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Ýlgili Makaleler
A search for 22-GHz H2O masers in supernova remnants
We have examined 18 known supernova remnants for evidence of the22.235-GHz water maser spectral line, using the 20-m telescope at theOnsala observatory in Sweden. No evidence of such emission wasdiscovered above the background noise in any of the observations. Wehave developed a radiative transfer water maser model for the shockedregions of supernova remnants in order to determine why this emissionwas not detected. The model indicates an insignificant amount of22.235-GHz gain; the gain lengths are far too short to allow theformation of detectable water masers from these objects. We predict thatonly in unusual environments can 22-GHz water masers be detected aroundsupernova remnants.
| Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| 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.
| Rotating elephant trunks
Aims.We investigate the structure and velocity of cold molecularpillars, "elephant trunks", in expanding H II regions. Methods:.The trunks are seen in silhouette against the bright background in ourHα images. All trunks are filamentary, and show signs of beingtwisted. Four such trunks in NGC 7822, IC 1805, the Rosette Nebula, andDWB 44 were selected, and then mapped mainly in 12CO and13CO. We determine the mass and density of the trunks. Mostof the mass is concentrated in a head facing the central cluster, and insub-filaments forming the body of the trunk that is connected toV-shaped filaments to the outer expanding shell. Results: .Wediscovered that all four trunks rotate as rigid bodies (to a firstapproximation) about their major axes, and that at least two trunks arestretching along their major axes, meaning that the massive heads arelagging behind in the general expansion of the H II regions. Therotational periods are of the order of a few million years - similar tothe age of the clusters. Rotation, then, is responsible for the twistedappearance of many elephant trunks, since they are rooted in the outershells. The trunks carry surprisingly large amounts of angular momentum,3× 1048{-}2× 1050 kg m2s-1, with corresponding rotational energies of up to 1037 J. However, we estimate the total magnetic energies tobe even larger. The trunks continuously reshape, and the formation oftwined, and in many cases helical, sub-filaments can be understood as aconsequence of electromagnetic and inertia forces inside the trunks. Atheory based on the concept of magnetically twisted trunks is developedfurther, where the initial angular momentum is a consequence of thetwisting of parent filaments containing mass condensations. Our resultsalso suggest a new process of removing angular momentum from parentmolecular clouds.
| Large Silicon Abundance in Photodissociation Regions
We have made one-dimensional raster scan observations of the ρ Ophand σ Sco star-forming regions with two spectrometers (SWS andLWS) on board the ISO. In the ρ Oph region, [Si II] 35 μm, [O I]63 μm, 146 μm, [C II] 158 μm, and the H2 purerotational transition lines S(0) to S(3) are detected, and thephotodissociation region (PDR) properties are derived as the radiationfield scaled by the solar neighborhood value G0~30-500, thegas density n~250-2500 cm-3, and the surface temperatureT~100-400 K. The ratio of [Si II] 35 μm to [O I] 146 μm indicatesthat silicon of 10%-20% of the solar abundance must be in the gaseousform in the PDR, suggesting that efficient dust destruction is ongoingeven in the PDR and that a fraction of the silicon atoms may becontained in volatile forms in dust grains. The [O I] 63 μm and [CII] 158 μm emissions are too weak relative to [O I] 146 μm to beaccounted for by standard PDR models. We propose a simple model, inwhich overlapping PDR clouds along the line of sight absorb the [O I] 63μm and [C II] 158 μm emissions, and show that the proposed modelreproduces the observed line intensities fairly well. In the σ Scoregion, we have detected three fine-structure lines, [O I] 63 μm, [NII] 122 μm, and [C II] 158 μm, and derived that 30%-80% of the [CII] emission comes from the ionized gas. The upper limit of the [Si II]35 μm is compatible with the solar abundance relative to nitrogen,and no useful constraint on the gaseous Si is obtained for the σSco region.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands, and the UK) and with the participation of ISAS and NASA.
