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Публикации по объекту
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.
| Pismis 24-1: The Stellar Upper Mass Limit Preserved
Is there a stellar upper mass limit? Recent statistical work seems toindicate that there is and that it is in the vicinity of 150Msolar. In this paper we use HST and ground-based data toinvestigate the brightest members of the cluster Pismis 24, one of which(Pismis 24-1) was previously inferred to have a mass greater than 200Msolar, in apparent disagreement with that limit. Wedetermine that Pismis 24-1 is composed of at least three objects, theresolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis24-1NE. The evolutionary zero-age masses of Pismis 24-1SW, theunresolved system Pismis 24-1NE, and the nearby star Pismis 24-17 areall ~100 Msolar, very large but under the stellar upper masslimit.This article is based on data gathered with three facilities: theNASA/ESA Hubble Space Telescope (HST), the 6.5 m Magellan Telescopes atLas Campanas Observatory (LCO), and the 2.15 m J. Sahade telescope atComplejo Astronómico El Leoncito (CASLEO). The HST observationsare associated with GO program 10602. HST is controlled from the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555. CASLEO is operated under agreement between CONICET, SeCyT, andthe Universities of La Plata, Córdoba and San Juan, Argentina.
| The Young Stellar Population in M17 Revealed by Chandra
We report here results from a Chandra ACIS observation of the stellarpopulations in and around the M17 H II region. The field reveals 886sources with observed X-ray luminosities (uncorrected for absorption)between ˜29.3 ergs s-1< log LX<32.8ergs s-1, 771 of which have stellar counterparts in infraredimages. In addition to comprehensive tables of X-ray source properties,several results are presented:1. The X-ray luminosity function is calibrated to that of the OrionNebula Cluster population to infer a total population of roughly8000-10,000 stars in M17, one-third lying in the central NGC 6618cluster.2. About 40% of the ACIS sources are heavily obscured withAV>10 mag. Some are concentrated around well-studiedstar-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, andM17-North -- but most are distributed across the field. As previouslyshown, star formation appears to be widely distributed in the molecularclouds. X-ray emission is detected from 64 of the hundreds of Class Iprotostar candidates that can be identified by near- and mid-infraredcolors. These constitute the most likely protostar candidates known inM17.3. The spatial distribution of X-ray stars is complex: in addition tothe central NGC 6618 cluster and well-known embedded groups, we find anew embedded cluster (designated M17-X), a 2 pc long arc of young starsalong the southwest edge of the M17 H II region, and 0.1 pc substructurewithin various populations. These structures may indicate that thepopulations are dynamically young.4. All (14/14) of the known O stars but only about half (19/34) of theknown B0-B3 stars in the M17 field are detected. These stars exhibitthe long-reported correlation between X-ray and bolometric luminositiesof LX˜10-7Lbol. While many O andearly-B stars show the soft X-ray emission expected from microshocks intheir winds or moderately hard emission that could be caused bymagnetically channeled wind shocks, six of these stars exhibit very hardthermal plasma components (kT>4 keV) that may be due to collidingwind binaries. More than 100 candidate new OB stars are found, including28 X-ray detected intermediate- and high-mass protostar candidates withinfrared excesses.5. Only a small fraction (perhaps 10%) of X-ray selected high- andintermediate-mass stars exhibit K-band-emitting protoplanetary disks,providing further evidence that inner disks evolve very rapidly aroundmore massive stars.
