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An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Observations of 14 young open star clusters with the HEGRA system of Cherenkov telescopes
Context: .A sample of 14 young open star clusters has been observed inthe TeV energy regime with the stereoscopic system of the HEGRA (HighEnergy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002,resulting in more than 300 h of observation time. Aims: .Youngopen star clusters may contribute to the acceleration of cosmic rays.The detection of γ-rays (from decaying π^0s produced inhadronic interactions) from these objects could be evidence for such acontribution. The results of our observations are compared to availableγ-ray data and to a simple hadronic model in the framework ofshock front acceleration of cosmic rays in the stellar winds of thecluster members to test the potential of the presently available data onyoung open star clusters to constrain this type of model. Methods:.The stereoscopic system of HEGRA Cherenkov telescopes makes use of theatmospheric imaging technique. Air showers initiated by primaryGamma-Rays are recorded as elliptical images in the telescope cameras.The images from the different telescopes are then superimposed toreconstruct the parameters of the primary particle. This technique(stereoscopy) was pioneered by the HEGRA experiment. Results: .Nosignificant excess has been found in the analysed data set of young openstar clusters. The derived upper limit on the TeV gamma-ray flux fromBerkeley 87 and the available EGRET data from the same direction do notallow us to fully constrain the simple hadronic model used here. Thecomparison of the upper limits derived for all 14 objects with the fluxdetected from TeV J2032+4130 (under the assumption of an association ofthe TeV-signal with the compact stellar association Cyg OB2) suggeststhat γ-ray emission from young open star clusters as an objectclass cannot be ruled out.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| Revisiting the population of Galactic open clusters
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
| Kinematics of the Gould belt based on open clusters.
Not Available
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster
We report an optical photometric survey of 0.89 deg2 of theOrion OB1b association centered on σ Ori. This region includesmost of the σ Ori cluster, the highest-density region within OrionOB1b. We have developed a statistical procedure to identify the young,low-mass, pre-main-sequence population of the association. We estimatethat the cluster has ~160 members in the mass range (0.2<=M<=1.0Msolar). The cluster has a radius of ~3-5 pc and an estimatedage of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster is225+/-30 Msolar. This mass is similar to the estimated massof the ~5×105 yr old cluster NGC 2024. NGC 2024 andσ Ori appear to be a well-matched pair of clusters, except for the~2 Myr difference in their ages.
| Wide-Area Mapping of 155 Micron Continuum Emission from the Orion Molecular Cloud Complex
We present the results of a wide-area mapping of the far-infraredcontinuum emission toward the Orion complex by using a Japaneseballoon-borne telescope. The 155-μm continuum emission was detectedover a region of 1.5 deg2 around the KL nebula with 3′resolution similar to that of the IRAS 100-μm map. Assuming asingle-temperature model of the thermal equilibrium dust, maps of thetemperature and the optical thickness were derived from the 155-μmintensity and the IRAS 100-μm intensity. The derived dust temperatureis 5-15K lower and the derived dust optical depth is 5-300 times largerthan those derived from the IRAS 60 and 100-μm intensities due to thesignificant contribution of the statistically heated very small grainsto the IRAS 60-μm intensity. The optical-thickness distribution showsa filamentary dust ridge that has a 1°.5 extent in the north-southdirection and well resembles the Integral-Shaped Filament (ISF)molecular gas distribution. The gas-to-dust ratio derived from the COmolecular gas distribution along the ISF is in the range 30-200, whichmay be interpreted as being an effect of CO depletion due to thephotodissociation and/or the freezing on dust grains.