| Number ratios of young stellar objects in embedded clusters
Embedded clusters usually contain young stellar objects in differentevolutionary stages. We investigate number ratios of objects in theseclasses in the star-forming regions ρ Ophiuchi, Serpens, Taurus,Chamaeleon I, NGC 7129, IC 1396A and IC 348. They are compared to thetemporal evolution of young stars in numerical simulations ofgravoturbulent fragmentation in order to constrain the models and todetermine the evolutionary stage of the clusters. Serpens is theyoungest and IC 348 the most evolved cluster, although the time when theobservations are best represented by the models varies stronglydepending on the model. We find an inverse correlation of the starformation efficiency (SFE) of the models with the Mach number. However,the observational SFE values cannot be reproduced by the currentisothermal models. This argues for models that take into accountprotostellar feedback processes and/or the effects of magnetic fields.
| The 90-110 μm dust feature in low to intermediate mass protostars: Calcite?
We present ISO spectra between 60 and 180 μm of 32 protostars of lowto intermediate mass. About half of the spectra present a dust featurebetween ~90 and ~110 μm. We describe the observationalcharacteristics of this feature, which seems to be due to one singlecarrier. In Class 0 sources the feature peaks around 100 μm, while inAeBe stars it peaks around 95 μm. The feature peak position seems tomostly depend on the temperature of the dust of the source, suggestingreprocessing of the dust. We present arguments for the identification ofthe observed feature as due to calcite, and estimate that about 10% to30% of elemental Ca is locked up in it. Therefore, calcite seems to beformed relatively easily around protostars despite the observation thaton Earth it needs aqueous solutions. This rises the question of whetherconditions simulating liquid water are common around forming stars andwhat creates them.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) with the participation of ISAS andNASA.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks
Observations of interstellar gas-phase and solid-state species in the2.4 200 m range obtained with the spectrometers on board the InfraredSpace Observatory (ISO) are reviewed. Lines and bands caused by ices,polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms andmolecules (in particular H2, CO, H2O, OH, andCO2) are summarized and their diagnostic capabilitiesillustrated. The results are discussed in the context of the physicaland chemical evolution of star-forming regions, includingphoton-dominated regions, shocks, protostellar envelopes, and disksaround young stars.
| Initial Results from the Spitzer Young Stellar Cluster Survey
We report initial results from IRAC observations of four young stellarclusters. These regions are part of a larger Spitzer Space Telescopesurvey of 31 young stellar groups and clusters within 1 kpc of the Sun.In each of the four clusters, there are between 39 and 85 objects withcolors inconsistent with reddened stellar photospheres. We identifythese objects as young stars with significant emission fromcircumstellar dust. Applying an analysis developed in a companion paper,we classify these objects as either pre-main-sequence stars with disks(Class II) or protostellar objects (Class I). These show that the sitesof recent star formation are distributed over multiparsec size scales.In two clusters, Cepheus C and S140, we find protostars embedded infilamentary dark clouds seen against diffuse emission in the IRAC bands.
| Infrared Array Camera (IRAC) Colors of Young Stellar Objects
We compare the infrared colors predicted by theoretical models ofprotostellar envelopes and protoplanetary disks with initialobservations of young stellar objects made with the Infrared ArrayCamera (IRAC) on the Spitzer Space Telescope. Disk and envelope modelscharacterized by infall and/or accretion rates found in previous studiescan quantitatively account for the range of IRAC colors found in fouryoung embedded clusters: S140, S171, NGC 7129, and Cep C. The IRACcolor-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to helpdistinguish between young stars with only disk emission and protostarswith circumstellar envelopes.