| The Galactic distribution of magnetic fields in molecular clouds and HII regions
Aims.Magnetic fields exist on all scales in our Galaxy. There is acontroversy about whether the magnetic fields in molecular clouds arepreserved from the permeated magnetic fields in the interstellar medium(ISM) during cloud formation. We investigate this controversy usingavailable data in the light of the newly revealed magnetic fieldstructure of the Galactic disk obtained from pulsar rotation measures(RMs). Methods: We collected measurements of the magnetic fields inmolecular clouds, including Zeeman splitting data of OH masers in cloudsand OH or HI absorption or emission lines of clouds themselves. Results: The Zeeman data show structures in the sign distribution of theline-of-sight component of the magnetic field. Compared to thelarge-scale Galactic magnetic fields derived from pulsar RMs, the signdistribution of the Zeeman data shows similar large-scale fieldreversals. Previous such examinations were flawed in the over-simplifiedglobal model used for the large-scale magnetic fields in the Galacticdisk. Conclusions: .We conclude that the magnetic fields in the cloudsmay still "remember" the directions of magnetic fields in the GalacticISM to some extent, and could be used as complementary tracers of thelarge-scale magnetic structure. More Zeeman data of OH masers in widelydistributed clouds are required.Tables 1 and 2 are only available in electronic form athttp://www.aanda.org
| An X-Ray Census of Young Stars in the Massive Southern Star-forming Complex NGC 6357
We present the first high spatial resolution X-ray study of the massivestar-forming region NGC 6357, obtained in a 38 ks Chandra/ACISobservation. Inside the brightest constituent of this large H II regioncomplex is the massive open cluster Pismis 24. It contains two of thebrightest and bluest stars known, yet remains poorly studied; only ahandful of optically bright stellar members have been identified. Weinvestigate the cluster extent and initial mass function and detect ~800X-ray sources with a limiting sensitivity of ~1030 ergss-1 this provides the first reliable probe of the richintermediate-mass and low-mass population of this massive cluster,increasing the number of known members from optical study by a factor of~50. The high-luminosity end (logLh[2-8 keV]>=30.3 ergss-1) of the observed X-ray luminosity function in NGC 6357 isclearly consistent with a power-law relation as seen in the Orion NebulaCluster and Cepheus B, yielding the first estimate of NGC 6357's totalcluster population, a few times the known Orion population. Weinvestigate the structure of the cluster, finding small-scalesubstructures superposed on a spherical cluster with 6 pc extent, anddiscuss its relationship to the nebular morphology. The long-standingLX-10-7Lbol correlation for O stars isconfirmed. Twenty-four candidate O stars and one possible new obscuredmassive YSO or Wolf-Rayet star are presented. Many cluster members areestimated to be intermediate-mass stars from available infraredphotometry (assuming an age of ~1 Myr), but only a few exhibit K-bandexcess. We report the first detection of X-ray emission from anevaporating gaseous globule at the tip of a molecular pillar; thissource is likely a B0-B2 protostar.
| 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.
| The open cluster G353.1+0.7 in NGC6357
The HII region NGC 6357 shows a very complex structure and it harborsthe interesting open cluster Pismis 24 with several O-type stars.Another HII structure known as G353.1+0.7 is located to the eastern sideof the nebula. This structure can be described like a Strömgrensphere surrounded by a dense dusty molecular shell with finger-likegaseous features pointing to the location of the O stars whichdemonstrates that these stars are the main ionizing source in the area.In this work we report new spectral types for the brightest starsprojected in the cavity of G353.1+0.7. Among them, two are classified asearly O-type stars: N49, O4 III((f*)) and N51, O3.5 V((f)), while twoother cluster members belong to spectral types O9 V (N50) and B1 V-III(N70). We also discuss UBV photometry for stars in the area leading tothe discovery of a young open cluster. >From the spectroscopicparallax of the early-type stars and MS fitting, we derive a DM ˜11.9+/-0.25 (2.4 kpc) in agreement with the value found by Massey et al.(2001), DM ˜ 12.0 for the Pismis 24 open cluster. This indicatesthat G353.1+0.7 and Pis 24 belong to the same HII complex. The stardensity in the core of the cluster (around N49) reaches 40 stars perarcmin^{-2} at V band and an inspection of the CM-D suggest the presenceof a population of PMS stars.
