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A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| Integrated spectral properties of 22 small angular diameter galactic open clusters
Aims.Flux-calibrated integrated spectra of a sample of 22 Galactic openclusters of small angular diameter are presented. With one exception(ESO 429-SC2), all objects have Galactic longitudes in the range208° < l < 33°. The spectra cover the range ≈3600-6800Å, with a resolution of ≈14 Å. The properties of thepresent cluster sample are compared with those of well-studied clusterslocated in two 90° sectors, centred at l = 257° and l =347°. The dissolution rate of Galactic open clusters in these twosectors is examined. Methods: Using the equivalent widths of the Balmerlines and comparing line intensities and continuum distribution of thecluster spectra with those of template cluster spectra with knownproperties, we derive both foreground reddening values and ages. Thus,we provide information independent of that determined throughcolour-magnitude diagrams. Results: The derived E(B-V) values for thewhole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, whilethe ages range from ~3 Myr (NGC 6604 and BH 151) to ~3.5 Gyr (Ruprecht2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2,BH 151 and Hogg 9) the foreground E(B-V) colour excesses and ages aredetermined for the first time. The results obtained for the remainingclusters show, in general terms, good agreement with previousphotometric results. Conclusions: The age and reddening distributionsof the present sample match those of known clusters in the two selectedGalactic sectors. The present results would favour a major dissolutionrate of star clusters in these two sectors. Two new solar-metallicitytemplates are defined corresponding to the age groups of (4-5) Myr and30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piattiet al. templates of 20 Myr and (3-4) Gyr are here redefined.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina (CONICET) and the National Universities of LaPlata, Córdoba and San Juan, Argentina. Tables [see full text]-[see full text] and Appendix are only availablein electronic form at http://www.aanda.org
| Discovery of Hot Supergiant Stars near the Galactic Center
We report new results of a campaign to find Wolf-Rayet and O (WR/O)stars and high-mass X-ray binaries (HMXBs) in the Galactic center. Wesearched for candidates by cross-correlating the Two Micron All SkySurvey with a deep Chandra X-Ray Observatory catalog of point sources inthe Radio Arches region. Following up with K-band spectroscopy, we foundtwo massive stellar counterparts to CXOGC J174555.3-285126 and CXOGCJ174617.0-285131, which we classify as a broad-lined WR star of subtypeWN6b and an O Ia supergiant, respectively. Their X-ray properties aremost consistent with those of known colliding-wind binaries in theGalaxy and the Large Magellanic Cloud, although a scenario involvinglow-rate accretion onto a compact object is also possible. The O Ia starlies 4.4 pc in projection from the Quintuplet cluster and has a radialvelocity consistent with that of the Quintuplet, suggesting that thisstar might have escaped from the cluster. We also present the discoveryof a B2 Ia supergiant, which we identified as a candidate massive starusing 8 μm Spitzer maps of the Galactic center in a region near theknown massive X-ray-emitting star CXOGC J174516.1-290315. We discuss theorigin of these stars in the context of evolving stellar clusters in theGalactic center.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| New runaway OB stars with HIPPARCOS
A Monte Carlo method for detection of runaway OB stars fromobservational data is proposed. 61 runaway OB star candidates have beendetected by an analysis of Hipparcos proper motions. The peculiartangential and total transverse velocities have been determined forthese stars. A list of the detected runaway star candidates ispresented. The evidence of a discrepancy between photometric andparallactic distances of runaway OB star candidates is presented.
| Pismis 20: One or two clusters?
We present UBVRI polarimetric observations of stars in the direction ofthe open cluster Pismis 20 and WR67. It is found that the observed members segregatepolarimetrically into two groups, with different associated meanPmax (4.31 and 6.08% respectively). Although in theliterature these stars are considered as members of the same cluster,the polarized light from these groups clearly differs. There are twopossible explanations: very efficient polarizing dust particles existsbetween certain members of the cluster; or else Pismis20 is composed of different groupings superimposed along theline of sight. WR 67 belongs to the group moreaffected by the dust.Based on observations obtained at Complejo Astronómico ElLeoncito, operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the Universities of La Plata,Córdoba, and San Juan.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The stellar content of obscured compact HII regions
Near-infrared, spectroscopic studies of central ionizing sources of veryyoung HII regions is presented in conjunction with a recently available,sophisticated atmospheric code to constrain the physical conditions andenvironment of very massive stars at very early stages of evolution. Combining high quality near-infrared spectroscopy of very young massivestars with model atmosphere calculations should allow for the mostaccurate quantitative determination of Teff, rotation, L, andlog g, and to search for binaries and possible disk or in-fallsignatures in forming or recently formed massive stars. Thesecharacteristics make up a vital boundary condition constraining theorieson massive star formation.
