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XMM-Newton Observations Reveal the X-Ray Counterpart of the Very High Energy Gamma-Ray Source HESS J1640-465
We present X-ray observations of the as of yet unidentified very highenergy (VHE) γ-ray source HESS J1640-465 with the aim ofestablishing a counterpart of this source in the keV energy range, andidentifying the mechanism responsible for the VHE emission. The 21.8 ksXMM-Newton observation of HESS J1640-465 in 2005 September represents asignificant improvement in sensitivity and angular resolution overprevious ASCA studies in this region. These new data show ahard-spectrum X-ray-emitting object at the centroid of the H.E.S.S.source, within the shell of the radio supernova remnant (SNR)G338.3-0.0. This object is consistent with the position and fluxpreviously measured by both ASCA and Swift XRT, but is now shown to besignificantly extended. We argue that this object is very likely thecounterpart to HESS J1640-465, and that both objects may represent thepulsar wind nebula of an as of yet undiscovered pulsar associated withG338.3-0.0.
| Optical multicolor polarization observations in the region of the open cluster NGC 5749
We present (UBVRI ) multicolor linear polarimetric data for 31 of thebrightest stars in the area of the open cluster NGC 5749 considered tostudy the properties of the interstellar medium (ISM) towards thecluster. Our data yield a mean polarization percentage of PV˜ 1.7%, close to the polarization value produced by the ISM withnormal efficiency (P(%)=3.5 {E(B-V)}0.8) for amean color excess of EB-V=0.42. The mean angle ofpolarization vectors, θ=74°, agrees quite well with theexpected angle produced by dust particles aligned in the direction ofthe galactic disk (and the magnetic field) in the region. Our analysisindicates that the visual absorption affecting the stars in NGC 5749 ispartially produced by a dust layer located up to 300 pc from the Sun andalso by a second layer of dust closer to the cluster (located at leastat 700 pc). The observed photometry and our polarization data areconsistent with the existence of dust within the cluster. We also showin this work how polarimetry could be an excellent technique foridentifying nonmember stars.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina. Table 1 is 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/462/621
| Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
Aims.We present new CORAVEL radial-velocity observations andphotoelectric photometry in the UBV and DDO systems for a sample ofpotential members of the red-giant branches of NGC 6192, NGC 6208 andNGC 6268, three open clusters projected close to the Galactic centerdirection. We also examine the properties of a sample of 42 inner diskopen clusters projected towards almost the same direction as the threeclusters. Methods: .Cluster members and red field giants werediscriminated by using the CORAVEL radial-velocity data and by applyingtwo photometric criteria. Interstellar reddening and metal content ofthe clusters were derived from combined BV and DDO data. Results:.Cluster membership for five red giants in NGC 6192, three in NGC 6208and three in NGC 6268 has been confirmed by the analysis of thephotometric and kinematic data. Photometric membership probabilitiesshow very good agreement with those obtained from CORAVEL radialvelocities. Three new spectroscopic binaries were discovered among thered giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B-V)colour excesses were derived. Conclusions: .The overallmetallicities were found to be nearly solar for NGC 6208 and above solarfor NGC 6192 and NGC 6268. Most of the clusters located closer than 2kpc from the Sun in the considered direction are slightly more reddenedthan the absorption resulting from the Baade's window absorption law.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| New fundamental parameters for the inner disk open cluster Lyngå 11
We have obtained CCD BVIKC photometry down to V ˜ 21.5for the open cluster Lyngå 11 and its surrounding field. Thispreviously unstudied object appears to be rich and highly absorbed. Acluster angular radius of 4.5′ ± 0.5′, equivalent to(3.0 ± 0.3) pc, is estimated from star counts in 100-pixel a sideboxes distributed throughout the whole observed field. Our analysissuggests that Lyngå 11 is moderately young and probably of solarmetallicity. Adopting the theoretical metal content Z = 0.02, whichprovides the best global fit, we derive an age of (630 ± 70) Myr.Simultaneously, colour excesses E(B ‑ V) = 0.70 and E(V ‑ I)= 0.85 and an apparent distance modulus V ‑ MV = 14.0are obtained. These results place Lyngå 11 at a distance of (2.3± 0.5) kpc from the Sun and ˜6.5 kpc from the Galacticcentre. The properties of a sample of clusters aligned along theline-of-sight of Lyngå 11 are examined as well.