| Studies of Clump Structure of Photodissociation Regions at Millimeter and Sub-millimeter Wavelengths
To interpret the millimeter and sub-millimeter line emissions of atomicand molecular species from galactic and extragalacticphotodissociationregions, warm gas components and molecular clouds,generally,escape probability formalism of Tielens & Hollenbach(herein referred as TH) are employed which is based on the assumption ofplane parallel geometry of infinite slab allowing photons to escape onlyfrom the front.Contrary to the assumption observationally it is foundthat these lines are optically thin except OI.63m/ and low rotationaltransitions of CO and some other molecules. This observational evidenceled us to assume that emitting regions are finite parallel plane slab inwhich photons are allowed to escape from both the surfaces (back andfront). Therefore, in the present study escape of radiations from bothsides of the homogeneous and also clumpy PDR/molecular clouds are takeninto consideration for calculating the line intensities at millimeterand sub-millimeter wavelengths (hereinafter referred as QA). Results arecompared with that of the TH model.It is found that thermal and chemicalstructures of the regions are almost similar in both the formalisms. Butline intensities are modified by differing factors. Particularly atlowdensity and lowkinetic temperature and also for optically thin linesline intensities calculated from TH and QA model differ substantially.But at density higher than the critical density and also for opticallythick lines TH and QA models converge to almost same values.An attempthas been made to study the physical conditions of the M17 regionemploying the present formalism.1?
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| A multiwavelength study of the S 106 region. II. Characteristics of the photon dominated region
The O star S 106 IR powers a bright, spatially extended 10'x 3' (1.75x0.5 pc at a distance of 600 pc) photon dominated region (PDR) traced byour observations of FIR fine structure lines and submm moleculartransitions. The [C II] 158 mu m, [C I] 609 and 370 mu m, CO 7->6,and CO 4->3 measurements probe the large scale (1.2 pc) PDR emission,whereas [O I] 63 mu m, CN N=3->2, and CS J=7->6 observations arefocused on the immediate ( ~ 1' (0.2 pc)) environment of S 106 IR. A hot(T>200 K) and dense (n>3 *E5 cm-3) gascomponent (emission peaks of [C I] 158 mu m, CO 7->6, and CO 4->3)is found at S 106 IR. Cooler gas associated with the bulk emission ofthe molecular cloud is characterized by two emission peaks (one close(20'' east) to S 106 IR and one 120'' to the west) seen in the [C I] andlow-J (Jup<4) CO emission lines. In the immediateenvironment of the star, the molecular and [C I] lines showhigh-velocity emission due to the interaction of the cloud with thestellar wind of S 106 IR. The intensities of the FIR lines measuredwith the KAO are compared to those observed with the ISO LWS towards twopositions, S 106 IR and 120'' west. We discuss intensities and lineratios of the observed species along a cut through the molecular cloud/HII region interface centered on S 106 IR. The excitation conditions(Tex, opacities, column densities) are derived from an LTEanalysis. We find that the temperature at the position of S 106 IRobtained from the [C I] excitation is high (>500 K), resulting insubstantial population of the energetically higher3P2 state; the analysis of the mid- and high-J COexcitation confirms the higher temperature at S 106 IR. At thisposition, the [O I] 63 mu m line is the most important cooling line,followed by other atomic FIR lines ([O III] 52 mu m, [C II] 158 mu m)and high-J CO lines, which are more efficient coolants compared to [C I]2->1 and 1->0. We compare the observed line ratios toplane-parallel PDR model predictions and obtain consistent results forUV fluxes spanning a range from 102 to 103.5G0 and densities around 105 cm-3 onlyat positions away from S 106 IR. Towards S 106 IR, we estimate a densityof at least 3*E5 at temperatures between 200 and 500 K fromnon-LTE modelling of the CO 16->15/14->13 ratio and the CO 7->6intensity. Our new observations support the picture drawn in the firstpart of this serie of papers that high-density (n>105cm-3) clumps with a hot PDR surface are embedded in low- tomedium density gas (nle10 4 cm-3).Appendix A is only available in electronic form athttp://www.edpsciences.org
| Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae
This paper presents Kuiper Airborne Observatory observations of thephotodissociation regions (PDRs) in nine reflection nebulae. Theseobservations include the far-infrared atomic fine-structure lines of [OI] 63 and 145 μm, [C II] 158 μm, and [Si II] 35 μm and theadjacent far-infrared continuum to these lines. Our analysis of thesefar-infrared observations provides estimates of the physical conditionsin each reflection nebula. In our sample of reflection nebulae, thestellar effective temperatures are 10,000-30,000 K, the gas densitiesare 4×102-2×104 cm-3, thegas temperatures are 200-690 K, and the incident far-ultravioletintensities are 300-8100 times the ambient interstellar radiation fieldstrength (1.2×10-4 ergs cm-2 s-1sr-1). Our observations are compared with current theory forlow-excitation PDRs. The [C II] 158 μm to [O I] 63 μm line ratiodecreases with increasing incident far-ultraviolet intensity. This trendis due in part to a positive correlation of gas density with incidentfar-ultraviolet intensity. We show that this correlation arises from abalance of pressure between the H II region and the surrounding PDR. The[O I] 145 to 63 μm line ratio is higher (greater than 0.1) thanpredicted and is insensitive to variations in incident far-ultravioletintensity and gas density. The stellar temperature has little effect onthe heating efficiency that primarily had the value3×10-3, within a factor of 2. This result agrees with amodel that modifies the photoelectric heating theory to account forcolor temperature effects and predicts that the heating efficiencieswould vary by less than a factor of 3 with the color temperature of theilluminating field. In addition to the single-pointing observations, an[O I] 63 μm scan was done across the molecular ridge of one of oursample reflection nebulae, NGC 1977. The result appears to supportprevious suggestions that the ionization front of this well-studied PDRis not purely edge-on.
