler candidates ofthe open clusters NGC 2516 and NGC6530, respectively. Reliable radial velocities for bothcomponents are measured even for orbital phases for which the separationbetween the spectral lines are about 0.5 times the quadratic sum of thefull-width at half-maximum of the lines. Both binaries have circularorbits and the orbital periods are 1.67 and 1.38 days for HD 66066A andHD 315031, respectively. We calculate minimum masses with errors of 3-5%and obtain the projected radii from the line widths. We derive absolutestellar parameters which are consistent with the age and distance of theclusters. Both binary systems are formed by main-sequence stars and itis expected that they will experience mass-transfer between theircomponents before the end of the core H-burning stage. HD 315031 islikely a triple system as suggested by the variation of thecenter-of-mass velocity.The observations presented here were obtained at the ComplejoAstronómico El Leoncito (CASLEO), which is operated underagreement between the Consejo Nacional de InvestigacionesCientíficas y Técnicas de la República Argentina(CONICET) and the National Universities of La Plata, Córdoba andSan Juan.
| Deepsky delights.
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| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| 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.
| Embedded Clusters in Molecular Clouds
Stellar clusters are born embedded within giant molecular clouds (GMCs)and during their formation and early evolution are often only visible atinfrared wavelengths, being heavily obscured by dust. Over the past 15years advances in infrared detection capabilities have enabled the firstsystematic studies of embedded clusters in galactic molecular clouds. Inthis article we review the current state of empirical knowledgeconcerning these extremely young protocluster systems. From a survey ofthe literature we compile the first extensive catalog of galacticembedded clusters. We use the catalog to construct the mass function andestimate the birthrate for embedded clusters within 2 kpc of the sun. Wefind that the embedded cluster birthrate exceeds that of visible openclusters by an order of magnitude or more indicating a high infantmortality rate for protocluster systems. Less than 4-7% of embeddedclusters survive emergence from molecular clouds to become boundclusters of Pleiades age. The vast majority (90%) of stars that form inembedded clusters form in rich clusters of 100 or more members withmasses in excess of 50 Mȯ. Moreover, observations ofnearby cloud complexes indicate that embedded clusters account for asignificant (70-90%) fraction of all stars formed in GMCs. We review therole of embedded clusters in investigating the nature of the initialmass function (IMF) that, in one nearby example, has been measured overthe entire range of stellar and substellar mass, from OB stars tosubstellar objects near the deuterium burning limit. We also review therole embedded clusters play in the investigation of circumstellar diskevolution and the important constraints they provide for understandingthe origin of planetary systems. Finally, we discuss current ideasconcerning the origin and dynamical evolution of embedded clusters andthe implications for the formation of bound open clusters.
| Star formation in unbound giant molecular clouds: the origin of OB associations?
We investigate the formation of star clusters in an unbound giantmolecular cloud, where the supporting kinetic energy is twice as largeas the cloud's self-gravity. This cloud manages to form a series of starclusters and disperse, all within roughly two crossing times (10Myr),supporting recent claims that star formation is a rapid process. Simpleassumptions about the nature of the star formation occurring in theclusters allows us to place an estimate for the star formationefficiency at about 5-10 per cent, consistent with observations. We alsopropose that unbound clouds can act as a mechanism for forming OBassociations. The clusters that form in the cloud behave as OBsubgroups. These clusters are naturally expanding from one another dueto the unbound nature of the flows that create them. The properties ofthe cloud we present here are consistent with those of classic OBassociations.
| MASSIVE STARS IN THE LOCAL GROUP: Implications for Stellar Evolution and Star Formation
The galaxies of the Local Group serve as important laboratories forunderstanding the physics of massive stars. Here I discuss what isinvolved in identifying various kinds of massive stars in nearbygalaxies: the hydrogen-burning O-type stars and their evolved He-burningevolutionary descendants, the luminous blue variables, red supergiants,and Wolf-Rayet stars. Primarily I review what our knowledge of themassive star population in nearby galaxies has taught us about stellarevolution and star formation. I show that the current generation ofstellar evolutionary models do well at matching some of the observedfeatures and provide a look at the sort of new observational data thatwill provide a benchmark against which new models can be evaluated.
| A search for TeV gamma-ray emission from SNRs, pulsars and unidentified GeV sources in the Galactic plane in the longitude range between -2 deg and 85 deg.