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| The Open Cluster NGC 6520 and the Nearby Dark Molecular Cloud Barnard 86
Wide-field BVI photometry and 12CO(1-->0) observations arepresented in the region of the open cluster NGC 6520 and the darkmolecular cloud Barnard 86. From the analysis of the optical data wefind that the cluster is rather compact, with a radius of1.0 arcmin +/-0.5 arcmin, smaller than previous estimates. The cluster ageis 150+/-50 Myr, and the reddening EB-V=0.42+/-0.10. Thedistance from the Sun is estimated to be 1900+/-100 pc, which is largerthan previous estimates. We finally derive basic properties of the darknebula Barnard 86 on the assumption that it lies at the same distance asthe cluster.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Pulsating Stars in the ASAS-3 Database. I. beta Cephei Stars
We present results of an analysis of the ASAS-3 data for short-periodvariables from the recently published catalog of over 38000 stars. Usingthe data available in the literature we verify the results of theautomatic classification related to \beta Cep pulsators. In particular,we find that 14 stars in the catalog can be classified unambiguously asnew beta Cep stars. By means of periodogram analysis we derive thefrequencies and amplitudes of the excited modes. The main modes in thenew beta Cep stars have large semi-amplitudes, between 35 and 80 mmag.Up to four modes were found in some stars. Two (maybe three) new betaCep stars are members of southern young open clusters: ASAS164409-4719.1 belongs to NGC 6200, ASAS 164630-4701.2 is a member ofHogg 22, and ASAS 164939-4431.7 could be a member of NGC 6216.We also analyze the photometry of four known beta Cep stars in theASAS-3 catalog, namely IL Vel, NSV 24078, V1449 Aql and SY Equ. Finally,we discuss the distribution of beta Cep stars in the Galaxy.
| Galactic Metallicity Gradients Derived from a Sample of OB Stars
The distribution of stellar abundances along the Galactic disk is animportant constraint for models of chemical evolution and Galaxyformation. In this study we derive radial gradients of C, N, O, Mg, Al,and Si, as well as S, from abundance determinations in young OB stars.Our database is composed of a sample of 69 members of 25 open clusters,OB associations, and H II regions with Galactocentric distances between4.7 and 13.2 kpc. An important feature of this abundance database is thefact that the abundances were derived self-consistently in non-LTE usinga homogeneous set of stellar parameters. Such an uniform analysis isexpected to reduce the magnitude of random errors, as well as theinfluence of systematics in the gradients defined by the abundance andGalactocentric distance. The metallicity gradients obtained in thisstudy are, in general, flatter than the results from previous recentabundance studies of early-type stars. The slopes are found to bebetween -0.031 (for oxygen) and -0.052 dex kpc-1 (formagnesium). The gradients obtained for the studied elements are quitesimilar, and if averaged they can be represented by a single slope of-0.042+/-0.007dexkpc-1. This value is generally consistentwith an overall flattening of the radial gradients with time.
| Optical polarization observations in Hogg 22 and NGC 6204
We present new (UBVRI) multicolor linear polarimetric data for 22 of thebrightest stars in the area of the open clusters Hogg 22 and NGC 6204 tostudy the properties of the ISM (interstellar medium) toward theseclusters and between them. The new data were incorporated in our dataset of previous observations (Waldhausen et al. \cite{waldhausen}),resulting in 28 observed stars in the region. Our data yield for NGC6204 a mean polarization percentage ofPλ_max1.8%, close to the polarization valueproduced by the ISM with normal efficiency (Pλ_max 5 EB-V) with a color excess of EB-V =0.51.Meanwhile for Hogg 22, located behind NGC 6204, the mean polarization isPλ_max 2.15%, lower than the expected value forthe observed color excess of EB-V =0.68 (Forbes et al. 1996)and the average efficiency of polarization for the interstellar dust.The mean angle of the polarization vectors of Hogg 22 is θ=44.9°, which agrees with the expected angle produce by dust particlesaligned in the direction of the Galactic Plane(θ=48°), while for NGC 6204 a lower value,θ=33.7 °, was found. Therefore, we believe that Hogg 22 isdepolarized by the same dust that is polarizing NGC 6204, due todifferent orientations of the dust particles (and magnetic fields) thatpolarize the starlight.Based on observations obtanined at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.