| Radio Continuum Structure of the Orion Nebula
We have imaged the large-scale radio continuum structure in the Orionregion with the Very Large Array at 330 MHz. Arcminute-resolutionmorphology of the extended emission in the H II regions M42 and M43 (NGC1976 and NGC 1982) and in the NGC 1973-75-77 nebulosity to the north arepresented. A low surface brightness thermal radio halo is detected inthe H II region M42: comparison with an optical photograph indicatesthat the radio emission distinguishes optical emission structures fromreflection nebulosities. In NGC 1977 we have discovered a compact,steep-spectrum radio source coincident with a bright optical rim.
| A Multiwavelength Study of Outflows in OMC-2/3
We present new v=1-0 S(1) H2, 12CO J=2-->1, and12CO J=3-->2 observations of the star-forming clouds OMC-2and OMC-3, one of the densest known groupings of outflows from low-massyoung stellar objects (YSOs) in the sky. High-velocity 12COJ=2-->1 gas in this region suggests that previously discoveredH2 flows are driving and entraining molecular outflows.However, the large number of sources and flows within the narrowmolecular filament means it is difficult to make a firm association ofmolecular outflow gas with H2 flows, except for in the caseof the bipolar east-west H flow. A number of Herbig-Haro (HH) objects,including ones far to the west and east of the main ridge, areidentified with H2 knots. High-resolution spectroscopy in thev=1-0 S(1) line of 10 H2 knots shows line profiles consistentwith dual forward and reverse shocks. C-shock modeling suggest thatasymmetries seen in suspected bow shocks could be evidence of varyingmagnetic field orientations throughout the cloud. One of the bow shocksin the H flow, YBD-5, can be successfully modeled by a 100 kms-1 C-shock propagating into a magnetized, 106cm-3 medium, although the observations and limitations withinthe computer code itself do not entirely rule out J-shocks. Mass spectraof the H flow are broken power laws, which might be evidence for a jetthat has two entrainment mechanisms for accelerating ambient moleculargas into the outflow. The H2 luminosity in this flow is manytimes smaller than the CO mechanical luminosity, but this fact cannotrule out the possibility that a narrow highly collimated jet drives themolecular outflow, owing to uncertainties in extinction, outflow dynamictimes, cooling contributions from other lines, and the wind model used.Outflows from OMC-2/3 are likely to contribute to the turbulent pumpingof gas within the molecular ridge north of the Orion Nebula.High-velocity gas clumps north of the sources investigated here mayrepresent evidence of additional undiscovered outflows from young stars.