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, onequarter of the Galactic plane (-2o < l <85o) was surveyed for TeV gamma-ray emission from pointsources and moderately extended sources (φ <= 0.8o).The region covered includes 86 known pulsars (PSR), 63 known supernovaremnants (SNR) and nine GeV sources, representing a significant fractionof the known populations. No evidence for emission of TeV gammaradiation was detected, and upper limits range from 0.15 Crab units upto several Crab units, depending on the observation time and zenithangles covered. The ensemble sums over selected SNR and pulsarsubsamples and over the GeV-sources yield no indication of emission fromthese potential sources. The upper limit for the SNR population is 6.7%of the Crab flux and for the pulsar ensemble is 3.6% of the Crab flux.
| Imaging Fabry-Perot spectrometric studies of velocity fields in gaseous nebulae
In order to study the spatio-kinematics of extended gaseous nebulae suchas the HII regions associated with giant molecular clouds and planetarynebulae, we had designed and built an Imaging Fabry-Perot Spectrometerfor the 1.2m Mt Abu Telescope. We describe here some of the significantscientific results that came out of these studies in the past one decadeat Mt Abu.
| Evidence of a Weak Galactic Center Magnetic Field from Diffuse Low-Frequency Nonthermal Radio Emission
New low-frequency 74 and 330 MHz observations of the Galactic center(GC) region reveal the presence of a large-scale(6deg×2deg) diffuse source of nonthermalsynchrotron emission. A minimum-energy analysis of this emission yieldsa total energy of~(φ4/7f3/7)×1052 ergs and amagnetic field strength of ~6(φ/f)2/7 μG (whereφ is the proton to electron energy ratio and f is the fillingfactor of the synchrotron emitting gas). The equipartition particleenergy density is 1.2(φ/f)2/7 eV cm-3, avalue consistent with cosmic-ray data. However, the derived magneticfield is several orders of magnitude below the 1 mG field commonlyinvoked for the GC. With this field the source can be maintained withthe supernova rate inferred from the GC star formation. Furthermore, astrong magnetic field implies an abnormally low GC cosmic-ray energydensity. We conclude that the mean magnetic field in the GC region mustbe weak, of order 10 μG (at least on size scales >~125").
| The Structure and Evolution of the Lagoon Nebula. I. Submillimeter Continuum and CO Line Mapping
We present submillimeter- and millimeter-wave maps tracing the moleculargas and dust around the edge of the H II region M8. The molecularmaterial is clumped into cores on the scale of the beam (about 0.1 pc)whose temperatures can be estimated from CO observations. The masses ofthe clumps, estimated from their continuum fluxes, are consistent with apower-law mass function with index -1.7+/-0.6, which agrees withdeterminations for other molecular clouds at similar resolutions, usingmolecular lines as tracers. The submillimeter clumps are sited at theinterface between the H II region and the background molecular cloud,where they are exposed to the ultraviolet flux of OB stars. The physicalparameters of the clumps are compared to published models of molecularclouds undergoing photoevaporation, suggesting that the pressure of theionized gas exceeds the internal pressure of the clumps and, therefore,that a shock front will be driven into the clumps. The clumps themselvescurrently appear to be gravitationally unbound, but the compression maybe sufficient to induce collapse.
| An XMM-Newton observation of the Lagoon Nebula and the very young open cluster NGC 6530
We report the results of an XMM-Newton observation of the Lagoon Nebula(M 8). Our EPIC images of this region reveal a cluster of point sources,most of which have optical counterparts inside the very young opencluster NGC 6530. The bulk of these X-ray sources are probablyassociated with low and intermediate mass pre-main sequence stars. Oneof the sources experienced a flare-like increase of its X-ray fluxmaking it the second brightest source in M 8 after the O4 star 9 Sgr.The X-ray spectra of most of the brightest sources can be fitted withthermal plasma models with temperatures of kT ~ a few keV. Only a few ofthe X-ray selected PMS candidates are known to display Hα emissionand were previously classified as classical T Tauri stars. This suggeststhat most of the X-ray emitting PMS stars in NGC 6530 are weak-line TTauri stars. In addition to 9 Sgr, our EPIC field of view contains alsoa few early-type stars. The X-ray emission from HD 164816 is found to betypical for an O9.5 III-IV star. At least one of the known Herbig Bestars in NGC 6530 (LkHα 115) exhibits a relatively strong X-rayemission, while most of the main sequence stars of spectral type B1 andlater are not detected. We also detect (probably) diffuse X-ray emissionfrom the Hourglass Region that might reveal a hot bubble blown by thestellar wind of Herschel 36, the ionizing star of the Hourglass Region.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).