| CCD photometric search for peculiar stars in open clusters. IV. Collinder 272, Pismis 20, Lyng\aa 14, NGC 6396 and NGC 6611
The time scale on which chemical peculiarity in upper main sequencestars begins is unknown. Results from the Hipparcos measurementsindicate that the classical chemically peculiar (CP) stars occupy thewhole area from the zero age main sequence to the terminal age mainsequence (Gómez et al. \cite{Gom98}). However, Hubrig et al.(\cite{Hub00}) reported that the magnetic fields appear only in starsthat have already completed at least approximately 30% of their mainsequence life time. Previous observations in the very young Orion OB1and Scorpio-Centaurus associations (Joncas & Borra 1981; Borra etal. 1982) reveal a percentage of chemically peculiar stars that issignificantly smaller (less than 5%) than that of the galactic field (upto 25%). This already indicates that the phenomenon needs at leastseveral 106 yr to appear. Since no further observations havebeen published since then, we have observed the five very young (1 Myr<= age <= 25 Myr) open clusters Collinder 272, Pismis 20, Lyng\aa14, NGC 6396 and NGC 6611. Photometry from 250 CCD frames in the threefilter Delta a-system results in very low detection limits (0.008 to0.015 mag). We find no single photometrically chemically peculiarcandidate in approximately 460 observed cluster members in the fiveprogram clusters. From the number of observed objects we deduce upperlimits between 0.6% and 2.4% for the incidence of CP stars for the fourobserved aggregates. Based on observations obtained at ComplejoAstronómico el Leoncito (CASLEO), operated under the agreementbetween the Consejo Nacional de Investigaciones Científicas yTécnicas de la República Argentina and the NationalUniversities of La Plata, Córdoba y San Juan; ESO-La Silla andUTSO-Las Campanas.
| A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253
We derive cluster parameters and mass functions from new UBVRI CCDphotometric observations of ~3500 stars reaching down to V~20mag for thedistant southern open star clusters NGC 3105, NGC 3603, Melotte 105,Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCDdata are presented for the first time. The reddening is non-uniformacross the face of the young (age <300Myr) clusters NGC 3105, NGC3603, Melotte 105, Hogg 15 and Pismis 20, with average values ofE(B-V)=1.06, 1.44, 0.52, 1.15 and 1.20mag respectively, while it isuniform with average values of E(B-V)=0.72 and 0.20mag for the olderclusters NGC 4815 and 6253 respectively. The values of colour excessratios indicate the presence of normal interstellar reddening across thecluster regions studied here. Well-defined main sequences can be seen inall the clusters. However, main-sequence turn-off points and subgiantbranches are well defined only in the older clusters NGC 4815 and 6253.The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15,NGC 4815, Pismis 20 and NGC 6253 are 9.5+/-1.5, 7.2+/-1.2, 2.3+/-0.2,3.0+/-0.3, 2.75+/-0.2, 3.55+/-0.35 and 1.8+/-0.12kpc respectively, whilethe corresponding ages derived using theoretical convective coreovershooting stellar evolutionary isochrones are 25+/-10, 3+/-2,250+/-30, 6+/-2, 400+/-50, 6+/-2 and 2500+/-600Myr respectively. In themass range ~1-75Msolar, the mass functions of all clustersexcept for NGC 6253 have been studied. The slopes of their mass spectraagree within errors with the Salpeter value (1.35). The slope of themass function for stars more massive than 10Msolar is almostthe same as for the lower mass stars. The mass function slopes ofclusters younger than 500Myr seem to have no dependence on Galacticlongitude, Galactocentric distance and cluster age. As the inherentuncertainties in the mass function determinations of young Galactic starclusters can produce internal scatter that is larger than the externalscatter, we conclude that, above 1Msolar, the initial massfunction is universal with a slope of Salpeter value.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations
In a previous paper on the Magellanic Clouds, we demonstrated thatcoeval clusters provide a powerful tool for probing the progenitormasses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs).Here we extend this work to the higher metallicity regions of the MilkyWay, studying 12 Galactic clusters. We present new spectral types forthe unevolved stars and use these, plus data from the literature, toconstruct H-R diagrams. We find that all but two of the clusters arehighly coeval, with the highest mass stars having formed over a periodof less than 1 Myr. The turnoff masses show that at Milky Waymetallicities some W-R stars (of early WN type) come from stars withmasses as low as 20-25 Msolar. Other early-type WN starsappears to have evolved from high masses, suggesting that a large rangeof masses evolve through an early WN stage. On the other hand, WN7 starsare found only in clusters with very high turnoff masses, over 120Msolar. Similarly, the LBVs are only found in clusters withthe highest turnoff masses, as we found in the Magellanic Clouds,providing very strong evidence that LBVs are a normal stage in theevolution of the most massive stars. Although clusters containing WN7stars and LBVs can be as young as 1 Myr, we argue that these objects areevolved, and that the young age simply reflects the very high massesthat characterize the progenitors of such stars. In particular, we showthat the LBV η Car appears to be coeval with the rest of theTrumpler 14/16 complex. Although the WC stars in the Magellanic Cloudswere found in clusters with turnoff masses as low as 45Msolar, the three Galactic WC stars in our sample are allfound in clusters with high turnoff masses (>70 Msolar)whether this difference is significant or due to small number statisticsremains to be seen. The bolometric corrections of Galactic W-R stars arehard to establish using the cluster turnoff method but are consistentwith the ``standard model'' of Hillier.
| 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
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| 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.
| Red and infrared colours of B stars and the reddening law in the Galaxy
The red and infrared intrinsic colours of B stars are derived fromphotometric observations through the UBV(RI)_CJHK and Hβ filters of257 early-type stars. Those stars for which the UBV and Hβmeasurements match the published spectral class, and which show no othersigns of peculiarity, are used to determine the intrinsic photometriccolours of B stars in the red and infrared. From these intrinsic coloursthe interstellar reddening relationships for the red and infraredcolours are evaluated, and the results are compared with previousestimates of these quantities. The values of R, E(B-V) and the distanceare then determined for the individual stars. R is confirmed to be closeto 3.1 in most cases, but was found to be much larger in somedirections. The relationship between R and the location of a star in theGalaxy is investigated. Usually the abnormally reddened stars seemed tobe associated with known regions of star formation. The paper alsoidentifies seven likely variable stars and a number of stars withpossible dust shells.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| PISMIS 20 - A Case Study of a Young Cluster
Existing photometric (photoelectric and CCD) and spectroscopic data forstars in the young open cluster Pismis 20 have been reanalyzed for thepurpose of examining the cluster extinction law as well as forrederiving the intrinsic parameters for the cluster and its more exoticmember stars, such as the luminous B2 Ia-O supergiant HD 134959 and theWolf-Rayet star WR 67. The line of sight to Pismis 20 is remarkablyclear of reddening material out to distances of ˜1.25 kpc, wheredust clouds, presumably associated with the Sagittarius-Carina (-I) armof the Galaxy, add more than 3m of extinction to more distantstars, including members of Pismis 20. The cluster extinction law isdescribed by a reddening relation of slopeEU-B/EB-V=O.767±O.O23 s.e. and by a valueof R=Av/EB-V=2.92±0.16 s.e. The derivedcluster distance of 3272±303 pc(V0-Mv=12.57±0.20), or 3272±33 pcwith fixed R value, is 11% smaller than the value obtained byVázquez et al. [A&AS, 111, 85 (1995)] in their recent study,the difference being accounted for mainly by the different approachesused to correct for the effects of extinction on the photometric data.The derived luminosity of Mv=-7.88±0.06 for the B2Ia-O star HD 134959 confirms its hypergiant status, while for WR 67 theinferred luminosity is Mv=-4.40±0.10(Mv=-4.48±O.10). This WN6 star lies on the edge of thecluster where the extinction is visibly larger and less well establishedthan near the cluster center. However, its derived space reddening of=1.28±0.03 leads to stellar parametersthat are consistent with those inferred for other Wolf-Rayet stars ofthe nitrogen sequence.
| A detailed study of the open cluster PISMIS 20.