| New variable stars in open clusters. I. Methods and results for 20 open clusters
We present high precision CCD photometry of 1791 objects in 20 openclusters with an age of 10 Myr to 1 Gyr. These observations wereperformed within the Δ a photometric system which is primarilyused to detect chemically peculiar stars of the upper main sequence.Time bases range from 30 min up to 60 days with data from severalnights. We describe the time series analysis reaching a detection limitof down to 0.006 mag for apparent variability. In total, we havedetected 35 variable objects of which four are not members of theircorresponding clusters. The variables cover the entireHertzsprung-Russell diagram, hence they are interesting targets forfollow-up observations.Based on observations obtained at Complejo Astronómico elLeoncito (CASLEO), operated under the agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba y San Juan; ESO-La Silla and UTSO-Las Campanas.
| Chemical Abundances for a Sample of Southern OB Stars
Chemical abundances are presented for carbon, nitrogen, oxygen,magnesium, aluminum, silicon, and sulfur in a sample of 25 OB stars thatare members of open clusters, OB associations, and H II regions havingGalactocentric distances that lie inside the solar Galactic orbit. Theelemental abundances are derived from fits of observed high-resolutionspectra to non-LTE synthetic line profiles. Our effective temperaturescale is based on calibrations of the Strömgren photometric indicescoupled to Hγ line profiles. For stars without Strömgrenindices, the reddening-free Q-parameter, defined from UBV photometry isused, with the Q-parameter calibrated against Teff for starsthat have Strömgren photometry. We also investigated othereffective temperature scales from the literature and conclude thatadoption of these other Teff scales, which typically resultin lower temperatures, produces dependences of the derived abundanceswith Teff. Our non-LTE abundances reveal dispersions that arevery similar to those expected from the analysis uncertainties. Thesolar abundances, some of which have been recently revised, fall withinthe abundance distributions defined by this sample of inner disk OBstars for all seven studied elements.Based on observations collected with the 1.52 m telescope at theEuropean Southern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| A multicolour CCD photometric study of the open clusters NGC 2866, Pismis 19, Westerlund 2, ESO96-SC04, NGC 5617 and NGC 6204
Accurate and deep BVI CCD photometry is presented for the core regionsof six southern compact open clusters (supplemented in one case by U andR bands), which generally have only scanty photometric data published,in some cases only photoelectric or photographic. Together with adoptionof modern reference isochrones, this has allowed us to derive improvedvalues for their distance, reddening, age and main sequence morphology.
| Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta asystem to derive astrophysical parameters such as the age, reddening anddistance modulus for open clusters. The Delta a system samples the fluxdepression at 520 nm which is highly efficient to detect chemicallypeculiar (CP) objects of the upper main sequence. The evolutionarystatus of CP stars is still a matter of debate and very important totest, for example, the dynamo and diffusion theories. In fact, thedynamo or fossil origin of the magnetic fields present in this kind ofstar is still not clear. Using the stellar evolutionary models by Claret(\cite{Cla95}), a grid of isochrones with different initial chemicalcompositions for the Delta a system was generated. The published dataof 23 open clusters were used to fit these isochrones with astrophysicalparameters (age, reddening and distance modulus) from the literature. Asan additional test, isochrones with the same parameters for Johnson UBVdata of these open clusters were also considered. The fits show a goodagreement between the observations and the theoretical grid. We findthat the accuracy of fitting isochrones to Delta a data without theknowledge of the cluster parameters is between 5 and 15%.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| Optical Polarization Observations of NGC 6231: Evidence for a Past Supernova Fingerprint
We present the first linear multicolor polarization observations for asample of 35 stars in the direction of the Galactic cluster NGC 6231. Wehave found a complex pattern in the angles of the polarimetric vectors.Near the core of this cluster the structure shows a semicircular patternthat we have interpreted as a reorientation of the dust particlesshowing the morphology of the magnetic field. We propose that asupernova event occurred some time ago and produced a shock on the localISM. We discuss in this paper independent confirmations of this event,both from the studies on the diffuse interstellar absorptions and theresults of the pre-main-sequence stars. We also show that a supernova issupported by the evolutionary status of the cluster.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.