| High-Mass Star Formation. I. The Mass Distribution of Submillimeter Clumps in NGC 7538
We present submillimeter continuum maps at 450 and 850 μm of a12'×8' region of the NGC 7538 high-massstar-forming region, made using the Submillimeter Common-User BolometerArray (SCUBA) on the James Clerk Maxwell Telescope. We used an automatedclump-finding algorithm to identify 67 clumps in the 450 μm image and77 in the 850 μm image. Contrary to previous studies, we find apositive correlation between high spectral index, α, and highsubmillimeter flux, with the difference being accounted for by differenttreatments of the error beam. We interpret the higher spectral index atsubmillimeter peaks as a reflection of elevated dust temperature,particularly when there is an embedded infrared source, although it mayalso reflect changing dust properties. The clump mass-radiusrelationship is well fitted by a power law of the form M~R-xwith x=1.5-2.1, consistent with theories of turbulently supportedclumps. According to our most reliable analysis, the high-mass end(~100-2700 Msolar) of the submillimeter clump mass functionin NGC 7538 follows a Salpeter-like power law with index 2.0+/-0.3. Thisresult agrees well with similar studies of lower mass regions ρ Ophand Orion B. We interpret the apparent invariance of the shape of theclump mass function over a broad range of parent cloud masses asevidence for the self-similarity of the physical processes thatdetermine it. This result is consistent with models that suggest thatturbulent fragmentation, acting at early times, is sufficient to set theclump mass function.
| Investigation of the Large-scale Neutral Hydrogen near the Supernova Remnant W28
The distribution and kinematics of neutral hydrogen have been studied ina wide area around the supernova remnant (SNR) W28. A 2.5d×2.5dfield centered at l=6.5d, b=0° was surveyed using the Parkes 64 mradio telescope (half-power beamwidth of 14.7′ at λ21 cm).Even though W28 is located in a complex zone of the Galactic plane, wehave found different H I features, which are evidence of the interactionbetween W28 and its surrounding gas. An extended cold cloud with about70 Msolar of neutral hydrogen was detected at the location ofW28 as a self-absorption feature, near the local standard of restvelocity of +7 km s-1. This H I feature is the atomiccounterpart of the molecular cloud shown by previous studies to beassociated with W28. From this detection, we can independently confirm akinematical distance of about 1.9 kpc for W28. In addition, the neutralhydrogen observed in emission around the SNR displays a ringlikemorphology in several channel maps over the velocity interval [-25.0,+38.0] km s-1. We propose that these features are part of aninterstellar H I shell that has been swept up by the supernova shockfront. Emission from this shell is confused with unrelated gas. Hence,we derive an upper limit for the shell mass of 1200-1600Msolar, a maximum radius of the order of 20 pc, an expansionvelocity of ~30 km s-1, an initial energy of about1.4-1.8×1050 ergs, and an age of~3.3×104 yr. The preexisting ambient medium has avolume density on the order of 1.5-2 cm-3. W28 is probably inthe radiative evolutionary phase, although it is not possible toidentify the recombined thin neutral shell expected to form behind theshock front with the angular resolution of the present survey.
| Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates. II. Outflow Properties
We present a study of molecular outflows toward a sample of 69 luminousIRAS point sources. The sample is associated with dense molecular gasand has far-infrared luminosities ranging from 102 to105 Lsolar, indicating these objects as regionslikely forming high-mass stars. Mapping in the CO J=2-1 line shows thatmolecular outflows are ubiquitous in these regions. Most of the outflowshave masses of tens of Msolar. The typical dynamicaltimescale of the flow, without correcting for inclination of the flowaxis, is a few times 104 yr. The typical energy in theoutflows is 1046 ergs, comparable to the turbulent energy inthe core. Nearly half of the outflows show spatially resolved bipolarlobes. This indicates that low-mass young stars that coexist in theregion are not responsible for the bipolar outflows observed. It is themore massive stars that drive the outflow. The large detection rate ofoutflows in the region favors an accretion process in the formation ofmassive stars. The maximum mass-loss rate in the wind is about10-4 Msolar yr-1. If these outflows aredriven via accretion, the accretion rate should be as high as a fewtimes 10-4 Msolar yr-1. We compare COoutflows with images at near-infrared wavelengths from the Two MicronAll Sky Survey (2MASS) archive and find that some outflows areassociated with extended emission in the K band, which may be partly dueto vibrationally excited H2 emission at 2.12 μm.
| The massive double-lined O-type binary HD 165052
We present a new optical spectroscopic study of the O-type binary HD165052 based on high- and intermediate-resolution CCD observations. Were-investigated the spectral classification of the binary components,obtaining spectral types of O6.5V and O7.5V for the primary andsecondary, respectively, finding that both stars display weakCIIIλ5696 emission in their spectra. We also determined aradial-velocity orbit for HD 165052 with a period of 2.95510+/-0.00001d,and semi-amplitudes of 94.8 and 104.7+/-0.5kms-1, resultingin a mass ratio Q=0.9. From a comparison with previous radial-velocitydeterminations, we found evidence of apsidal motion in the system.Several signatures of wind-wind collision, such as phase-lockedvariability of the X-ray flux and the Struve-Sahade effect, are alsoconsidered. It was also found that the reddening in the region should benormal, in contrast with previous determinations.