| Origin of diffuse C II 158 micron and Si II 35 micron emission in the Carina nebula
We present the results of mapping observations with ISO of [O I] 63μm, 145 μm, [N II] 122 μm, [C II] 158 μm, [Si II] 35 μm,and H_2 9.66 μm line emissions for the Carina nebula, an activestar-forming region in the Galactic plane. The observations were madefor the central 40 arcmin × 20 arcmin area of the nebula,including the optically bright H II region and molecular cloud lying infront of the ionized gas. Around the center of the observed area is theinterface between the H II region and the molecular cloud which createsa typical photodissociation region (PDR). The [C II] 158 μm emissionshows a good correlation with the [O I] 63 μm emission and peaksaround the H II-molecular region interface. The correlated component hasthe ratio of [C II] 158 μm to [O I] 63 μm of about 2.8. Weestimate from the correlation that about 80% of [C II] emission comesfrom the PDR in the Carina nebula. The photoelectric heating efficiencyestimated from the ratio of the ([C II] 158 μm + [O I] 63 μm)intensity to the total far-infrared intensity ranges from 0.06 to 1.2%.[O I] 145 μm is detected marginally at 10 positions. The averageratio of [O I] 145 μm to [O I] 63 μm of these positions is about0.09 ± 0.01 and is larger than model predictions. The observed [CII] 158 μm to [O I] 63 μm ratio indicates a relatively lowtemperature ( <500 K) of the gas, while the large [O I] 145 μm to63 μm ratio suggests a high temperature (˜ 1000 K). Thisdiscrepancy cannot be accounted for consistently by the latest PDR modelwith the efficient photoelectric heating via polycyclic aromatichydrocarbons (PAHs) even if absorption of [O I] 63 μm by foregroundcold gas is taken into account. We suggest that absorption of [C II] 158μm together with [O I] 63 μm by overlapping PDRs, in which theheating via PAHs is suppressed due to the charge-up effect, may resolvethe discrepancy. Quite strong [Si II] 35 μm emission has beendetected over the observed area. It shows a good correlation with [N II]122 μm, but the correlation with [O I] 63 μm is very weak,indicating that [Si II] 35 μm comes mainly from the diffuse ionizedgas rather than the PDR. The ratio of [Si II] 35 μm to [N II] 122μm is about 8 and Si of about 50% of the solar abundance relative toN should be present in the gas phase. The present results suggest thatefficient dust destruction takes place and a large fraction of Sireturns to the gas in the Carina star-forming region.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/579
| Defective Carbon Onions in Interstellar Space as the Origin of the Optical Extinction Bump at 217.5 Nanometers
It is known that carbonaceous dust in interstellar space shows a strongextinction bump at 217.5 nm (4.6 μm-1, 5.7 eV). One of thepossible candidates for such carbonaceous interstellar dusts is a newmember of the fullerene family called the carbon onion. Recently, weconducted a laboratory experiment to study the optical extinctionproperties of carbon onions. In order to explain the experimentalresults, a new dielectric model of the onions, the ``defective''spherical onion model, was proposed. Here we show that the defectivecarbon onions are likely to be one of the origins of the interstellarextinction bump. This report also suggests that carbonaceousinterstellar dust particles may be replicated by onions produced in thelaboratory.