| Interstellar bubbles and photodissociation regions in NGC 6357
We investigate the distribution of the ionized, atomic neutral andmolecular gas, and of the interstellar dust in the complex star formingregion NGC 6357, with the aim of studying the interplay between thestars and the surrounding interstellar matter.We based the study of the ionized gas on narrow-band optical images(Hα, [SII], [OIII]) obtained with the Curtis-Schmidt Camera atCTIO, Chile, and on radio continuum observations at 1465 MHz taken withthe VLA, USA, with a synthesized beam of 40 arcsec. The distribution ofthe neutral atomic gas is analyzed using HI 21cm line data from theSouthern Galactic Plane Survey obtained with the ATCA and Parkesradiotelescopes (synthesized beam = 2.3 arcmin). The distribution of themolecular gas was investigated using CO(1-0) data obtained with theNanten radiotelescope (angular resolution = 2.7 arcmin), in Chile. Theinterstellar dust distribution was studied using infrared data from theIRAS satellite (HIRES).The images show neutral atomic and molecular gas with velocities in therange -12 to +1 km s^{-1} associated with the outer ionized nebula. Adetailed analysis of the region reveals interstellar bubbles andphotodissociation regions created by the massive stars in Pis 24 and byundiscovered ionizing sources.
| Abundances in planetary nebulae: Mz 3
ISO spectra of the bipolar planetary nebula Mz 3 areused to determine the element abundances in the bright lobes of thenebula. The ISO spectra alone are sufficient to determine nitrogen,neon, argon, sulfur and iron abundances. These spectra are combined withspectra in the visual wavelength region (taken from the literature) toobtain an extinction corrected spectrum which is used to determine theabundance of oxygen and some other elements using a classicaldetermination. We have tried abundance determination usingphotoionization modeling using cloudy, which is essential fordetermining the helium, silicon and chlorine abundances. It was founddifficult to model the entire spectrum. New information about thecentral star could be determined. The abundances determined are found todiffer somewhat from earlier results using only visual spectra. Thereasons for this difference are discussed. An elevated helium abundanceis found, agreeing with the determination of Smith 2003. Taken togetherwith the high nitrogen abundance found, it is concluded that theexciting star of Mz 3 had a high progenitor mass.
| A Survey of N IV and O IV Features near 3400 Å in O2-O5 Spectra
We have conducted a survey of little-known N IV and O IV multiplets near3400 Å in an extensive sample of well-classified, very earlyO-type spectra. The initial motivation was to search for additionaluseful classification criteria for these types, but an unexpected resultis the high sensitivity of these features to evolutionary CNOprocessing. We have found a useful discriminant between O2 and latertypes in the relative strengths of the O IV multiplets, one of which issubject to selective emission in the hottest spectra; the overallstrengths of these lines also decrease between spectral types O4 and O5.More remarkable, however, are the variations in the N/O ratios amongboth individual stars and clusters. For instance, several O4 If+ spectrahave very large ratios, while main-sequence stars in the Carina Nebulagenerally have smaller values than others of the same spectral types inother regions. These effects correspond to different degrees of mixingof processed material as a function of evolutionary age and initialrotational velocities; the second effect provides significant furtherevidence that very massive stars mix while still on the main sequence.Thus, further analysis of these features will likely provide valuablediagnostics of important evolutionary parameters.
| 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").
| A Very Large Array Search for Water Masers in Six H II Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated withyoung low-mass protostars in six H II regions-M16, M20, NGC 2264, NGC6357, S125, and S140. The survey was conducted with the NRAO Very LargeArray from 2000 to 2002. For several of these H II regions, ours are thefirst high-resolution observations of water masers. We detected 16 watermasers: eight in M16, four in M20, three in S140, and one in NGC 2264.All but one of these were previously undetected. No maser emission wasdetected from NGC 6357 or S125. There are two principle results to ourstudy. (1) The distribution of water masers in M16 and M20 does notappear to be random but instead is concentrated in a layer of compressedgas within a few tenths of a parsec of the ionization front. (2)Significantly fewer masers are seen in the observed fields than expectedbased on other indications of ongoing star formation, indicating thatthe maser-exciting lifetime of protostars is much shorter in H IIregions than in regions of isolated star formation. Both of theseresults confirm predictions of a scenario in which star formation isfirst triggered by shocks driven in advance of ionization fronts andthen truncated ~105 yr later when the region is overrun bythe ionization front.