| Carbon, Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,nitrogen, and oxygen yields adopted, have been computed to reproduce theGalactic O/H values obtained from H II regions. All the models fit theoxygen gradient, but only two models also fit the carbon gradient, thosebased on carbon yields that increase with metallicity owing to stellarwinds in massive stars (MSs) and decrease with metallicity owing tostellar winds in low- and intermediate-mass stars (LIMSs). Thesuccessful models also fit the C/O versus O/H evolution history of thesolar vicinity obtained from stellar observations. We also compare thepresent-day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Feversus Fe/H evolution histories of the solar vicinity predicted by ourtwo best models with those derived from H II regions and from stellarobservations. While our two best models fit the C/H and O/H gradients,as well as the C/O versus O/H history, only model 1 fits well the N/Hgradient and the N/O values for metal-poor stars but fails to fit theN/H values for metal-rich stars. Therefore, we conclude that our twobest models solve the C enrichment problem but that further work needsto be done on the N enrichment problem. By adding the C and O productionsince the Sun was formed predicted by models 1 and 2 to the observedsolar values, we find an excellent agreement with the O/H and C/H valuesof the solar vicinity derived from H II region O and C recombinationlines. Our results are based on an initial mass function (IMF) steeperthan Salpeter's a Salpeter-like IMF predicts C/H, N/H, and O/H ratioshigher than observed. One of the most important results of this paper isthat the fraction of carbon due to MSs and LIMSs in the interstellarmedium is strongly dependent on time and on the galactocentric distance;at present about half of the carbon in the interstellar medium of thesolar vicinity has been produced by MSs and half by LIMSs.
| The blue to red supergiant ratio in young clusters at various metallicities
We present new determinations of the blue to red supergiant ratio (B/R)in young open clusters at various metallicities. For this purpose, weexamine the HR diagrams of 45 clusters in the Galaxy and of 4 clustersin the Magellanic Clouds. The identification of supergiants is based onspectroscopic measurements (with photometric counts to check theresults). The new counts confirm the increase of the B/R ratio when themetallicity increases with the following normalized relation:(B/R)/((B/R)sun) =~ 0.05* e3(Z)/(Zsun)}, where Zsun=0.02 and(B/R)sun is the value of B/R at Zsun which dependson the definition of B and R and on the age interval considered (e.g.for spectroscopic counts including clusters with log age between 6.8 and7.5, (B/R)sun =~ 3 when B includes O, B and A supergiants).
| Carbon and Oxygen Galactic Gradients: Observational Values from H II Region Recombination Lines
We present results of deep echelle spectrophotometry of eight Galactic HII regions located at Galactocentric distances between 6.3 and 10.4 kpc.The data have been taken with the Very Large Telescope UltravioletEchelle Spectrograph in the 3100-10360 Å range. We have derivedC++ and O++ abundances from recombination linesfor all the objects as well as O+ abundances from this kindof line for three of the nebulae. The intensity of recombination linesis almost independent of the assumed electron temperature as well as ofthe possible presence of spatial temperature variations or fluctuationsinside the nebulae. These data allow the determination of the gas-phaseC and O abundance gradients of the Galactic disk, of paramountimportance for chemical evolution models. This is the first time the Cgradient is derived from such a large number of H II regions distributedin such a wide range of Galactocentric distances. Abundance gradientsare found of the form Δlog(O/H)=-0.044+/-0.010 dexkpc-1, Δlog(C/H)=-0.103+/-0.018 dex kpc-1,and Δlog(C/O)=-0.058+/-0.018 dex kpc-1.Based on observations collected at the European Southern Observatory,Chile; proposals ESO 68.C-0149(A) and ESO 70.C-0008(A).
| Search for star clusters close to the Galactic plane with DENIS
An automated search for star clusters close to the Galactic plane (|b|< 5o) was carried out on the Point Source Catalogue of theDENIS survey. 44% of the Galactic plane have been observed andcalibrated. The method allowed to retrieve 22 known star clusters and toidentify two new ones, not published yet although previously presentedin the 2MASS web site as embedded clusters in HII regions. Extinction inthe field and in front of the clusters are estimated using a model ofpopulation synthesis. We present the method and give the properties ofthese clusters. Based on observations collected at ESO La Silla.