| Distribution of Cold Dust in Orion A and B
Large-scale far-infrared (FIR) observations of the Orion complex at 205and 138 μm are presented with the aim of studying the distribution ofcold (<25 K) dust. The maps in these FIR bands extend over ~3600arcmin2 and cover regions around OMC-1, 2, and 3 in Orion Aand NGC 2023 and NGC 2024 in Orion B. Some limited regions have alsobeen mapped at 57 μm. A total of 15 sources in Orion A and 14 inOrion B (south) have been identified from our FIR maps. Dust temperaturedistribution in both Orion A and Orion B (south) have been determinedreliably using the maps at 205 and 138 μm obtained from simultaneousobservations using almost identical beams (1.6′ diameter). Thesetemperatures have been used to generate the map of τ150,the optical depth at 150 μm, for the Orion B region. The coldestsource detected is in OMC-3 and has temperature ~15 K. The diffuse FIRemission in the different subregions is found to vary between 25% and50% of the total FIR emission from that subregion.
| 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
| Far-Infrared Study of IRAS 00494+5617 and IRAS 05327-0457
High angular resolution far-infrared observations at 143 and 185 μm,using the Tata Institute of Fundamental Research 1 m balloon-bornetelescope, are presented for two Galactic star-forming complexesassociated with IRAS 00494+5617 and IRAS 05327-0457. The latter map alsoreveals the cold dust in OMC-3. Both these regions are illuminated atthe edges by high-mass stars with substantial UV flux. The HIRES(High-Resolution processing using Maximum Correlation Method) processedIRAS maps at 12, 25, 60, and 100 μm have also been presented forcomparison. The present study is aimed at quantifying the role of thenearby stars vis-à-vis embedded young stellar objects in theoverall heating of these sources. Based on the FIR observations at 143and 185 μm carried out simultaneously with almost identical angularresolution, reliable dust temperature and optical depth maps have beengenerated for the brighter regions of these sources. Radiative transfermodeling in spherical geometry has been carried out to extract physicalparameters of these sources by considering the observationalconstraints, such as spectral energy distribution, angular size atdifferent wavelengths, dust temperature distribution, etc. It isconcluded that for both IRAS 00494+5617 and IRAS 05327-0457, theembedded energy sources play the major role in heating them with finitecontribution from the nearby stars. The best-fit model for IRAS00494+5617 is consistent with a simple two-phase clump-interclumppicture with ~5% volume filling factor (of clumps) and a densitycontrast of ~80.
| Mid-and far-infrared studies of galactic compact H II regions
Several Galactic H II region complexes have been observed in the farinfrared (FIR) wavebands using the TIFR 1-m balloon borne telescope, andin the mid infrared (MIR) using the Infrared Space Observatory (ISO).Many of these FIR maps show interesting structural details. The MIRobservations have clearly detected PAH features in two compact H IIregions and one molecular clump. A self consistent radiative transferscheme (1-D) has been developed which has been successful in extractingimportant details like geometric sizes, radial density distribution,dust composition, etc, from these and other similar measurements. A newscheme (2-D) has been developed in cylindrical geometry to model IRAS19181 + 1349, which has been resolved into two sources. The modelparameters in this scheme have been constrained by the observed spectralenergy distribution (SED) and radial profiles at MIR & FIRwavebands. Non-equilibrium processes have also been incorporated inspherical geometry, to explain the PAH emission features. This schemehas successfully explained high resolution spectra (ISO-SWS) of severalGalactic compact H II regions.
| Magnetic Fields in Star-forming Molecular Clouds. I. The First Polarimetry of OMC-3 in Orion A
The first polarimetric images of the OMC-3 region of the Orion Afilamentary molecular cloud are presented. Using the new imagingpolarimeter on the Submillimeter Common User Bolometric Array at theJames Clerk Maxwell Telescope, we have detected polarized thermalemission at 850 μm from dust along a 6' length of the dense filament.The polarization pattern is highly ordered and is aligned with the longaxis of the filament throughout most of the region, diverging only nearthe southern boundary by 30 deg-50deg. If the polarizationarises from thermal emission of dust grains aligned via eitherparamagnetic inclusions or radiative torques, this configurationindicates a plane-of-sky magnetic field which is normal to the filamentalong most of its length. The mean percentage polarization is 4.2% witha 1 σ dispersion of 1%. This region is part of the integral-shapedfilament, and active star formation is ongoing along its length, withonly two of nine dust condensations in our field lacking evidence ofoutflow activity. The outflow directions do not appear to beconsistently correlated with the direction of the plane-of-sky field orthe filament structure itself. Depolarization toward the filamentcenter, previously detected in many other star-forming cores andprotostars, is also evident in our data.