| 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.
| Spatially Resolved Very Large Array 74 MHz Observations Toward the Galactic Center
Not Available
| New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups onthe 2MASS J, H and Ks all-sky release Atlas in the Northernand Equatorial Milky Way (350deg < l < 360deg, 0deg < l < 230 deg). Thesearch in this zone complements that in the Southern Milky Way (Dutra etal. \cite{Dutra03}a). The method concentrates efforts on the directionsof known optical and radio nebulae. The present study provides 167 newinfrared clusters, stellar groups and candidates. Combining the twostudies for the whole Milky Way, 346 infrared clusters, stellar groupsand candidates were discovered, whereas 315 objects were previouslyknown. They constitute an important new sample for future detailedstudies.
| A New Spectral Classification System for the Earliest O Stars: Definition of Type O2
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.
| New OH Zeeman Measurements of Magnetic Field Strengths in Molecular Clouds
We present the results of a new survey of 23 molecular clouds for theZeeman effect in OH undertaken with the Australia Telescope NationalFacility Parkes 64 m radio telescope and the National Radio AstronomyObservatory Green Bank 43 m radio telescope. The Zeeman effect wasclearly detected in the cloud associated with the H II region RCW 38,with a field strength of 38+/-3 μG, and possibly detected in a cloudassociated with the H II region RCW 57, with a field strength of-203+/-24 μG. The remaining 21 measurements give formal upper limitsto the magnetic field strength, with typical 1 σ sensitivitiesless than 20 μG. For 22 of the molecular clouds we are also able todetermine the column density of the gas in which we have made asensitive search for the Zeeman effect. We combine these results withprevious Zeeman studies of 29 molecular clouds, most of which werecompiled by Crutcher, for a comparison of theoretical models with thedata. This comparison implies that if the clouds can be modeled asinitially spherical with uniform magnetic fields and densities thatevolve to their final equilibrium state assuming flux freezing, then thetypical cloud is magnetically supercritical, as was found by Crutcher.If the clouds can be modeled as highly flattened sheets threaded byuniform perpendicular fields, then the typical cloud is approximatelymagnetically critical, in agreement with Shu et al., but only if thetrue values of the field for the nondetections are close to the 3σ upper limits. If instead these values are significantly lower(for example, similar to the 1 σ limits), then the typical cloudis generally magnetically supercritical. When all observations of theZeeman effect are considered, the single-dish detection rate of the OHZeeman effect is relatively low. This result may be due to low meanfield strengths, but a more realistic explanation may be significantfield structure within the beam. As an example, for clouds associatedwith H II regions, the molecular gas and magnetic field may be swept upinto a thin shell, which results in a nonuniform field geometry andmeasurements of the beam-averaged field strength, which aresignificantly lower than the true values. This effect makes it moredifficult to distinguish magnetically subcritical and supercriticalclouds.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations
In a previous paper on the Magellanic Clouds, we demonstrated thatcoeval clusters provide a powerful tool for probing the progenitormasses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs).Here we extend this work to the higher metallicity regions of the MilkyWay, studying 12 Galactic clusters. We present new spectral types forthe unevolved stars and use these, plus data from the literature, toconstruct H-R diagrams. We find that all but two of the clusters arehighly coeval, with the highest mass stars having formed over a periodof less than 1 Myr. The turnoff masses show that at Milky Waymetallicities some W-R stars (of early WN type) come from stars withmasses as low as 20-25 Msolar. Other early-type WN starsappears to have evolved from high masses, suggesting that a large rangeof masses evolve through an early WN stage. On the other hand, WN7 starsare found only in clusters with very high turnoff masses, over 120Msolar. Similarly, the LBVs are only found in clusters withthe highest turnoff masses, as we found in the Magellanic Clouds,providing very strong evidence that LBVs are a normal stage in theevolution of the most massive stars. Although clusters containing WN7stars and LBVs can be as young as 1 Myr, we argue that these objects areevolved, and that the young age simply reflects the very high massesthat characterize the progenitors of such stars. In particular, we showthat the LBV η Car appears to be coeval with the rest of theTrumpler 14/16 complex. Although the WC stars in the Magellanic Cloudswere found in clusters with turnoff masses as low as 45Msolar, the three Galactic WC stars in our sample are allfound in clusters with high turnoff masses (>70 Msolar)whether this difference is significant or due to small number statisticsremains to be seen. The bolometric corrections of Galactic W-R stars arehard to establish using the cluster turnoff method but are consistentwith the ``standard model'' of Hillier.