| Hubble Space Telescope and United Kingdom Infrared Telescope Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by bands of obscuring dust lanes and excited by an O7.5 star,HD 164492A. Previous near-IR, mid-IR, and radio continuum observationsof the cluster of stars at the center of the Trifid Nebula indicated thepresence of circumstellar disks associated with hot stars with envelopesthat are photoionized externally by the UV radiation from the hotcentral star, HD 164492A. Using the WFPC2 on the Hubble Space Telescope,we present evidence of a resolved proplyd in Hα and [S II] lineemission from a stellar source emitting cool dust emission. Using theUnited Kingdom Infrared Telescope, an infrared observation of thestellar source with a proplyd indicates a late F to mid-G spectral type.We also note a remarkable complex of filamentary and sheetlikestructures that appear to arise from the edge of a protostellarcondensation. These observations are consistent with a picture in whichthe bright massive star HD 164492A is responsible for thephotoevaporation of protoplanetary disks of other less massive membersof the cluster, as well as the closest protostellar condensation facingthe central cluster. Using the evidence for a proplyd, we argue that themassive and intermediate-mass members of the cluster, HD 164492C (B6star) and HD 164492 (Herbig Be star), have disks associated with them.
| UBVI and Hα Photometry of the Young Open Cluster NGC 2244
New UBVI and Hα photometry has been performed for the young opencluster NGC 2244. We classified 30 OB stars as being members of thecluster using proper-motion data and spectral types from previousinvestigators, along with photometric diagrams obtained in this study.We measured Hα emission strength of the stars by Hαphotometry and set up a selection criterion to select pre-main-sequence(PMS) stars with Hα emission. Fourteen PMS stars and seven PMScandidates were found using the criterion. In addition, six stars foundnear the positions of ROSAT HRI X-ray sources were assumed to be PMSstars and the optical counterparts of these X-ray sources. We determineda reddening of =0.47+/-0.04 for the cluster and atotal-to-selective extinction ratio of RV=3.1+/-0.2. Thedistance modulus derived was V0-MV=11.1. Bycomparing our photometric results with theoretical evolution models, wederived a main-sequence turnoff age of 1.9 Myr and a PMS age spread ofabout 6 Myr. The slope of the initial mass function, Γ, calculatedin the mass range 0.5<=logm<=2.0 could be flat(Γ=-0.7+/-0.1).
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours
We have obtained high resolution (R = 10 000) K-band spectra ofcandidate young massive stars deeply embedded in (ultra-) compact H IIregions (UCHIIs). These objects were selected from a near-infraredsurvey of 44 fields centered on IRAS sources with UCHII colours. Often,the near-infrared counterpart of the IRAS source is a young embeddedcluster hosting massive stars. In these clusters, three types of objectsare identified. The first type (38 objects) consists of "naked" OB starswhose K-band spectra are dominated by photospheric emission. We classifythe K-band spectra of the OB-type cluster members using near-infraredclassification criteria. A few of them have a very early (O3-O4 V)spectral type, consistent with a young age of the embedded clusters. Thespectral classification provides an important constraint on the distanceto the embedded cluster. The ionising power of the population thusderived is compared to the information obtained from the infrared andradio flux of these sources. In most cases these two differentdeterminations of the ionising flux are consistent, from which weconclude that we have identified the ionising star(s) in about 50% ofthe embedded clusters. The second type (7 objects) are point sourcesassociated with UCHII radio emission, that exhibit nebular emissionlines in the near-infrared. Six of the objects in this group produce HeI emission indicative of an embedded O-type star. These objects are moreembedded than the OB stars and probably do not dominate the infraredflux as measured by IRAS. They may emit the bulk of their reprocessed UVradiation at mm wavelengths. The third type (20 objects) ischaracterised by broad (100-200 km s-1) Brγ emissionand no photospheric absorption profiles. Bik et al. (2005, A&A,submitted) show that these objects are massive YSO candidates surroundedby dense circumstellar disks.
| The Open Cluster NGC 2516. I. Optical Photometry
We present UBVI CCD photometry of the intermediate-age open cluster NGC2516. From this new photometry we derive the cluster parameters[V0-MV=7.77+/-0.11 mag, E(B-V)=0.112+/-0.024 mag],age [log(age)=8.2+/-0.1], and a photometric abundance([Fe/H]=-0.10+/-0.04) from a comparison with theoretical isochrones ofthe Padua group. We find that the X-ray bright stars[logLX(ergs s-1)>~29.0] are systematicallybluer in B-V and U-B. It was necessary to modify the photometricmembership criterion used by Sung & Bessell due to the UV excess ofX-ray bright stars and due to the abrupt increase in the slope of the(MV, B-V) ZAMS relation at B-V>~1.4. A minimum binaryfrequency 40% (+/-5%) was estimated from the distribution of distancemoduli of unevolved photometric members. The photometric abundanceobtained in this study is similar to that of the Pleiades and thereforethe lower metallicity is not the cause of strong X-ray emission ofF-type stars in NGC 2516. We believe that this results from them beingclose binaries, as suggested by their different distribution of distancemoduli compared to other cluster members and surmise that the tidalforces between the two stars must increase their X-ray activity.