| The Rotation Period Distribution of Pre-Main-Sequence Stars in and around the Orion Nebula
We report rotation periods for 254 stars in an area 40'x80' centered onthe Orion Nebula. We show that these stars are likely members of theyoung (~10^6 yr) Orion OBIc/d association. The rotation perioddistribution we determine, which is sensitive to periods 0.1
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Photodissociation regions in the interstellar medium of galaxies
The interstellar medium of galaxies is the reservoir out of which starsare born and into which stars inject newly created elements as they age.The physical properties of the interstellar medium are governed in partby the radiation emitted by these stars. Far-ultraviolet (6eV
| 350 Micron Continuum Imaging of the Orion A Molecular Cloud with the Submillimeter High Angular Resolution Camera
We have used the Submillimeter High Angular Resolution Camera (SHARC)bolometer camera at the Caltech Submillimeter Observatory to map thedistribution of the broadband 350 μm continuum emission toward theOrion A molecular cloud. A comparison of the 350 μm flux densities inOMC-1 with previous 1100 μm measurements indicates a strong spatialvariation of the grain emissivity exponent, beta. The lowest value ofthe exponent (beta ~= 1.75) is found toward the Orion Barphoton-dominated region (PDR), while the highest value (beta ~= 2.5) isfound toward the Orion Ridge north of IRc2. This variation is consistentwith the destruction of grain mantles by the UV photons from theTrapezium cluster. The observed spatial variation of beta in OMC-1suggests that the long-wavelength grain emissivity may also varysignificantly in GMC cores on small linear scales (<~0.5 pc),affecting H_2 column density and mass estimates. The 350 μm continuumemission in the Orion Bar region correlates well with the CO (6-5) peakbrightness temperature and is shifted by ~10" from the molecularcomponent traced by the ^13CO (6-5) emission. This indicates that the350 μm dust emission in this region originates predominantly in theouter high-temperature PDR layers. Several filamentary structurespreviously detected in molecular tracers are also seen in our map atflux levels comparable to those seen in the Bar. Over 30 compact dustsources are detected in the OMC-2 and OMC-3 clouds, including a dozensources not previously known. The average 350/1300 μm flux ratiobased on our data and previous observations of this region (63 +/- 19)indicates low dust temperatures (17 +/- 4 K, assuming beta = 2) for mostof the sources. The brightest 350 μm source in OMC-3 has a low350/1300 μm ratio (~23), indicating a very low dust temperature (~10K), or a significant opacity at 350 μm (~2.5). This source appears tobe a deeply embedded and cold young protostar. A comparison of the massestimate for the OMC-2/3 filament based on the 350 μm continuumemission with previous C^18O mass estimates indicates a relatively highgrain emissivity, Q(350) = 4 x 10^-4, in this region.
| A Search for Star Clusters from the HIPPARCOS Data
We present results of a search for nearby star clusters and associationsusing Hipparcos Catalogue data, restricting the sample to stars withparallaxes above 2 mas (d <~ 500 pc). Two new OB associations havebeen identified in the Carina-Vela and Cepheus-Cygnus-Lyra-Vulpecularegions. A very probable new open cluster has been discovered in Carina.The cluster, a Car, named after its brightest member, is young (60 Myr)and nearby (d = 132 pc). However, only seven bona fide members can bedrawn from the Hipparcos data. We report a detection of nine opencluster candidates in the distance range of 150 to 400 pc, and sixpossible associations almost all located within the Gould belt, althoughslightly older than the known nearby associations. In all cases, wepresent Yale theoretical isochrone fits to the color-magnitude diagrams,which indicate a moderate spread of ages between 60 to 200 Myr.Evidently, these young open cluster and association candidates arerelated to the overall distribution of young OB and A-type stars in thesolar neighborhood.