The very young open cluster Pis 20 has been investigated using CCD UBVRphotometry. New blue faint members were found down to V=19mag. With themwe obtain a better definition of the lower cluster main sequence. Usingthe fitting method, we derive a cluster distance of d=3600pc(V_0_-M_V_=12.80). The mean color excess is =1.24mag. Fromthe observed characteristics and spatial location of the bluest clusterstars we conclude that the star WR 67 has a good chance to be a clustermember. In that case, the mass of its progenitor should be greater than50Msun_ according to evolutionary models. We obtain for Pis20 an age of 5(+/-1)x10^6^yr from isochrone fitting. We discuss thecluster's mass spectrum whose slope, within the observationaluncertainties, appears to be similar to the one in Salpeter's law. Wenotice that high WR/OB and WC/WN number ratios are present not onlywithin the cluster itself but also in the association where Pis 20 issituated. Although the cluster distance derived here places Pis 20nearer than the supernova remnant MSH 15-52, we cannot/rule out thepossibility that they are spatially related. The presently observedcluster mass is around 600Msun_.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| UBV photometry of OB+ stars in the southern Milky Way
One thousand two hundred and twenty six new observations are combinedwith previously published results of the author to yield an internalyconsistent set of magnitudes and colors on the international UBV systemfor 666 stars classified as OB+ in the Stephenson-Sanduleak OB starsurvey. The U - B, B - V diagram indicates that these stars consistprimarily of O-type stars and early B-type supergiants, reddened by upto E(B - V) = 2.1 mag.
| 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.
| UBV photometry in four southern open clusters with suspected supernova remnants
UBV photoelectric photometry is presented for four southern openclusters with suspected supernova remnants: Stock 14, Trumpler 21, Lynga1, and Pismis 20. The extinctions and distance moduli for the clustersare determined. It is found that all of the clusters except for Lynga 1are good candidates for spiral tracers. Rough age estimates for theclusters are made on the basis of postmain sequence turn-offs andsupergiant members. The color-magnitude diagrams of the clusters arecompared with the age groups of Mermilliod (1981).
| The region of the supernova remnant MSH 15-52 revisited - A new thermal H II region, H II G 320.5-1.4
The authors revisited the stellar and nebular content in the directionof MSH 15-52. This search was initiated by the discovery of a newH? thermal region H II G 320.5-1.4 with velocity VLSR =-43 km s-1, extending over an area similar to MSH 15-52, andclearly distinct from the foreground H II region BBW 28802. From arediscussion of the reddening and distances of hot stars with availablespectra in the direction l = 320°, it is found that they constitutea single stellar association (Cir OB1) at a distance about 4 kpc,probably not much more extended than 80×80 pc. This associationcontains the cluster Pis 20, four WR stars and a number of stars withages in the range 4 - 10×106yr. It is the excitationsource of H II G 320.5-1.4. In such an association, bubbles may haveformed previously to the explosion of supernovae; also, severalsupernovae may have exploded recently. Thus it is not unlikely that MSH15-52 originated from the same SN explosion as PSR 1509-58 and expandedfreely into a bubble; on the other hand, it would not be surprising thattwo different supernovae exploded close in time and space.
| Radial velocities for 28 southern young open clusters
Radial velocities for 83 OB-stars in 28 southern young open clusters arepresented. The internal and external accuracy of the data is in goodagreement with results obtained by Ardeberg and Maurice (1977) with thesame equipment. From a comparison with radial velocities existing in theliterature no systematic radial-velocity differences for stars in commonare found.
| The brightest stars in the Magellanic Clouds and other late-type galaxies
Observational data on the most luminous stars in the Galaxy, theMagellanic Clouds, and other nearby late-type galaxies are compiled intables, characterized, and used to construct a schematic HR diagram.Luminosity calibrations are performed; the position of the S Dor orHubble-Sandage variables on the HR diagram is indicated; and anempirical upper luminosity limit for normal stars, declining withtemperature for the hotter stars but becoming constant for the coolersupergiants, is determined.
| Exciting stars and the distances of the diffuse nebulae
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