| Infrared portrait of the nearby massive star-forming region IRAS 09002-4732
We present high-resolution near-infrared and mid-infrared imaging,mid-infrared spectroscopy and millimetre-wavelength continuumobservations of the nearby massive star-forming complex IRAS 09002-4732.A rich cluster of young stars displaying near-infrared excess emissionis detected. We identify the ionising source of the ultracompact H IIregion G268.42-0.85 and show that this star is the dominant heating andilluminating source of the region. Spectral type estimates based ondifferent methods are consistent with a star of spectral type O9. Thecombination of the new observations with literature data allows us toset up the first structural model for the region. We argue that theultracompact H II region is embedded in the rear side of the southern CSclump. Additionally, we detect several interesting objects. Among theseobjects are a network of dark dust filaments, an elongated, externallyheated object with strong infrared excess inside the H II region andobjects seen as silhouettes in the foreground of the large southernreflection nebulosity. The filamentary structures may play an importantrole in the star formation process.
| Radial velocities, binarity, and kinematic membership in open clusters with blue straggler candidates
Not Available
| 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
| Molecular Hydrogen in the Lagoon: H2 Line Emission from Messier 8
The 2.12μmv=1-0 S(1) line of molecular hydrogen has been imaged inthe Hourglass region of M8. The line is emitted from a roughly bipolarregion, centred around the O7 star Herschel 36. The peak H21-0 S(1) line intensity is 8.2 ×10-15ergs-1cm-2arcsec-2. Theline centre emission velocity varies from -25kms-1 in the SElobe to +45kms-1 in the NW lobe. The distribution is similarto that of the CO J=3-2 line. The H2 line appears to beshock-excited when a bipolar outflow from Herschel 36 interacts with theambient molecular cloud. The total luminosity of all H2 linesis estimated to be ~16Lsolar and the mass of the hotmolecular gas ~9 × 10-4Msolar (without anycorrection for extinction).
| MSX mid-infrared imaging of massive star birth environments - II. Giant HII regions
We conduct a Galactic census of giant HII (GHII) regions, based on theall-sky 6-cm data set of Kuchar & Clark, plus the kinematicdistances obtained by Russeil. From an inspection of mid-infrared (MIR)Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlasimages, we identify a total of 56 GHII regions in the Milky Way, ofwhich 15 per cent (65 per cent) can be seen at optical (near-IR)wavelengths. The mid to far-IR fluxes from each GHII region aremeasured, and sample the thermal emission from the ubiquitous dustpresent within the exciting clusters of OB stars, arising from theintegrated luminosity of the hot stars heating the cluster dust, forwhich we obtain log L(IR) = 5.5-7.3Lsolar. The MIR 21-μmspatial morphology is presented for each GHII region, and oftenindicates multiple emission sources, suggesting complicated clusterformation. IR colour-colour diagrams are presented, providinginformation concerning the temperature distribution and the opticaldepth of the dust. For the clusters of our study, the dust is notoptically thick to all stellar radiation, thus the measured infraredluminosity is lower than Lbol. As the dust environment of acluster begins to dissipate, the thermal emission and its optical depthought to decrease even before the stars evolve appreciably. We seeevidence of this in our empirical relationship between the integrated IRand Lyman continuum luminosities.
| Non-uniform extinction in young open star clusters
The extinction law and the variation of colour excess with position,luminosity and spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14,Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. Thedifference in the minimum and maximum values of E(B-V) of clustermembers has been considered as a measure of the presence of non-uniformgas and dust inside the clusters. Its value ranges from 0.22 to 1.03magin the clusters under study, which indicates that non-uniform extinctionis present in all the clusters. It has been noticed for the first timein NGC 1502 and Tr 37. It is also found that the differential colourexcess in open clusters, which may be caused by the presence of gas anddust, decreases systematically with the age of clusters, indicating thatmatter is either used in star formation or blown away by hot stars orboth. There is no uniformity in the variation of E(B-V) with position orspectral class or luminosity. Except in Tr 14, all clusters show arandom spatial distribution of E(B-V), indicating a random distributionof gas and dust inside the clusters. The E(B-V) value correlates withboth luminosity and spectral class only in the case of Coll 228, Tr 16and Be 86. The members of these clusters atλ>=λR show larger values of colour excessratios than the normal ones. The value of E(U-V)/E(B-V) for most of thecluster members is close to the normal interstellar value of 1.73.However, the colour excess ratios with E(B-V) atλ>=λJ are smaller than the normal value forNGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in therelationship of extinction properties amongst the clusters under study.
