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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
| Interstellar absorption lines toward Cep OB4
An analysis of optical echelle spectra towards nine stars in the Cep OB4association is presented. Interstellar absorption lines which arise inthe (1, 0) and (2, 0) bands of the CN A 2Π - X2Σ+ red system towards BD+66o1661, BD+66o 1674, and BD+66o 1675 are used toinfer accurate CN column densities N(CN). A comparison with earliermeasurements in the CN violet system allows to infer a CN Doppler bparameter of b(CN) = 1.2{-}2.4 km s-1. Molecular carbonabsorption lines which arise in the (1, 0), (2, 0) and (3, 0) bands ofthe C2 A1Πu -X1Σg+ Phillips system is used toinfer gaskinetic temperatures of 35±10 K and densities of n = 700± 200 cm-3 towards BD+66o 1661 andBD+66o 1675, and a temperature of 60±10 K and adensity of 800 ± 400 cm-3 towards BD+66o1674. The R(1) line of the (0, 0) band of the CH A2Δ -X2Π system is detected towards 6 stars. A tightcorrelation exists between N(C2) and N(CH). N(CN) increaseswith N(CH) and with N(C2). Interstellar CH+ ismarginally detected towards four stars. Inferred CH+ columndensities are significantly lower than towards other lines of sight withsimilar reddening. The velocity structure towards Cep OB4 and thechemical abundances suggest that CN, C2, CH, andCH+ are formed in quiescent material. A previous suggestionthat the molecules form in a photon-dominated region close to the starsis not supported by the observations.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofisica deAndalucia (CSIC).
| XID: Cross-Association of ROSAT/Bright Source Catalog X-Ray Sources with USNO A-2 Optical Point Sources
We quantitatively cross-associate the 18,811 ROSAT Bright Source Catalog(RASS/BSC) X-ray sources with optical sources in the USNO A-2 catalog,calculating the probability of unique association (Pid)between each candidate within 75" of the X-ray source position, on thebasis of optical magnitude and proximity. We present catalogs ofRASS/BSC sources for which Pid>98%, Pid>90%,and Pid>50%, which contain 2705, 5492, and 11,301 uniqueUSNO A-2 optical counterparts respectively down to the stated level ofsignificance. Together with identifications of objects not cataloged inUSNO A-2 due to their high surface brightness (M31, M32, ...) andoptical pairs, we produced a total of 11,803 associations to aprobability of Pid>50%. We include in this catalog a listof objects in the SIMBAD database within 10" of the USNO A-2 position,as an aid to identification and source classification. This is the firstRASS/BSC counterpart catalog which provides a probability of associationbetween each X-ray source and counterpart, quantifying the certainty ofeach individual association. The catalog is more useful than previouscatalogs which either rely on plausibility arguments for association ordo not aid in selecting a counterpart between multiple off-band sourcesin the field. Sources of high probability of association can beseparated out, to produce high-quality lists of classes (Seyfert 1/2s,QSOs, RS CVns) desired for targeted study, or for discovering newexamples of known classes (or new classes altogether) through thespectroscopic classification of securely identified but unclassifiedUSNO A-2 counterparts. Low Pid associations can be used forstatistical studies and follow-on investigation-for example, performingfollow-up spectroscopy of the many low-mass stars to search forsignatures of coronal emission, or to investigate the relationshipbetween X-ray emission and classes of sources not previouslywell-studied for their X-ray emissions (such as pulsating variablestars). We find that a fraction ~65.8% of RASS/BSC sources have anidentifiable optical counterpart, down to the magnitude limit of theUSNO A-2 catalog which could be identified by their spatial proximityand high optical brightness.
| 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
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| On the Connection Between Radial Systems of Dark Globules and Stellar Associations
We study the connection between radial systems of dark globules andstellar associations. It is shown that of the 17 systems of type 1 inTable 1 of [1] 16 radial systems are connected with known associations.A new association is found (missing from the catalogs) connected withthe remaining system (System No. 2). Four systems of the six systems oftype 2 (Table 2 of [1]) are connected with known associations. A newmethod of determining the distance to associations is proposed, usingthe mean linear thickness of dark globules of radial systems connectedwith these associations as the criterion for distance. Using this methodwe make the distance to the association Cyg OB 9 more precise and answerthe question whether several radial systems belong to the correspondingstellar associations.