| Optical polarization observations in the region of Stock 16
We present (UBVRI) multicolor linear polarimetric data for 26 of thebrightest stars in the area of the open cluster Stock 16 that wereconsidered to study the properties of the ISM (interstellar medium)towards the cluster. Our data yield a mean polarization percentage of P~ 2.5%, close to the polarization value produced by the ISM with normalefficiency (Plambda_max ~ 5 EB-V) undergoing acolor excess of EB-V =0.51. The mean angle of thepolarization vectors, theta = 74fdg9 , agrees quite well with theexpected angle produced by dust particles aligned in the direction ofthe galactic disk (and the magnetic field) in the region. A study of theextinction suffered by the stars in the zone was also performedcombining our new data with previous photometric data. In this sense,our analysis indicates that the visual absorption affecting Stock 16stars is mainly produced in front of the cluster by a dust cloud atapproximately 500 pc from the sun. The large polarization value of thenonmember star, WR 51, confirms its background star nature.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.Tables 1 and 2 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/409/933
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Deep-sky dedication.
Not Available
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Two highly reddened young open clusters located beyond the Sagittarius arm
We present the results of CCD BVI Johnson-Cousins photometry down to V ~19 mag in the regions of the unstudied stellar groups Pismis 23 and BH222, both projected close to the direction towards the Galactic centre.We measured V magnitude and B-V and V-I colours for a total of 928 starsin fields of about 4arcmin x4arcmin . Pismis 23 is conclusively aphysical system, since a clear main sequence and other meaningfulfeatures can be seen in the colour-magnitude diagrams. The reality ofthis cluster is also supported by star counts carried out within andoutside the cluster field. For Pismis 23 we derive colour excessesE(B-V) = 2.0 +/- 0.1 and E(V-I) = 2.6 +/- 0.1, a distance from the Sunof 2.6 +/- 0.6 kpc (Z = -19 pc) and an age of 300 +/- 100 Myr (assumingsolar metal content). BH 222 appears to be a young open cluster formedby a vertical main sequence and by a conspicuous group of luminous,typically red supergiant stars. We derived for this cluster a colourexcess of E(V-I) = 2.4 +/- 0.2, a distance from the Sun of 6.0 +/- 2.7kpc (Z = -46 pc) and an age of 60 +/- 30 Myr. The resulting reddeningand distance estimates place these two young objects among the mostreddened and distant open clusters known in the direction towards theGalactic centre. They are located beyond the Sagittarius arm, close tothe direction where this arm probably bifurcates into two arms. Based onobservations made at the University of Toronto (David DunlapObservatory) 24-inch telescope, Las Campanas, Chile. Tables \ref{t2} and3 are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/179
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| The Highly Polarized Open Cluster Trumpler 27
We have carried out multicolor linear polarimetry (UBVRI) of thebrightest stars in the area of the open cluster Trumpler 27. Our datashow a high level of polarization in the stellar light with aconsiderable dispersion, from P=4% to P=9.5%. The polarization vectorsof the cluster members appear to be aligned. Foreground polarization wasestimated from the data of some nonmember objects, for which twodifferent components were resolved: the first one associated with a dustcloud close to the Sun producing Pλmax=1.3% andθ=146°, and a second component, the main source ofpolarization for the cluster members, originating in another dust cloud,which polarizes the light in the direction of θ=29.5d. From adetailed analysis, we found that the two components have associatedvalues EB-V<0.45 for the first one andEB-V>0.75 for the other. Due the difference in theorientation of both polarization vectors, almost 90° (180° atthe Stokes representation), the first cloud (θ~146°)depolarizes the light strongly polarized by the second one(θ~29.5d). Based on observations obtanined at ComplejoAstronómico El Leoncito (CASLEO), operated under agreementbetween CONICET and the National Universities of La Plata,Córdoba, and San Juan, Argentina.
| 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
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Unidentified 3EG gamma-ray sources at low galactic latitudes
We present a study on the possible association of unidentified gamma-ray sources in the Third EGRET (3EG) catalog with different types ofgalactic objects such as Wolf-Rayet and Of stars, supernova remnants(SNRs), and OB associations (considered as pulsar tracers). We have madeuse of numerical simulations of galactic populations of gamma -ray pointsources in order to determine the statistical significance of thepositional coincidences. New constraints on pure chance association arepresented for SNRs and OB associations. There is overwhelmingstatistical support for the hypothesis that some SNRs are gamma -raysources (we obtain probabilities of pure chance association <10(-5)). OB star associations present higher, but still negligibleprobabilities (<10(-3) ). It is also shown that although massivestars present just marginally significant correlation with 3EG sources,there is a posteriori support for at least three candidates.
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