| Integrated photometry of galactic H Ii regions
Integrated photoelectric measurements of the equivalent width W_Hβ, the [O Iii]/Hβ ratio and the Hβ emission line flux arepresented for 31 southern hemisphere galactic H Ii regions. The Lymancontinuum photon fluxes are obtained for some of these objects. Theintegrated [O Iii]/Hβ ratios have not shown any statisticallysignificant deviation from non-integrated measurements found in theliterature.
| Stellar Contents of the Galactic Giant H { II} Region NGC 3603
We present CCD UBVRI and Hα photometry for the starburst clusterassociated with the galactic giant H II region NGC 3603. The meanreddening of the cluster members comes out to be < E(B-V)>=1.48+/- 0.21 (s.d.). The surface distribution of the reddening shows aradial variation with a minimum near to the cluster center and two localmaxima at radii ~ 30'' and ~ 70''. Our photometricdata indicate an anomalous reddening law for the dust inside the clusterregion, and the ratio of the total to the selective extinction, R, isfound to be ~ 4.3. A comparison of the observational and intrinsiccolour-magnitude digrams (CMDs) yields a mean distance modulus of (m -M)_0 = 14.0, which corresponds to a distance of 6.3 +/- 0.6 kpc. Thisdistance is smaller than those obtained in earlier works (e.g., 7.2 kpc,Melnick et al. 1989, AAA 49.132.013) but is in good agreement with thekinematic distance (6.1 +/- 0.6 kpc) recently reported by De Pree et al.(1999, AAA 71.132.041). The cluster shows a large age spread, containingstars as old as up to several Myr. However, the mean age of the clusteris found to be <= 1 Myr. We estimate a minimum star formationefficiency of ~ 25 % and a gas removal time of tau ~ 4.6 Myr.
| 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.
| NH_3(1,1) survey toward southern hemisphere HII regions. I
The (J,K) = (1,1) metastable ammonia line was searched for in thedirection of more than one hundred southern HII regions located between270 degr and 15degr in galactic longitude, resulting in new detectionstoward 21 of them. Ammonia detection rate was about 30 % towards alldirections except in the longitude interval 270degr -290degr where noammonia lines were found in the 15 selected HII regions. A highdetection rate (>= 0.6) was observed toward positions where IRASpoint sources with color indices of ultra-compact HII regions were inthe beam of the telescope. An anticorrelation between the number ofdetected NH_3 sources and the distance to the Galactic Center was found,compatible with the observed gradient in N abundance. From the value ofthis gradient and the detection rate of ammonia lines toward HIIregions, it is inferred that the number of ammonia sources must beproportional to the inverse of the column density of the molecularcloud.