| Chemical transitions for interstellar C2 and CN in cloud envelopes
Observations were made of absorption from CH, C2, and CN towardmoderately reddened stars in Sco, OB2, Ceo OB3, and Taurus/Auriga. Forthese directions, most of the reddening is associated with a singlecloud complex, for example, the rho Ophiuchus molecular cloud, and as aresult, the observations probe moderately dense material. When combinedwith avaliable data for nearby directions, the survey provides the basisfor a comprehensive analysis of the chemistry for these species. Thechemical transitions affecting C2 and CN in cloud envelopes wereanalyzed. The depth into a cloud at which a transition takes place wascharacterized by tauuv, the grain optical depth at 1000 A.One transition at tauuv approx. = 2, which arises from, theconversion of C(+) into CO, affects the chemistries for both moleculesbecause of the key role this ion plays. A second one involvingproduction terms in the CN chemistry occurs at tauuv ofapprox. = 3; neutral reactions which C2 and CH is more important atlarger values for tauuv. The transition fromphotodissociation to chemical destruction takes place attauuv approx. = 4.5 for C2 and CN. The observational data forstars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemicalrate equations containing the most important production and destructionmechanisms. Because the sample of stars in Sco OB2 includes sight lineswith Av ranging from 1-4 mag, sight lines dominated byphotochemistry could be analyzed separately from those controlled bygas-phase destruction. The analysis yielded values for two poorly knownrate constants for reactions involved in the production of CN; thereactions are C2 + N yields CN + C and C(+) + NH yields all products.The other directions were analyzed with the inferred values. Thepredicted column densities for C2 and CN agree with the observed valuesto better than 50%, and in most instances 20%. When combining theestimates for density and temperature derived from chemical modeling andmolecular excitation for a specific cloud, such as the rho Ophiuchusmolecular cloud, the portion of the cloud envelope probed by C2 and CNabsorption was found to be in pressure equilibrium.
| A CO study of Sharpless 171 - Evidence for interaction between the H II region and its neighboring molecular cloud
Observations of the S171 region were made using the J = 1-0 lines of(C-12)O, (C-13)O, and (C-18)O emission. The large-scale molecular gasdistribution in S171 and its neighborhood has been mapped. Two densemolecular clumps have been revealed near the young star cluster Be 59 inthe central region of S171. The mass of these clumps is estimated intotal to be 600 solar masses. A comparison with radio continuumobservations shows that the dense gas is contacting with the continuumsource. Spectra of (C-13)O and (C-18)O exhibit highly asymmetric shapes,indicating that the dense clumps are suffering dynamic disturbance dueto the H II region. The disturbed gas component of one of the clumpsshows good correspondence with the luminous ridge of the continuumemission. The interpretation is made that ionization fronts generated bythe nearby star cluster drive shocks into the molecular clumps. Theevolutionary effects of the molecular clumps under the ionization fieldof massive stars in the region are discussed.
| More radial-velocity measurements in young open clusters
Further high resolution radial-velocity measurements are reported in 23young open clusters using the Kitt Peak CCD coude spectrograph on the0.9-m feed telescope. The radial velocities for the cluster stars arederived with the technique of cross correlation. The internal precisionof the velocity measurements is typically 2 km/s for early type stars.From these new data and previously published velocities, the observedstars in two clusters, NGC 663 and NGC 2287, were found to show arelatively small dispersion in the measured mean velocities. Furtherobservations of stars in young clusters will be useful in helping toestablish an early-type-star-velocity standard system.
| The upper main sequence of OB associations. II - The single-lined O stars: Spectral classification of northern stars and lines of C and N
The properties of the stars close to the upper main sequence of OBassociations are studied in order to provide constraints on the theoryof evolution of massive stars. The spectral classification of northernsingle-lined O stars, most of them belonging to the associations CygOB1, Per OB1 and Cas OB6, is presented. The classification is performedaccording to Conti's classification scheme for O stars, amended byMathys (1988). The behavior of the C and N lines of the stars isstudied, in order to determine whether CNO-processed material is visibleat their surface. Three new ON stars have been discovered: BD + 36 deg4063 (O9.71), HD 13268 (O8V), and HD 110360 (O7V).
| Radial-velocity measurements in 20 young open clusters
The further results of a program to determine the radial velocities ofyoung open clusters are presented. Using the KPNO coude spectrographcoupled with the 1-m feed and 2.1-m telescopes, radial velocities havebeen measured for nearly one hundred stars, most of which are ofspectral type B and A, in 20 young clusters. The combination ofinstruments and the use of cross-correlation techniques show that radialvelocities of B and A type stars as faint as 10th magnitude can bedetermined with an internal precision of less than about 2 km/s. Asexpected, the uncertainties in the velocity determination for the youngclusters are dominated by spectroscopic binary stars in these clusters.A third of the stars in the sample are found to be spectroscopicbinaries, but with a large variation in the frequency of binaries fromcluster to cluster. Because the time coverage is still limited, thisshould be considered a lower limit to the binary frequency. Clustervelocities are determined after eliminating binaries and known nonmemberstars. The new velocities are compared with a model galactic rotationcurve, as well as with previous velocity determinations.