| Far-infrared loops in the 2nd Galactic Quadrant
We present the results of an investigation of the large-scale structureof the diffuse interstellar medium in the 2nd Galactic Quadrant(90°≤l≤180°). 145 loops were identified on IRAS-basedfar-infrared maps. Our catalogue lists their basic physical properties.The distribution clearly suggests that there is an efficient processthat can generate loop-like features at high Galactic latitudes.Distances are provided for 30 loops. We also give an observationalestimate of the volume filling factor of the hot gas in the Local Arm,4.6%≤f2nd<6.4%.Appendices A-C 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/418/131
| Mid- to far-infrared spectroscopy of Sharpless 171
We have collected one-dimensional raster-scan observations of the activestar-forming region Sharpless 171 (S171), a typical H Iiregion-molecular cloud complex, with the three spectrometers (LWS, SWS,and PHT-S) on board ISO. We have detected 8 far-infrared fine-structurelines, [O Iii] 52 mu m, [N Iii] 57 mu m, [O I] 63 mu m, [O Iii] 88 mu m,[N Ii] 122 mu m, [O I] 146 mu m, [C Ii] 158 mu m, and [Si Ii] 35 mu mtogether with the far-infrared continuum and the H2 purerotation transition (J=5-3) line at 9.66 mu m. The physical propertiesof each of the three phases detected, highly-ionized, lowly-ionized andneutral, are investigated through the far-infrared line and continuumemission. Toward the molecular region, strong [O I] 146 mu m emissionwas observed and the [O I] 63 mu m to 146 mu m line ratio was found tobe too small ( ~ 5) compared to the values predicted by currentphotodissociation region (PDR) models. We examine possible mechanisms toaccount for the small line ratio and conclude that the absorption of the[O I] 63 mu m and the [C Ii] 158 mu m emission by overlapping PDRs alongthe line of sight can account for the observations and that the [O I]146 mu m emission is the best diagnostic line for PDRs. We propose amethod to estimate the effect of overlapping clouds using thefar-infrared continuum intensity and derive the physical properties ofthe PDR. The [Si Ii] 35 mu m emission is quite strong at almost all theobserved positions. The correlation with [N Ii] 122 mu m suggests thatthe [Si Ii] emission originates mostly from the ionized gas. The [Si Ii]35 mu m to [N Ii] 122 mu m ratio indicates that silicon of 30% of thesolar abundance must be in the diffuse ionized gas, suggesting thatefficient dust destruction is undergoing in the ionized region.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the UK) and with the participation of ISAS and NASA.
| A Catalog of Young Stellar Groups and Clusters within 1 Kiloparsec of the Sun
We present a catalog of near-infrared surveys of young (<~ a few106 yr) stellar groups and clusters within 1 kpc from theSun, based on an extensive search of the literature from the past tenyears. We find 143 surveys from 69 published articles, covering 73different regions. The number distribution of stars in a region has amedian of 28 and a mean of 100. About 80% of the stars are in clusterswith at least 100 members. By a rough classification of the groups andclusters based on the number of their associated stars, we show thatmost of the stars form in large clusters. The spatial distribution ofcataloged regions in the Galactic plane shows a relative lack ofobserved stellar groups and clusters in the range270°
| A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters within 1 Kiloparsec of the Sun
As the first step of a multiwavelength investigation into therelationship between young stellar clusters and their environment, wepresent fully sampled maps in the J=1-0 lines of 13CO andC18O and the J=2-1 line of C18O for a selectedgroup of 30 young stellar groups and clusters within 1 kpc of the Sun.This is the first systematic survey of these regions to date. Theclusters range in size from several stars to a few hundred stars. Thirtyfields ranging in size from 8'×8' to 30'×60' were mappedwith 47" resolution simultaneously in the two J=1-0 lines at the FiveCollege Radio Astronomy Observatory. Seventeen sources were mapped overfields ranging in size from 3'×3' to 13'×13' in the J=2-1line with 35" resolution at the Submillimeter Telescope Observatory. Wecompare the cloud properties derived from each of the three tracers inorder to better understand systematic uncertainties in determiningmasses and line widths. Cloud masses are determined independently usingthe 13CO and C18O transitions; these masses rangefrom 30 to 4000 Msolar. Finally, we present a simplemorphological classification scheme, which may serve as a roughindicator of cloud evolution.