| A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry
We present Very Large Telescope (VLT) UVES echelle spectrophotometry ofthe Orion nebula in the 3100-10400 Årange. We have measured theintensity of 555 emission lines, many of them corresponding to permittedlines of different heavy-element ions. This is the largest set ofspectral emission lines ever obtained for a Galactic or extragalacticHII region. We have derived He+, C2+,O+, O2+ and Ne2+ abundances from purerecombination lines. This is the first time that O+ andNe2+ abundances have been obtained from these kinds of linesin the nebula. We have also derived abundances from collisionallyexcited lines for a large number of ions of different elements. In allcases, ionic abundances obtained from recombination lines are largerthan those derived from collisionally excited lines. We have obtainedremarkably consistent independent estimations of the temperaturefluctuation parameter, t2, from different methods, which arealso similar to other estimates from the literature. This resultstrongly suggests that moderate temperature fluctuations (t2between 0.02 and 0.03) are present in the Orion nebula. We have comparedthe chemical composition of the nebula with those of the Sun and otherrepresentative objects. The heavy-element abundances in the Orion nebulaare only slightly higher than the solar ones, a difference that can beexplained by the chemical evolution of the solar neighbourhood.
| Estimation of Errors in the Distances to Intrinsically Reddened Stars
Not Available
| 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.
| Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
| Revised masses of dust and gas of SCUBA Local Universe Survey far-infrared bright galaxies based on a recent CO survey
Recent CO measurements of an essentially complete subsample of galaxiesfrom the SCUBA Local Universe Survey (SLUGS) are used to examine theirimplications for dust and gas masses in this sample. Estimates of dustmasses are affected by a contribution to the SCUBA brightnessmeasurements by CO(3-2) emission, and molecular gas masses by the use ofa modified value of the CO-to-H2 conversion factor X. Theaverage dust mass is reduced by 25-38 per cent, which has no bearing onearlier conclusions concerning the shape of the dust mass luminosityfunction derived from the SLUGS. The value of X found from the COsurvey, when applied together with the reduction in dust masses, leadsto lower estimates for the mean gas-to-dust mass ratios, where the gasincludes both H2 and H I. For the CO sample, the mean globalratio is reduced from approximately 430 to about 320-360, but is furtherreduced to values near 50 when applied to the nuclear regions relevantto the CO observations. We discuss these results and suggest that thedifferences between the nuclear and outer regions may simply reflectdifferences in metallicity or the existence of considerable amounts ofunobserved cold dust in the outer regions of these galaxies.
| Radio Continuum and Recombination Line Study of Ultracompact H II Regions with Extended Envelopes
We have carried out 21 cm radio continuum observations of 16ultracompact H II regions using the VLA (DnC array) in a search forassociated extended emission. We have also observed the H76αrecombination line toward all the sources and the He76α line atthe positions with strong H76α line emission. The UC H II regionshave simple morphologies and large (>~10) ratios of single-dish toVLA fluxes. We detected extended emission toward all the sources. Theextended emission consists of one to several compact (~1' or 0.5-5 pc)components and a diffuse extended (2'-12' or 4-19 pc) envelope. All theUC H II regions but two are located in the compact components, where theUC H II regions always correspond to their peaks. The compact componentswith UC H II regions are usually smaller and denser than those withoutUC H II regions. For individual sources, we derive the spectral types(O7-O4) of the ionizing stars and the fractions of UV photons absorbedby dust within the nebulae, which are significantly different fromprevious estimates based on the UC H II regions alone. Our recombinationline observations show that the ultracompact, compact, and extendedcomponents have approximately the same velocity in the individualsources with one exception (G25.72+0.05), implying that they arephysically associated. The compact components in each object appear tobe ionized by separate sources, while the UC H II regions and theirassociated compact components are likely to be ionized by the samesources on the basis of the morphological relations mentioned above.This suggests that almost all of the observed UC H II regions are not``real'' UC H II regions but ultracompact cores of more extended H IIregions, and that their actual ages are much greater than theirdynamical age (<~104 yr). We find that most of simple UC HII regions previously known have large ratios of single-dish to VLAfluxes, similar to our sources. Therefore, the ``age problem'' of UC HII regions does not seem to be as serious as earlier studies argued. Wepresent a simple model in which the coexistence of the ultracompact,compact, and extended components for a long (>105 yr) time iseasily explained by combining the champagne flow model with thehierarchical structure of massive star-forming regions. The well-knownrelation between the density and diameter of H II regions,ne~D-1, is a natural consequence of thehierarchical structure according to our model. We discuss someindividual sources.