| A survey of radio emission from Galactic OB stars
Radio continuum observations of 88 O-type and early B-type stars areexamined. Results are presented for a subset of these stars which formsa distance-limited sample of all OB stars within 2.5 kpc of the sun.Mass loss rates are derived for HD 15570, HD 166734, HD 151804, HD152408, Alpha Cam, HD 169454, and Zeta Sco. All of these very luminousOB stars are found to be losing mass at a rate on the order of 10 to the-5 solar masses/yr. Multifrequency observations confirm the free-freeinterpretation for Zeta Pup and provide evidence for variability in thefree-free sources P Cygni and Cyg OB2 No. 12.
| Interstellar C2, CH, and CN in translucent molecular clouds
Optical absorption-line techniques have been applied to the study of anumber of translucent molecular clouds in which the total columndensities are large enough that substantial molecular abundances can bemaintained. Results are presented for a survey of absorption lines ofinterstellar C2, CH, and CN. Detections of CN through the A 2Pi-X2Sigma(+) (1,O) and (2,O) bands of the red system are reported andcompared with observations of the violet system for one line of sight.The population distributions in C2 provide diagnostic information ontemperature and density. The measured column densities of the threespecies can be used to test details of the theory of molecule formationin clouds where photoprocesses still play a significant role. The C2 andCH column densities are strongly correlated with each other and probablyalso with the H2 column density. In contrast, the CN column densitiesare found to vary greatly from cloud to cloud. The observations arediscussed with reference to detailed theoretical models.
| Interstellar molecular carbon in distant supergiants
The results of a search for interstellar C2 lines in the spectrum ofsouthern reddened supergiants are reported. The C2 excitation parametersalong the lines of sight to HD 169454 and BD-14 deg 5037, both in theSct OB3 association, have been determined from spectra of high S/N.Radiative excitation effects of high rotational levels were found in HD169454. The C2 lines in BD-14 deg 5037 are clearly double, but only showmarginal deviations from thermal level populations. The column densitiesfor all stars so far observed, except of HD 92693 in Car OB1, appearlinearly correlated with E(B-V). A mean columnar abundance of 3.0 x 10to the -8th is estimated, which agrees with theoretical predictions forthe C2 abundance in dense molecular clouds.
| O-B2-STARS with Large Circumstellar Obscuration
Not Available
| The evolutionary status of OB stars with peculiar nitrogen spectra
The majority (60 percent) of OB stars exhibiting nitrogen abundancepeculiarities are members of stellar associations. The rate ofmembership is typical of O and WR stars. Bolometric magnitudes for the46 known associated peculiar OB stars (25 overabundant and 21 deficient)demonstrated that most of the anomalous stars are the brightest in theassociations. Grouping the anomalous stars in an aggregate age-classwith the next-brightest stars in the associations and then examining thecluster age reveals that nitrogen-rich OB stars are found in young andold clusters and nitrogen-poor objects are in intermediate ageassociations. Further details show that 30 solar mass OB have a nitrogenenrichment in the main sequence. Old clusters with nitrogen-rich OBstars can contain blue stragglers, which are also nitrogen-enriched.Some of the nitrogen-enriched stars are WNE progenitors or are a branchof a late evolutionary phase. Finally, nitrogen-rich OB stars areusually hotter than nitrogen-poor OB stars.
| Exciting stars and the distances of the diffuse nebulae
Not Available
| Molecular carbon in interstellar clouds.
Not Available
| Abundances of carbon-bearing diatomic molecules in diffuse interstellar clouds
In order to test in detail theories concerning the gas-phase chemistryof light interstellar molecules, it is necessary to have a coherent bodyof observational data for a large number of sight lines. In the presentinvestigation new and previously published data for 22 stellar lines ofsight are combined to provide a basis for the study of the relativeabundances of CO, CN, CH, and CH(+). New data include millimeter-waveobservations of CO toward several stars in both the 2.6 and 1.3 mmrotational lines. It is found that CO column densities (deduced largelyfrom radio data) correlate very well with those of CN (determined purelyby optical means) in regions possessing measurable CN absorptionfeatures. This correlation extends over nearly two orders of magnitudein the column densities of both species.