| The Rosette molecular complex. II. [CII] 158 MU M observations
We present an extended map in the [CII] 158 mu m fine structure line ofthe Rosette Molecular Cloud Complex, obtained withthe Far Infrared Fabry-Perot Imaging Spectrometer FIFI onboard the NASAKuiper Airborne Observatory. Very weak but significant C(+) emission wasfound deep in the cloud at a distance of around 25 pc from the centralOB cluster. The observed intensity distribution is consistent with thepicture of a clumpy molecular cloud, exposed to an UV flux of around 200G_deg from the central OB cluster NGC 2244 at thecloud edge. The UV radiation creates Photon Dominated Regions (PDRs) onthe clump surfaces throughout the cloud which give rise to the observedweak [CII] 158 mu m line emission (peak intensity 5x10(-4) erg s(-1)sr(-1) cm(-2) ). In contrast to more luminous PDR regions, the emergentC(+) intensity scales logarithmically with the incident UV field but isin agreement with PDR models for a weak UV illumination. The density ofthe [CII] emitting gas is around 10(4) cm(-3) . The rather high value of1% for the [CII] to FIR flux ratio in Rosette indicates a large grainphotoelectrical heating efficiency which might be due to the lowincident UV field. With an average density of 300 cm(-3) , obtained fromCO observations, we derive a high (30-300) clump to interclump densitycontrast, where the UV radiation can penetrate deep into the cloud andinduces emission from many PDRs along the line of sight.
| From Head to Sword: The Clustering Properties of Stars in Orion
We investigate the structure in the spatial distributions of opticallyselected samples of young stars in the Head (lambda Orionis) and in theSword (Orion A) regions of the constellation of Orion with the aid ofstellar surface density maps and the two-point angular correlationfunction. The distributions of young stars in both regions are found tobe nonrandom and highly clustered. Stellar surface density maps revealthree distinct clusters in the lambda Ori region. The two-pointcorrelation function displays significant features at angular scalesthat correspond to the radii and separations of the three clustersidentified in the surface density maps. Most young stars in the lambdaOri region (~80%) are presently found within these three clusters,consistent with the idea that the majority of young stars in this regionwere formed in dense protostellar clusters that have significantlyexpanded since their formation. Over a scale of ~0.05d-0.5d thecorrelation function is well described by a single power law thatincreases smoothly with decreasing angular scale. This suggests that,within the clusters, the stars either are themselves hierarchicallyclustered or have a volume density distribution that falls steeply withradius. The relative lack of Hα emission-line stars in the onecluster in this region that contains OB stars suggests a timescale foremission-line activity of less than 4 Myr around late-type stars in thecluster and may indicate that the lifetimes of protoplanetary disksaround young stellar objects are reduced in clusters containing O stars.The spatial distribution of young stars in the Orion A region isconsiderably more complex. The angular correlation function of the OBstars (which are mostly foreground to the Orion A molecular cloud) isvery similar to that of the Hα stars (which are located mostlywithin the molecular cloud) and significantly different from that of theyoung stars in the lambda Ori region. This suggests that, althoughspatially separated, both populations in the Orion A region may haveoriginated from a similar fragmentation process. Stellar surface densitymaps and modeling of the angular correlation function suggest thatsomewhat less than half of the OB and Hα stars in the Orion Acloud are presently within well-defined stellar clusters. Although allthe OB stars could have originated in rich clusters, a significantfraction of the Hα stars appear to have formed outside suchclusters in a more spatially dispersed manner. The close similarity ofthe angular correlation functions of the OB and Hα stars towardthe molecular cloud, in conjunction with the earlier indications of arelatively high star formation rate and high gas pressure in this cloud,is consistent with the idea that older, foreground OB stars triggeredthe current episode of star formation in the Orion A cloud. One of theOB clusters (Upper Sword) that is foreground to the cloud does notappear to be associated with any of the clusterings of emission-linestars, again suggesting a timescale (<4 Myr) for emission-lineactivity and disk lifetimes around late-type stars born in OB clusters.