| The Canadian Galactic Plane Survey
The Canadian Galactic Plane Survey (CGPS) is a project to combine radio,millimeter, and infrared surveys of the Galactic plane to providearcminute-scale images of all major components of the interstellarmedium over a large portion of the Galactic disk. We describe in detailthe observations for the low-frequency component of the CGPS, the radiosurveys carried out at the Dominion Radio Astrophysical Observatory(DRAO), and summarize the properties of the merged database of surveysthat comprises the CGPS. The DRAO Synthesis Telescope surveys haveimaged a 73° section of the Galactic plane, using ~85% of thetelescope time between 1995 April and 2000 June. The observationsprovide simultaneous radio continuum images at two frequencies, 408 and1420 MHz, and spectral-line images of the λ=21 cm transition ofneutral atomic hydrogen. In the radio continuum at 1420 MHz,dual-polarization receivers provide images in all four Stokesparameters. The surveys cover the region 74.2d
| Detection of Far-Infrared Features in Star-Forming Regions
We report the detection of a feature at 65 μm and a broad featurearound 100 μm in the far-infrared spectra of the diffuse emissionfrom two active star-forming regions, the Carina Nebula and Sharpless171. The features are seen in the spectra over a wide area of theobserved regions, indicating that the carriers are fairly ubiquitousspecies in the interstellar medium. A similar 65 μm feature has beendetected in evolved stars and attributed to diopside, a Ca-bearingcrystalline silicate. The present observations indicate the firstdetection of a crystalline silicate in the interstellar medium if thisidentification also holds true for the interstellar feature. A similarbroad feature around 90 μm reported in the spectra of evolved starshas been attributed to calcite, a Ca-bearing carbonate mineral. Theinterstellar feature seems to be shifted to longer wavelengths and havea broader width, although the precise estimate of the feature profile isdifficult. As a carrier for the interstellar 100 μm feature, weinvestigate a possibility that the feature originates from carbononions, grains consisting of curved graphitic shells. Because of thecurved graphitic sheet structure, the optical properties in thedirection parallel to the graphitic plane interact with those in thevertical direction in the carbon onion grains. This effect enhances theinterband transition feature in the direction parallel to the graphiticplane in the carbon onions, which is suppressed in the graphiteparticles. Simple calculations suggest that carbon onion grains are alikely candidate for the observed 100 μm feature carrier, but theappearance of the feature is sensitive to the assumed opticalproperties.Based on observations with ISO, an ESA project with instruments fundedby ESA members states (especially the PI countries France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083
| Multiwavelength Study of the Cas OB5 Supershell
We present the results of a multiwavelength study of a large, expandingshell around the Cas OB5 association. Based on the analysis of HI, COand infrared data the main parameters of the shell were determined. Weestimated the total mass in the shell to be≍7.5x105Mo.
| Hα Emission Stars and Herbig-Haro Objects in the Vicinity of Bright-rimmed Clouds
Bright-rimmed clouds (BRCs) found in H II regions are probably sites oftriggered star formation due to compression by ionization/shock fronts.Many BRCs harbor IRAS point sources of low dust temperature. They alsofrequently contain a small cluster of near-IR stars that is elongatedalong the axis of the BRC. Here we present the results of our Hαgrism spectroscopy and narrowband imaging observations of BRCs in searchof candidate pre-main-sequence stars of the T Tauri, Herbig Ae/Be andrelated types, and Herbig-Haro (HH) objects. We have detected a largenumber (460) of Hα emission stars down to a limiting magnitude ofabout R=20 in and around all but two of the 28 BRCs observed. Thepresent study has, for the first time, reached down nearly to thefaintest classical T Tauri star population in OB associations. A totalof 12 new HH objects have been found. Most are of small apparent size,emphasizing the need for deep searches at high spatial resolution, butHH 588 associated with BRC 37 represents a huge HH complex composed oftwo-staged bow shocks on both sides of a tiny central knot. These starsand HH objects are concentrated near the tip of bright rims (i.e., inthe head of the BRCs and just outside the rims) and often make looseaggregates similar to those of near-IR stars, thus supporting ourhypothesis of ``small-scale sequential star formation.'' The presence ofsuch a large number of Hα emission stars in the immediatevicinities of BRCs implies that second-generation formation of low-massstars is relatively extensive and further supports the notion ofcohabitation of high- and low-mass populations in OB associations.