| Observations of interstellar C2 toward three heavily reddened stars
Observations have been made of the 2-0 band of the Phillips system ofinterstellar C2 toward the heavily obscured early-type stars VI CygniNo. 12, HD 29647, and BD +66.1675 deg. The first direct proof that therotational excitation temperatures of interstellar C2 are nonthermal wasobtained. Toward VI Cygni No. 12, the rotational distribution cannot becharacterized by a single, unique excitation temperature; thedistribution is consistent with radiative pumping models. A very stronglinear correlation was found between N(C2) and E(B-V), which suggeststhat the relative abundance of C2 is insensitive to a wide variation inphysical conditions.
| The initial mass function for massive stars
A machine readable catalog of over 750 galactic O stars with publishedphotometry, spectral types, and luminosity classes has been compiled.The catalog is probably complete to a distance of about 2.5 kpc. Fromthis volume-limited data, the initial mass function (IMF) for stars moremassive than 20 solar masses has been derived. This IMF differs fromthat of Miller and Scalo (1979) and of Lequeux (1979), in havingproportionately more O type stars and not as steep a fall-off in numberof stars with increasing mass. Dividing the sample into stars inside andoutside the solar circle, a substantial difference in the IMF of themost massive stars is found. There are proportionally more toward thegalactic center. This gradient in the IMF may be related to the observedspace density of Wolf-Rayet stars, which are descendants of O typestars.
| Nitrogen anomalies in O-type stars - A new spectroscopic criterion
A spectroscopic analysis was performed on 95 O-type stars to detectnitrogen overabundance in an investigation of spectroscopic criteria. Itis found that the N III 4514 A line is a good indicator of the abundancevariations of N in O-type stars, and it is suggested that the ONphenomenon is a continuous one rather than a marked discontinuitybetween normal O stars and those showing nitrogen overabundance.Anomalies are investigated for several early O stars, and it isconcluded that mass transfer in close binary systems and high mass lossrates due to stellar winds are the most probable mechanisms to produceON stars.
| Radio-wavelength observations of CH in the direction of nearby bright stars
Observations of the 9 cm F = 1-1 Lambda doublet transition of the CHmolecule have been made in the direction of 22 stars which lie in thevicinity of diffuse interstellar clouds. Positive detections were madetoward 14 of the regions. In some cases more than one emission featurewas detected. The ratio of the column densities of CH in the radio andoptical spectral regions is 1.11 + or - 0.4. The velocities and widthsof the CH lines differ from those of the CH(+) lines in the direction ofZeta Oph, Chi 2 Ori and Chi Oph, providing support for the theory thatthe CH(+) molecules are formed in a hot compressed gas produced by apassing interstellar shock wave.
| 21-cm observations of the CEP IV star-formation region
The Cep IV star-formation region has been mapped at 21 cm. The resultingH I distribution indicates that the available gas forms a broken ringcoincident with the optical nebulosity. Comparison of this feature withother kinematic data for both stars and gas suggests that sequentialstar formation is an ongoing process in this region.
| Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...38..309H&db_key=AST
| 2.8 and 6 CM wavelength observations of NGC 7822
Abstract image available at:http://adsabs.harvard.edu/abs/1977A&A....61..285A
| Four-color and H beta photometry of 0-type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASP...87..481C&db_key=AST
| Isolated Stroemgren spheres as a source of galactic H alpha emission
The isolated H alpha emission features reported by Reynolds et al. areidentified with Stroemgren spheres surrounding observed O stars; theroot mean square electron densities and Stroemgren radii are calculatedfrom the observed emission measure and stellar type. The density has anaverage value of 2 per cu cm but increases to 3.3 per cu cm for starswithin 200 pc of the galactic plane. An analysis of the Catalogue ofGalactic O Stars shows that enhanced H alpha emission is observed aroundall known O stars. The diffuse component of H alpha emission is what onewould expect from the line-of-sight overlapping of isolated Stroemgrenspheres surrounding B stars and appears to correlate with features onextinction maps, as derived. There is no compelling reason to suggestany large-scale intercloud ionization from H alpha data alone.
| Interstellar CN at radio wavelengths
Seven of the nine hyperfine lines in the 2.6-mm K = 1 to 0 transition ofCN have been observed in many interstellar sources. CN is detected inthe direction of infrared sources (with and without associated H IIregions), toward at least one star which shows interstellar lines of CHand CH(+), toward several objects regarded as protostellar in nature,but not in dark dust clouds or in several strong sources of HCN andHC3N. Hyperfine transition frequencies are accurately determined byassuming CN velocities are those of several other millimeter-wavelengthmolecules in the 22 sources we have detected (of 45 searched). New, moreaccurate values are derived for the rotation constant, spin-doublingconstants, and magnetic hyperfine and electric quadrupole couplingconstants for the CN radical.
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