| A complex bipolar outflow in the Wolf-Rayet BCDG He 2-10
We present results from multi-wavelength observations of the Wolf-Rayet(WR) blue compact dwarf galaxy He 2-10. These observations includeHα and optical red continuum imaging, high resolution Hαspectroscopy, high resolution radio-continuum mapping at 4.80 and 8.64GHz and ROSAT X-ray observations. The deep Hα images reveal thatthe galaxy is surrounded by a complex kiloparsec-scale bipolarsuperbubble centered on the most intense star forming knot.High-resolution spectroscopy of this feature indicates that it has acomplex structure of possibly different kinematic components expandingat velocities from <=50 km s(-1) up to 250 km s(-1) . These kinds ofoutflows are likely to be produced by the mechanical action of stellarwinds and supernova (SN) explosions in the intense starbursts that thegalaxy hosts. This scenario is consistent with the finding of a steepoverall radio spectral index (alpha =-0.59) that confirms the presenceof a large number of supernova remnants ( ~ 3750) in the galaxy. Thestudy of radio spectral index throughout the galaxy shows that theeastern parts have significantly steeper spectra. These radio regionshave no counterparts in our Hα or X-ray images. The X-ray emissionextends to a similar scale as the Hα emission. The extensions ofthe X-ray distribution towards northeast and southwest appears to bewell spatially correlated with the position of the bipolar lobesindicating the hot gas is confined inside the bubbles. The comparisonbetween the energetics of the outflows and the expected mechanicalenergy released by the supernovae and stellar winds indicates that thelarge-scale expanding structures can be powered by the massive starpopulation of the galaxy and, therefore, can be considered a galacticwind. Estimates of the escape velocity of the galaxy indicate that partof the large-scale outflows can escape the weak potential well of thegalaxy, reinforcing the possibility of a blowout in He 2-10.
| Magnetic Fields in Molecular Clouds: Observations Confront Theory
This paper presents a summary of all 27 available sensitive Zeemanmeasurements of magnetic field strengths in molecular clouds togetherwith other relevant physical parameters. From these data inputparameters to magnetic star formation theory are calculated, andpredictions of theory are compared with observations. Results for thiscloud sample are the following: (1) Internal motions are supersonic butapproximately equal to the Alfvén speed, which suggests thatsupersonic motions are likely MHD waves. (2) The ratio of thermal tomagnetic pressures beta_p~0.04, implying that magnetic fields areimportant in the physics of molecular clouds. (3) The mass-to-magneticflux ratio is about twice critical, which suggests but does not requirethat static magnetic fields alone are insufficient to support cloudsagainst gravity. (4) Kinetic and magnetic energies are approximatelyequal, which suggests that static magnetic fields and MHD waves areroughly equally important in cloud energetics. (5) Magnetic fieldstrengths scale with gas densities as |B|~rho^kappa with kappa~0.47 thisagrees with the prediction of ambipolar diffusion driven star formation,but this scaling may also be predicted simply by Alfvénicmotions. The measurements of magnetic field strengths in molecularclouds make it clear that magnetic fields are a crucial component of thephysics governing cloud evolution and star formation.
| Clouds of turbulent interstellar plasma in the spiral arms of the galaxy
It is shown that the interstellar turbulent plasma in the arms of theGalaxy has two components: component BII, which is extremely nonuniformand associated with HII regions and supernova remnants, and componentBI, which is more homogeneous and corresponds to the numerousStrömgren zones formed by 07-B0 stars. The following parameters areobtained for component BI: cloud size <= 50 kpc, mean electrondensity in the clouds N_e = 0.1-1 cm^-3, characteristic thickness of thecloud layer 300-500 pc, volume density of the cloud distribution ~= 1 x10^-6 pc^-3, scale height of the cloud distribution above the Galacticplane ~= 80-250 pc. For N_e < N_e,_rit ~= 0.7 cm^-3, the turbulenceis weak; the level of turbulence grows with N_e.
| Far-Infrared [C ii] Line Survey Observations of the Galactic Plane
We present results of our survey observations of the [C II] 158 mu mline emission from the Galactic plane using the Balloon-borne InfraredCarbon Explorer (BICE). Our survey covers a wide area (350 deg <~ l<~ 25 deg, |b| <~ 3 deg) with a spatial resolution of 15'. Weemployed a new observing method called "fast spectral scanning" to makelarge-scale observations efficiently. Strong [C II] line emission wasdetected from almost all areas we observed. In the general Galacticplane, the spatial distribution of the [C II] line emission correlatesvery well with that of far-infrared continuum emission, but diffusecomponents are more prominent in the [C II] line emission; the I[CII]/IFIR ratio is ~0.6% for diffuse components but is ~0.2% for compactsources such as active star-forming regions. In the Galactic centerregion, on the other hand, the distribution of the [C II] line emissionis quite different from that of the far-infrared continuum emission, andthe I[C II]/IFIR ratio is systematically lower there. The FWHM velocityresolution of our instrument is 175 km s-1, but we determined thecentral velocity of the line at each observed point very precisely withstatistical errors as small as +/-6 km s-1. The longitudinaldistribution of the central velocity clearly shows the differentialrotation pattern of the Galactic disk and also violent velocity fieldsaround the Galactic center.
| Molecular cloud/HII region interfaces in the star forming region NGC 6357.