| Chemical Abundances of the Galactic H II Region NGC 3576 Derived from Very Large Telescope Echelle Spectrophotometry
We present echelle spectrophotometry of the Galactic H II region NGC3576. The data have been taken with the VLT UVES echelle spectrograph inthe 3100-10400 Å range. We have measured the intensities of 458emission lines, 344 are permitted lines of H0,He0, C+, N0, N+,N++, O0, O+, Ne+,S++, Si0, Si+, Ar0, andAr+; some of them are produced by recombination and othersmainly by fluorescence. Electron temperatures and densities have beendetermined using different continuum and line intensity ratios. We havederived He+, C++, O+, O++,and Ne++ ionic abundances from pure recombination lines. Wehave also derived abundances from collisionally excited lines for alarge number of ions of different elements. Remarkably consistentestimations of t2 have been obtained by comparing Balmer andPaschen with [O III] temperatures, and O++ andNe++ ionic abundances obtained from collisionally excited andrecombination lines. The chemical composition of NGC 3576 is comparedwith those of other Galactic H II regions and with the one from the Sun.A first approach to the gas-phase Galactic radial abundance gradient ofC as well as of the C/O ratio has been made.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 68.C-0149(A).
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| A Deep Chandra X-Ray Observation of the Rich Young Cluster NGC 6530. I. The X-Ray Source Catalog and the Cluster Population
In a deep 60 ks Chandra ACIS X-ray observation of the very young clusterNGC 6530, we detect 884 X-ray point sources and argue that a very largefraction of them (90%-95%) must be pre-main-sequence (PMS) clustermembers, mostly of low masses. This is a significant enlargement of theknown NGC 6530 stellar population with respect to previous opticalstudies, including Hα surveys. We identify 220 X-ray sources withcataloged stars down to V=17, while most unidentified sources havefainter counterparts. Moreover, we find an infrared counterpart in the2MASS catalog for 731 X-ray sources. The optically identified clusterX-ray sources are found in a band in the H-R diagram above the mainsequence, in the locus of 0.5-1.5 Myr PMS stars, with masses down to0.5-1.5 Msolar. We find evidence of an age gradient acrossthe field from northwest to south, suggesting a sequence of starformation events qualitatively similar to that found in earlier studiesof the same region, but differing in the details. A group of X-raysources showing frequent flares may be associated with the youngeststars in the cluster, suggesting that X-ray flaring activity isespecially intense in the youngest PMS phases of low-mass stars.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The central part of the young open cluster NGC 6383
The spectral and extinction properties of 14 pre-main sequencecandidates in the central part of the very young open cluster NGC 6383were investigated. None of these stars shows evidence for anomalouscircumstellar extinction. However, six out of 14 programme stars do showan infrared excess, indicative of the presence of circumstellar dust,heated up by the central star. One of these stars (number 4), also showsHα in emission and shows some indications for the presence ofcircumstellar gas in its spectrum, and might therefore be a newlyidentified Herbig Ae star. Based on observations collected at theEuropean Southern Observatory, La Silla, Chile.
| Optical Imaging and Spectroscopy of the H II Region G353.2+0.9 in NGC 6357 and Its Relation to Pismis 24
G353.2+0.9 is the brightest H II region in NGC 6357. The presentobservations imply that it is optically thin and contains ~300Msolar of ionized gas. It is probably expanding into thesurrounding medium as a result of its higher thermal pressure. Itschemical composition is similar to that found in other H II regionslocated at comparable galactocentric distances. The inner regions areprobably made of thin shells and filaments, whereas extended slabs ofmaterial, probably shells seen edge-on, are found in the periphery.Extinction, although abnormal, is quite uniform but somewhat larger inthe brightest optical regions. The radio continuum and Hα emissionmaps are very similar, indicating that most of the optical nebula is notembedded in the denser regions traced by molecular gas and by thepresence of IR sources. About 1050 UV photons per second arerequired to produce the Hβ flux from the11.3m×10' region surrounding Pis 24. Thus, most ofthe energy powering G353.2+0.9 and the surrounding environment isproduced by the O3-O7 stars in Pis 24. Practically all Two Micron AllSky Survey sources with large near-infrared excesses are withinG353.2+0.9, indicating that the most recent star-forming processoccurred in this region. Some amount of heating and ionization is due tothese stars. The formation of the Pis 24 cluster preceded and caused theformation of this new generation of stars and may be responsible for thepresent-day morphology of the entire NGC 6357 region.
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