| Hydrogen Recombination Lines near 327 MHz. III. Physical Properties and Origin of the Low-Density Ionized Gas in the Inner Galaxy
We present constraints on the physical properties of the ionized gasresponsible for hydrogen radio recombination lines (RRLs) near 327 MHzdetected in a recent Galactic plane survey made with the Ooty RadioTelescope. To obtain these constraints, we combined the data at 327 MHzwith previously published RRL observations near 1.4 GHz. The density ofthe ionized gas is well constrained and is in the range of 1 to 10cm-3. The data implies upper limits to the temperature andsize of the line emitting regions of ~12,000 K and ~500 pc,respectively. Assuming an electron temperature of 7000 K, the derivedpath lengths of the line emitting region are in the range of 20 to 200pc. The derived properties of the ionized gas responsible for the RRLemission near 327 MHz suggest that most of the [N II] 205 μm emissionand a considerable fraction of the [C II] 158 μm emission observed inthe Galactic plane by the COBE satellite could also originate in thesame gas. The Hα emission from these ionized gases is mostlyundetected in the existing Hα surveys because of largeinterstellar extinction. About 50% of the free-free absorption of theGalactic nonthermal radiation observed at frequencies less than 100 MHzcan be accounted for by the same ionized gas. We also discuss the originof this low-density ionized gas in the inner Galaxy. The derived lowline-of-sight filling factor (<1%) for this ionized gas indicatesthat it does not form a pervasive medium. On the basis of the similarityof the distribution of this gas in the Galactic disk with that of thestar-forming regions and the range of derived physical properties, wesupport the earlier suggestion that the low-frequency RRL emissionoriginates from low-density ionized gas, which forms envelopes of normalH II regions.
| 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.
| 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 First 50 Years at Palomar, 1949-1999 Another View: Instruments, Spectroscopy and Spectrophotometry and the Infrared
We review the research on a wide variety of topics using data obtainedwith the 200-inch Hale telescope. Using state-of-the-art spectrographs,photometers, spectrometers and infrared detectors, the Palomarastronomers investigated the spectra of stars, interstellar matter, AGNsand quasars in great detail. Spectral resolutions ranged from 1000 A forbroad-band photometry to 0.04 A using interferometric techniques.
| Outflow activity of a YSO and thermal properties of the gas in IRAS 23385+6053.
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| The radio emission from the Galaxy at 22 MHz
We present maps of the 22 MHz radio emission between declinations -28degr and +80 degr, covering ~ 73% of the sky, derived from observationswith the 22 MHz radiotelescope at the Dominion Radio AstrophysicalObservatory (DRAO). The resolution of the telescope (EW x NS) is 1.1degr x 1.7 degr secant(zenith angle). The maps show the large scalefeatures of the emission from the Galaxy including the thick non-thermaldisk, the North Polar Spur (NPS) and absorption due to discrete H Iiregions and to an extended band of thermal electrons within 40 degr ofthe Galactic centre. We give the flux densities of nine extendedsupernova remnants shown on the maps. A comparison of the maps with the408-MHz survey of \cite[Haslam et al. (1982]{haslam82}) shows aremarkable uniformity of spectral index (T~ nu (-beta ) ) of most of theGalactic emission, with beta in the range 2.40 to 2.55. Emission fromthe outer rim of the NPS shows a slightly greater spectral index thanthe distributed emission on either side of the feature. The mean localsynchrotron emissivity at 22 MHz deduced from the emission toward nearbyextended opaque H Ii regions is ~ 1.5 10(-40) Wm(-3) Hz(-1) sr(-1) ,somewhat greater than previous estimates.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Two new outflow sources: S 146 and IRAS 23545+6508
A 12CO (J = 1 - 0) line-mapping observation using the newlybuilt 3mm system of the 13.7m radio telescope at the Qinghai Station ofPurple Mountain Observatory showed there are high velocity molecularoutflows near IRAS 22475+5939 (S 146) and 23545+6508. The morphologiesof the two outflows are analyzed and their physical and dynamicalparameters are calculated and discussed. Using the IRAS data, theevolutionary status of the central sources and the thermalcharacteristics of the ambient gas are also investigated.
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