| Shock-Excited H 2 Flows in OMC-2 and OMC-3
We report the discovery of nearly a dozen collimated outflows from youngstellar objects embedded in the molecular filament that extends north ofthe Orion Nebula towards the H II region NGC 1977. The large number ofnearly coeval outflows and embedded class 0 young stellar objectsindicates that the OMC-2/3 region is one of the most active sites ofongoing low- to intermediate-mass star formation known. These outflowswere identified in the 2.12 mu m v = 1--0 S(1) H2 line during a surveyof a 6' x 16' region containing the OMC-2 and OMC-3 cloud cores and overa dozen recently discovered class 0 protostars. We also observefilamentary emission that is likely to trace possible fluorescent H2 inphotodissociation regions associated with M43 and NGC 1977. Neither thesuspected outflows nor the fluorescent emission are seen at thecontinuum wavelength of 2.14 mu m, which confirms their emission-linenature. Several of the new H2 flows are associated with recentlydiscovered bipolar molecular outflows. However, the most prominentbipolar CO outflow from the region (the MMS 8 flow) has no clear H2counterpart. Several H2 flows consist of chains of knots and compact bowshocks that likely trace highly collimated protostellar jets. Ourdiscovery of more than 80 individual H2-emitting shocks demonstrate thatoutflows from young stars are churning this molecular cloud.
| Physical models of photon-dominated regions: influence of clumpiness and geometry for S 140.
Model calculations for the photon-dominated region S 140 are presented.The overall geometry and an inhomogeneous density distribution areincorporated and the chemical and thermal balance is solved in two andthree dimensions. The observed widespread neutral carbon emission in theextended molecular cloud is investigated for a range in clump size andvolume filling factor. It is found that the clump volume filling factorneeds to be less than 30% and the clump size larger than 0.2 pc in orderto reproduce the observed [CI] data and their correlation with ^13^CO.Because of the presence of a dense core near the edge of the cloud, alarge part of the stellar radiation cannot penetrate the extendedmolecular cloud. Therefore, to explain the [CI] extent, the star must belocated closer to the sun, i.e. off the direction defined by the linecenter cloud - dense core. The dense core itself contains three embeddedyoung stars, which influence the source morphology of the denseinterface region. Detailed models including these internal sources areconstructed and the results compare well with high spatial resolutionobservations. Maps of the [C II], [C I], [O I] and CO line emission arepresented for various cases, and their relation with the underlyinggeometry and density distribution is discussed. It is found thatgeometrical effects play a central role in the resulting intensity andmorphology of line maps.
| Dust Filaments and Star Formation in OMC-2 and OMC-3
We have mapped the 1300 mu m dust emission of a 6' x 16' field in theOMC-2 and -3 region and detected an extended filamentary structure withat least 11 embedded condensations in OMC-2 and 10 in OMC-3. Six havebeen observed previously at 1300 mu m, and two were also detected byIRAS. We observed eight of the new sources at six wave bands from 350 to2000 mu m and derived upper limits for their far-infrared (FIR) emissionfrom the IRAS raw data. The millimeter/submillimeter emission originatesfrom cold dust (Td ~ 20 K), and the individual components have gasmasses of 5 Msolar < Mgas < 36 Msolar. The ratio Lbol/Lsmm isbelow 70 for six sources associated with OMC-3, indicating that they areclass 0--type objects. Condensations in the OMC-2 region have Lbol/Lsmm<= 360, suggesting an evolutionary effect from north to south. Wereport the discovery of a highly collimated bipolar CO outflow, mostlikely driven by the source OMC-3 MMS 8. The flow is over 5' (0.7 pc)long and less than 1' (0.15 pc) wide and is oriented nearly east-west.Less prominent outflows are associated with OMC-2 FIR 2/3 and OMC-3 MMS6. The 1300 mu m continuum emission is confined to a ridge less than 1'wide while the emission in J = 2--1 13CO, C18O, and CS is between 5' and10' wide. The continuum emission is displaced toward the eastern side ofthe molecular ridge that contains it. Most 1300 mu m sources lie in orclose to line emission peaks. However, not all line emission peakscontain prominent 1300 mu m continuum sources. The curved filamentarystructure of the large-scale dust and molecular emission is likely theresult of compression by the superbubble centered ~25--70 pc furthernorth, plus the impact of energy from the younger 1c subgroup of theOrion OB association, which lies 10--50 pc in front of our mappedregion, and by the very recent expansion of the NGC 1977 and M42 H IIregions.
| Exploring southern nebulae.
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