We have performed ^12^CO(1-0), ^12^CO(2-1), ^13^CO(1-0), ^13^CO(2-1),C^18^O(1-0), C^18^O(2-1), HCO^+^(1-0) and H^13^CO^+^(1-0) observationstowards two selected areas in the star forming complex NGC 6357 withangular resolutions from 21" to 55". In particular, we have mapped themolecular gas around the two HII regions G353.1+0.6 and G353.2+0.9 inthe ^12^CO(1-0) and ^13^CO(1-0) transitions with a resolution of ~43".This improves on the coarser ^12^CO(1-0) observations previously carriedout by others. We have also studied the physical properties of gas alongstrips through the molecular cloud/HII region interfaces. ForG353.1+0.6, the observations confirm that an ionization front is erodinga warm, dense molecular cloud located to the north of the opticalnebula. The molecular gas appears fragmented on a scale size >0.5pcand not all the components are related to the HII region. There isevidence of a density increase near the front and indications oftemperature gradients toward the exciting stars. This is furthersuggested by the presence of ^12^CO(1-0) self-absorption produced by acooler external layer. The isotopic ratio X(^13^CO)/X(C^18^O) decreasesfrom the inner part of the clouds towards the front, contrary to what istheoretically predicted and observed in many other regions. This may bedue to excitation or optical depths effects. An evolutionary scenario isproposed where the exciting stars form at the edge of a molecular cloud.The morphology of G353.2+0.9 is rather different from what previouslybelieved. Only a weak "bar" of molecular material was found to the southof the sharp ionization front observed optically and in theradio-continuum, and most of the molecular emission arises from regionsbehind or to the north of the HII region. This indicates that we areviewing a late stage "blister" configuration face-on. This region isfragmented on a scale size >0.5pc, and a warm, dense and compactmolecular fragment coincides with the elephant trunk visible in Hαimages. Other clouds with high (~40K) ^12^CO(1-0) brightnesstemperatures surround the nebula to the north. Around their peaks, the^12^CO(2-1)/^12^CO(1-0) main beam temperatures ratio is significantly<1, contrary to what is observed in G353.1+0.6. In both G353.1+0.6and G353.2+0.9 a large range of radial velocities is observed close tothe ionization fronts, and the molecular structures interacting with theionized gas have virial masses greater than masses calculated under theassumption of LTE. These observations have allowed us to betterunderstand the morphology of the two regions and to sketch the physicalproperties of molecular clouds exposed to UV radiation. Dynamicalinteractions between ionized and molecular gas are used in order toestimate the age (<~ a few 10^5^yrs) of both nebulae. Also theeffects of UV radiation in determining the morphology of molecular gasare considered. The heating sources of the molecular clouds are theearly type stars of the HII regions, at the edges of the clouds. The^12^CO(1-0) opacity, τ, seems to affect ^12^CO(1-0) main beamtemperatures and an empirical relation between τ and the visualextinction A_V_ has been determined.
| Possible sites of nuclear line emission from massive OB associations.
Intense γ-ray line emission at 4.44 and 6.13MeV from the nucleardeexcitation of ^12^C^*4.44^ and ^16^O^*6.13^ has recently beendiscovered (Bloemen et al. 1994A&A...281L...5B) by the COMPTELexperiment from the Orion complex, a nearby (~450pc) molecular cloudwith a resident association of massive OB stars. This emission isapparently produced by low energy (<30MeV/nucleon) cosmic rays, whichare accelerated by shocks and undergo nuclear interactions with thedense cloud gas. To test this hypothesis, we have identified severalsimilar candidate OB associations from which we would also expectcomparable fluxes in these two nuclear γ-ray lines.
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